Ph.D Rhonda L Quinn
email: rhonda.quinn@shu.edu

Jason Lewis

Jean-Philip Brugal

Christopher J Lepre

Alexandr Trifonov

Sonia Harmand

Influences of dietary niche expansion and Pliocene environmental changes on the origins of stone tool making

Keywords: Omo-Turkana Depression, paleosol isotopes, enamel isotopes, Pliocene climate, Lomekwi 3 archaeological site

The world's oldest known stone tools discovered at the Pliocene site of Lomekwi 3 (LOM3) in the Nachukui Formation, northern Kenya, signals a prodigious behavioral change in the hominin lineage. LOM3, dated to 3.3 Ma, is significantly earlier than the first appearance of genus Homo and coincides with the onset of the Mid-Pliocene Warm Period, warranting new phylogenetic and environmental explanations for the origins of hominin stone tool making behaviors. Here we examined Pliocene paleoenvironments (4.3-3.0 Ma) within the Omo-Turkana Depression (OTD) with paleosol pedogenic carbonate (PC) and faunal enamel carbonate (EC)  13 C and  18 O values. Amidst the gradual shift toward more grassy environments, intranodular  13 CPC and  18 OPC variation slightly increased through time. Three of fourteen ungulate herbivore taxa yielded statistically significant increases in  13 CEC values. When all ungulate herbivore  13 CEC values are grouped by feeding categories (C3-browsers, C3-C4-mixed feeders, C4-grazers) there is evidence that the C3-C4-mixed feeding niche contracted and shifted toward the C4-grazing niche as hominins expanded their dietary breadth during the middle Pliocene. Only one of nine taxa showed a significant decrease in  18 OEC values across the study interval. Abrupt, significant environmental perturbations are not evident during LOM3 times in the OTD, casting doubt on variability selection or pulsed humidity as selective forces for middle Pliocene technological innovations. We propose that a long-term feedback system involving gradual environmental changes and biotic dietary competition influenced a suite of hominin adaptations to a broad dietary niche, culminating in the origins of stone tool making behaviors.

Introduction

Repositioning the adaptive milestones of early African hominins

Stone tool making is characterized as a critical adaptive threshold, which once crossed, paved the way toward language, complex social interactions and global dispersal [START_REF] Ambrose | Paleolithic technology and human evolution[END_REF]. The ability to make stone tools was originally thought to be one of the defining characteristics of genus Homo; its earliest member, Homo habilis ("handy man"), was named for the adaptation [START_REF] Leakey | A new species of the genus Homo from Olduvai Gorge[END_REF]. The origin and evolution of stone tool technologies associated with genus Homo have been linked to environmental perturbations [START_REF] Clark | Human ecology during Pleistocene and later times in Africa south of the Sahara[END_REF][START_REF] Potts | Evolution and climate variability[END_REF][START_REF] Potts | Variability selection and hominid evolution[END_REF][START_REF] Potts | Environmental and behavioral evidence pertaining to the evolution of early Homo[END_REF][START_REF] Potts | Hominin evolution in settings of strong environmental variability[END_REF][START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF][START_REF] Ziegler | Development of Middle Stone Age innovation linked to rapid climate change[END_REF]. [START_REF] Potts | Alternating high and low climate variability: the context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF] showed that early members of genus Homo were bombarded with near consecutive periods of environmental instability during the late Pliocene-early Pleistocene associated with global climatic forcing factors via teleconnections to the tropics. Global climatic and large-scale environmental changes emphasizing the increasing presence of open and variable ("mosaic") landscapes were traditionally and continue to be evoked as primary selective pressures for hominin speciation events [START_REF] Alemseged | Fossils from Mille-Logya, Afar, Ethiopia, elucidate the link between Pliocene environmental changes and Homo origins[END_REF][START_REF] Brain | The evolution of man in Africa: was it a consequence of Cainozoic cooling? Alex L. du Toit Memorial Lectures 17[END_REF][START_REF] Bromage | Biogeographic and climatic basis for a narrative of early hominid evolution[END_REF][START_REF] Demenocal | Plio-Pleistocene African climate[END_REF][START_REF] Demenocal | African climate change and faunal evolution during the Pliocene-Pleistocene[END_REF][START_REF] Stanley | An ecological theory for the origin of Homo[END_REF][START_REF] Trauth | Late Cenozoic moisture history of East Africa[END_REF][START_REF] Trauth | High-and low-latitude forcing of Plio-Pleistocene African climate and human evolution[END_REF][START_REF] Trauth | Trends, rhythms and events in Plio Pleistocene African climate[END_REF][START_REF] Vrba | Ecological and adaptive changes associated with early hominid evolution[END_REF][START_REF] Vrba | The fossil record of African antelopes (Mammalia, Bovidae) in relation to human evolution and paleoclimate[END_REF][START_REF] Vrba | Habitat theory in relation to the evolution in African Neogene biota and hominids[END_REF] and influences of specific adaptations including thermoregulation [START_REF] Ruff | Climate and body shape in hominid evolution[END_REF][START_REF] Ruff | Climatic adaptation and hominid evolution: the thermoregulatory imperative[END_REF][START_REF] Ruff | Body size and body shape[END_REF][START_REF] Wheeler | The thermoregulatory advantages of hominid bipedalism in open equatorial environments: the contribution of increased convective heat loss and cutaneous evaporative cooling[END_REF]Wheeler, , 1992aWheeler, , 1992b[START_REF] Wheeler | The thermoregulatory advantages of heat storage and shadeseeking behaviour to hominids foraging in equatorial savannah environments[END_REF], energetic efficiency [START_REF] Aiello | The expensive-tissue hypothesis[END_REF][START_REF] Aiello | Energetics and the evolution of the genus Homo[END_REF]Bramble and Leiberman, 2004;[START_REF] Leonard | Comparative primate energetics and hominid evolution[END_REF][START_REF] Wheeler | The influence of stature and body form on hominid energy and water budgets: a comparison of Australopithecus and early Homo physiques[END_REF], and encephalization (Ash and Gallop, 2007;[START_REF] Bailey | Hominid brain evolution: testing climatic, ecological, and social competition models[END_REF][START_REF] Shultz | Early human speciation, brain expansion and dispersal influenced by African climate pulses[END_REF].

Published in 2015 [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF] the Lomekwi 3 archaeological site, (LOM3; Figure 1) located in the Nachukui Formation of the Omo-Turkana Depression (OTD; northern Kenya-southern Ethiopia; Figure 2) and dated to 3.3 Ma, revealed that Pliocene hominins made stone tools substantially earlier than previously thought. As compared to the evidence of stone tool use with the discovery of cutmarked bone at Dikika dated to 3.4 Ma (McPherron et al., 2010), LOM3 provided evidence of stone tool manufacturing. Prior to the discovery of LOM3, the earliest known stone tool making behaviors were preserved at Gona (Ethiopia) and dated to 2.6-2.5 Ma [START_REF] Semaw | 2.5million-year-old stone tools from Gona[END_REF]. The LOM3 site and the surrounding paleontological collecting area are substantially older than the known origins of genus Homo dated to 2.8 Ma from Ledi-Geraru (Ethiopia) [START_REF] Villmoare | Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia[END_REF]. Genus Homo identified as the first, and perhaps only, stone toolmaker in the earliest Pleistocene has been questioned for some time (e.g., [START_REF] Panger | Older than the Oldowan? rethinking the emergence of hominin tool use[END_REF][START_REF] Susman | Who made the Oldowan tools? Fossil evidence for tool behavior in Plio-Pleistocene hominids[END_REF], but these finds likely place the tools in the hands of australopithecine-like hominins. In eastern and north-central Africa, hominin species that date to the middle Pliocene have been designated as Australopithecus afarensis [START_REF] Kimbel | Lucy" redux: a review of research on Australopithecus afarensis[END_REF], A. deyiremeda [START_REF] Haile-Selassie | New species from Ethiopia further expands Middle Pliocene hominin diversity[END_REF], A. bahrelghazali [START_REF] Brunet | Australopithecus bahrelghazali, a new species of early hominid from Koro Toro region, Chad[END_REF] and Kenyanthropus platyops [START_REF] Leakey | New hominin genus from eastern Africa shows diverse middle Pliocene lineages[END_REF].

Although specimens were initially identified as A. afarensis [START_REF] Brown | New hominids from the Lake Turkana Basin, Kenya[END_REF][START_REF] Kimbel | Identification of a partial cranium of Australopithecus afarensis, from the Koobi Fora Formation, Kenya[END_REF]Ward et al., 1999a), the majority of middle Pliocene hominin specimens found in the OTD has been designated as K. platyops [START_REF] Wood | The Omo-Turkana Basin fossil hominins and their contribution to our understanding of human evolution in Africa[END_REF].

From excavations in 2011, 2012, and 2014, the West Turkana Archaeological Project (WTAP) recovered 165 surface and in situ artifacts at LOM3 [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Lewis | An earlier origin for stone tool making: implications for cognitive evolution and the transition to Homo[END_REF]. WTAP excavations recovered cores and flakes that are significantly larger than those from the Early Oldowan [START_REF] Braun | Earliest known Oldowan artifacts at >2.58 Ma from Ledi-Geraru, Ethiopia, highlight technological diversity[END_REF][START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Roche | Early hominid stone tool production and technical skill 2.34 myr ago in West Turkana, Kenya[END_REF][START_REF] Stout | Technological variation in the earliest Oldowan from Gona, Afar, Ethiopia[END_REF] and include heavy anvils, percussors, and worked and split cobbles made predominantly from local basalts. Although the LOM3 artifacts do not indicate a mastery of stone knapping demonstrated by Oldowan toolmakers, as detailed in [START_REF] Lewis | An earlier origin for stone tool making: implications for cognitive evolution and the transition to Homo[END_REF], they provide evidence of sensorimotor performance and an effective control of elementary percussive gestures beyond what has been observed in chimpanzees either in the wild or during experiments in captivity.

Stone tool making and use were not the only novel behaviors of Pliocene African hominins. Several species show evidence for a significant dietary shift with the incorporation of C4 foods as revealed by stable carbon isotopic values of enamel carbonate (denoted here as  13 CEC) (Cerling et al., 2013b;[START_REF] Lee-Thorp | Diet of Australopithecus robustus at Swartkrans from stable carbon isotopic analysis[END_REF]Sponheimer and Lee-Thorp, 1999a). Middle Pliocene hominin  13 CEC values grouped by identified species and/or separated into temporal bins (e.g., [START_REF] Patterson | Comparative isotopic evidence from East Turkana supports a dietary shift within the genus Homo[END_REF] show a directional shift toward consumption of C4 foods, which may have included C4 sedges and grass seeds, underground storage organs, termites and other arthropods, bird's eggs, as well as C3-C4-mixed-and C4-feeding vertebrates [START_REF] Peters | Africa's wild C4 plant foods and possible early hominid diets[END_REF][START_REF] Sponheimer | Differential resource utilization by extant great apes and Australopithecines: towards solving the C4 conundrum[END_REF][START_REF] Yaekel | The isotopic ecology of African mole rats informs hypotheses on the evolution of human diet[END_REF]. There is also an increase in  13 CEC variance, which has been interpreted to indicate dietary niche expansion relative to the earliest hominins (i.e., Ardipithecus and A. anamensis) and extant chimpanzees [START_REF] Antón | Evolution of early Homo: an integrated biological perspective[END_REF][START_REF] Levin | Dietary change among hominins and cercopithecids in Ethiopia during the early Pliocene[END_REF][START_REF] Lee-Thorp | What do stable isotopes tell us about hominin dietary and ecological niches in the Pliocene?[END_REF][START_REF] Kohn | Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate[END_REF][START_REF] Sponheimer | Isotopic evidence of early hominin diets[END_REF][START_REF] Van Der Merwe | The carbon isotope ecology and diet of Australopithecus africanus at Sterkfontein, South Africa[END_REF]. The earliest mixed C3-C4 feeding hominins with measured  13 CEC values and identified to species are A. bahrelghazali at 3. 6 Ma (Lee-Thorp et al., 2012), A. afarensis at 3.4 Ma [START_REF] Wynn | Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia[END_REF], and in the OTD, K. platyops at 3.4 Ma (Cerling et al., 2013b) (Figure 3). Consumption of C4 food resources by hominins has been found as early as 3.7 Ma at Woranso-Mille (Ethiopia); these hominin specimens were not identified to species but are likely australopithecines [START_REF] Levin | Dietary change among hominins and cercopithecids in Ethiopia during the early Pliocene[END_REF].

Searching for environmental forces of hominin evolution

Finding evidence for stone tool making behaviors at 3.3 Ma potentially by australopithecine-like hominins complicates hypotheses of temporally discrete environmental perturbations as primary selective pressures for this adaptive milestone (Figure 4A-D). The formation of deep lakes forced by eccentricity-modulated precession has been proposed as a mechanism for environmental change influencing human evolution [START_REF] Donges | Nonlinear detection of paleoclimate-variability transitions possibly related to human evolution[END_REF][START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Trauth | Late Cenozoic moisture history of East Africa[END_REF][START_REF] Trauth | High-and low-latitude forcing of Plio-Pleistocene African climate and human evolution[END_REF][START_REF] Trauth | Trends, rhythms and events in Plio Pleistocene African climate[END_REF]. Deep African lakes are interpreted to have occurred throughout the Pliocene (e.g., [START_REF] Trauth | Late Cenozoic moisture history of East Africa[END_REF], but an identified lake phase in the OTD, Lake Lokochot [START_REF] Feibel | A geological history of the Turkana Basin[END_REF], ends approximately 200 Ka prior to the LOM3 site date (Figure 4D). [START_REF] Potts | Evolution and climate variability[END_REF][START_REF] Potts | Variability selection and hominid evolution[END_REF] links high environmental variability to several hominin speciation events and technological innovations, but the middle Pliocene appears to encompass a long period of low climatic variability [START_REF] Potts | Alternating high and low climate variability: the context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF] (Figure 4C). Modeled global pCO2 concentrations by [START_REF] Stap | CO2 over the past 5 million years: continuous simulation and new d11B-based proxy data[END_REF] (Figure 4A) suggests a steady decrease through time aligning with trends in sea level, temperature, and global ice volume (Figure 4B). The LOM3 site is coincident with the start of the mid-Pliocene Warm Period (mPWP: 3.264-3.025 Ma: Dowsett et al., 2010;[START_REF] Haywood | Pliocene model intercomparison project (PlioMIP): experimental design and boundary conditions (Experment1)[END_REF], which is commonly used as an analogue for anthropogenic conditions of today [START_REF] Burke | Pliocene and Eocene provide best analogs for near-future climates[END_REF]. The mPWP is characterized as "a warm blip," potentially ~2-3C degrees warmer in the tropics than present day [START_REF] Haywood | Large-scale features of Pliocene climate: results from the Pliocene Model Intercomparison Project[END_REF], high atmospheric CO2 levels, and low seasonal variability in rainfall [START_REF] Pagani | High Earth-system climate sensitivity determined from Pliocene carbon dioxide concentrations[END_REF][START_REF] Raymo | Mid-Pliocene warmth: stronger greenhouse and stronger conveyor[END_REF]. The warm and wet Pliocene world [START_REF] Salzmann | Climate and environment of a Pliocene warm world[END_REF] is often used to contrast environmental proxy records of the late Pliocene when Earth begins its major climatic reorganization near the onset of the Quaternary ice age [START_REF] Maslin | The contribution of orbital forcing to the progressive intensification of Northern Hemisphere glaciation[END_REF][START_REF] Mudelsee | Slow dynamics of the Northern Hemisphere glaciation. Paleooceanogr. 20[END_REF][START_REF] Shackleton | Oxygen isotope and palaeomagnetic evidence for early Northern Hemisphere glaciation[END_REF][START_REF] Willeit | The role of CO2 decline for the onset of Northern Hemisphere glaciation[END_REF]. Compared to the early Pliocene, however, mPWP climatic proxy records appear to indicate relatively constant conditions [START_REF] Haywood | Integrating geological archives and climate models for the mid-Pliocene warm period[END_REF]. Permanent El Niño conditions are interpreted for the early and middle Pliocene due to a deep eastern Pacific thermocline and shallow sea-surfacetemperature gradient across the equatorial Pacific [START_REF] Fedorov | The Pliocene paradox (mechanisms for a permanent El Niño)[END_REF][START_REF] Molnar | El Niño's tropical climate and teleconnections as a blueprint for pre-Ice Age climates Paleoceanog[END_REF][START_REF] Wara | Permanent El Nino-like conditions during the Pliocene warm period[END_REF].

Characterization of the LOM3 site environment with the pedogenic carbonate  13 C-based paleo-shade proxy of Cerling et al. (2011) positioned the site in C3-dominated environments in the grassy woodland/bushland/shrubland category (fraction of woody canopy cover, WC =60-40%) [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF]. WC comparisons to other middle Pliocene (3.4-3.2 Ma) eastern African hominin locales from the Awash Basin (Ethiopia), Tugen Hills (Kenya), and elsewhere in the OTD found that none were significantly different from one another, but all were woodier than the earliest Oldowan site of Gona [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF]. Study of the Fe oxides in paleosols from the northeast Turkana Basin at 3.3 Ma indicate a mean annual rainfall estimate of 500-1000 mm/year [START_REF] Lepre | Constraints on Fe-oxide formation in monsoonal Vertisols of Pliocene Kenya using rock magnetism and spectroscopy[END_REF]. These estimates are comparable to those interpreted for the middle Pliocene OTD based on faunal ecomorphic analysis [START_REF] Fortelius | An ecometric analysis of the fossil mammal record of the Turkana Basin[END_REF] and the  18 OEC-based aridity index [START_REF] Blumenthal | Aridity and hominin environments[END_REF]. Current OTD middle Pliocene environmental reconstructions imply that stone tool making occurred in more humid and closed environments than widely found for genus Homo (e.g., [START_REF] Plummer | Oldest evidence of toolmaking hominins in a grasslanddominated ecosystem[END_REF][START_REF] Reed | Early hominid evolution and ecological change through the African Plio-Pleistocene[END_REF][START_REF] Sikes | Early hominid habitat preferences in East Africa: paleosol carbon isotopic evidence[END_REF][START_REF] Sikes | Early Pleistocene habitat in Member 1 Olorgesailie based on paleosol stable isotopes[END_REF].

Additionally, through-time records show little evidence for substantial environmental shifts from the early to middle Pliocene, especially compared to the late Pliocene (Figure 5). From the compiled pedogenic carbonate (subscript PC)  13 C datasets of the OTD, there is a gradual increase in grassy environments from 4-1 Ma (Figure 5A); however the largest increase occurs after 2 Ma (Cerling et al., 2011a,b;[START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF]. The compiled herbivore ungulate OTD  13 CEC database demonstrates a relatively steady increase in C4-grazers from 4-1 Ma (Figure 5B); although sample sizes are not uniform and some time periods have small sample sizes, the trend appears gradual.

Faunal abundance data of [START_REF] Bobe | The expansion of grassland ecosystems in Africa in relation to mammalian evolution and the origin of the genus Homo[END_REF] from the Lower Omo Valley shows a major shift in grassland indicators after 3 Ma; the middle Pliocene appears stable (Figure 5C). [START_REF] Passey | Hightemperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates[END_REF] estimated Plio-Pleistocene OTD soil temperatures with pedogenic carbonate clumped isotopes and found consistently high temperatures but no directional trend from 4-1 Ma (Figure 5D). [START_REF] Fortelius | An ecometric analysis of the fossil mammal record of the Turkana Basin[END_REF] estimated air temperatures in the OTD from 7-1 Ma via ecomorphic proxies and similarly found no directional shift through time. [START_REF] Blumenthal | Aridity and hominin environments[END_REF] found that aridity does not increase from 4-1 Ma, but rather variable water deficits are found throughout the OTD (Figure 5E). [START_REF] Fortelius | An ecometric analysis of the fossil mammal record of the Turkana Basin[END_REF] found a general decreasing trend in mean annual precipitation in the OTD based on ecometric analysis, but the largest decrease in rainfall was in the upper portion of the record from 3-1 Ma.

Detecting environmental change relevant to questions of human evolutionary milestones depends on the scale of each proxy record (e.g., [START_REF] Behrensmeyer | Climate change and human evolution[END_REF][START_REF] Kingston | Shifting adaptive landscapes: progress and challenges in reconstructing early hominid environments[END_REF]. [START_REF] Kingston | Shifting adaptive landscapes: progress and challenges in reconstructing early hominid environments[END_REF] and [START_REF] Potts | Environmental and behavioral evidence pertaining to the evolution of early Homo[END_REF][START_REF] Potts | Hominin evolution in settings of strong environmental variability[END_REF] suggested that, in order to test if key evolutionary events in the hominin lineage occurred during intervals of high (global/regional) climatic variability or in response to significantly increased aridity or humidity, climate proxy records from terrestrial hominin environments at resolutions of seasonal-to orbital-scale should be evaluated. The search for high-resolution records of environmental change relevant to patterns and processes of human evolution motivated the Hominin Sites and Paleolakes Drilling Project (HSPDP; [START_REF] Campisano | The Hominin Sites and Paleolakes Drilling Project: High-resolution paleoclimate records from the East African rift system and their implications for understanding the environmental context of hominin evolution[END_REF][START_REF] Cohen | Understanding paleoclimate and human evolution through the Hominin Sites and Paleolakes Drilling Project[END_REF][START_REF] Cohen | The Hominin Sites and Paleolakes Drilling Project: inferring the environmental context of human evolution from eastern African rift lake deposits[END_REF]. The drill project comprised several fossil hominin locales in eastern Africa. In the OTD, however, the target interval was focused on the younger part of the record [START_REF] Lupien | A leaf wax biomarker record of early Pleistocene hydroclimate from West Turkana, Kenya[END_REF][START_REF] Sier | The top of the Olduvai Subchron in a high-resolution magnetostratigraphy from the West Turkana core WTK13, hominin sites and Paleolakes Drilling Project (HSPDP)[END_REF] during the tenure of Homo erectus [START_REF] Lepre | Chronostratigraphy of KNM-ER 3733 and other Area 104 hominins from Koobi Fora[END_REF]. HSPDP environmental reconstructions reported to date suggest changes in vegetation structure and aridity were associated with the both high-and lowlatitude climate forcing and coincide with major evolutionary events in genus Homo (e.g., [START_REF] Lupien | Vegetation change in the Baringo Basin, East Africa across the onset of Northern Hemisphere glaciation 3[END_REF][START_REF] Potts | Environmental dynamics during the onset of the Middle Stone Age in eastern Africa[END_REF]. HSPDP cores from the Awash and Baringo Basins reached maximum depths that date to the middle/late Pliocene [START_REF] Deino | Chronostratigraphy of the Baringo-Tugen-Barsemoi (HSPDP-BTB13-1A) core: 40 Ar/ 39 Ar dating, magnetostratigraphy, tephrostratigraphy, sequence stratigraphy and Bayesian age modeling[END_REF][START_REF] Lupien | Vegetation change in the Baringo Basin, East Africa across the onset of Northern Hemisphere glaciation 3[END_REF][START_REF] Westover | Diatom paleolimnology of late Pliocene Baringo Basin (Kenya) paleolakes. Palaeogeog, Palaeoclim[END_REF], which are highly relevant for the environments during the rise of genus Homo but postdate the complete time interval of interest for Pliocene hominin diet change and stone tool making origins.

Approach of study

The early flickering of environmental interactions with unmodified or minimally worked stones has not likely been discovered given the subtle archaeological traces and current modes of field-based artifact search and recovery methods (e.g., [START_REF] Mcpherron | Tool-marked bones from before the Oldowan change the paradigm[END_REF][START_REF] Thompson | Origins of the human predatory pattern[END_REF]. To date, LOM3 is the earliest known evidence for stone tool making behaviors by early African hominins and represents an important datum to identify selective pressures for this adaptation. Our approach in this study relies on current data suggesting that hominin tool making behaviors arose within the 4-3 Ma interval. Naturally, future archaeological discoveries may change the supposed time period of interest, similar to the evidence that has come to light within the last decade [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Mcpherron | Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia[END_REF].

Since the original publication of the LOM3 site only included pedogenic carbonate  13 C-based vegetation reconstructions and vertebrate specimen identification to characterize the environment, we examined other proxy records to detect environmental changes during the middle Pliocene. Paleosol  13 CPC values provide spatially limited (~1 m 2 ) and time averaged (~10 1 -10 3 years) estimates of vegetation structure [START_REF] Birkeland | Soils and Geomorphology[END_REF][START_REF] Srivastava | Paleoclimatic implications of pedogenic carbonates in Holocene soils of the Gangetic Plains, India[END_REF] and may not have the resolution to reveal the vegetation structural diversity available with  13 CEC values of fauna [START_REF] Du | Stable carbon isotopes from paleosol carbonate and herbivore enamel document differing paleovegetation signals in the eastern African Plio-Pleistocene[END_REF]. Here we contribute new  18 OPC data of paleosols and faunal  13 CEC and  18 OEC values of specimens collected at LOM3 and in the surrounding paleontological collecting areas.

We focus on the interval of 4.3-3.0 Ma, which is before a major paleohydrographic reconfiguration of the OTD [START_REF] Lepre | Early Pleistocene lake formation and hominin origins in the Turkana-Omo rift[END_REF] and the detected significant environmental changes there and across eastern Africa after 3 Ma (Cerling et al., 2011a,b;[START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF]. At present and even with this study, sampling across the time bracketing the rise of hominin tool making behaviors and dietary changes (4.3-3.0 Ma) is not uniformly distributed, neither temporally nor spatially, primarily due to differential preservation of paleosols and fossil material suitable for isotopic analyses. Research focus has also impacted sample density. Much environmental research has historically focused on the rise and evolution of Homo. Early and middle Pliocene environments in eastern African hominin locales have been used as the "before" interval to emphasize the magnitude of directional shifts and changes in climatic variability in the "after" interval during the late Pliocene and early Pleistocene. We shifted the comparison to the early and middle Pliocene by examining isotopic proxy records in two 400-kya intervals of time (4.3-3.9 Ma and 3.4-3.0 Ma). These relatively large temporal bins are comparable to those used by [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF] and have considerable sample sizes of paleosols [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF][START_REF] Quinn | Revisiting the pedogenic carbonate isotopes and paleoenvironmental interpretation of Kanapoi[END_REF][START_REF] Wynn | Paleosols, stable carbon isotopes and paleoenvironmental interpretation of Kanapoi, Northern Kenya[END_REF][START_REF] Wynn | Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols from the Turkana Basin, Kenya[END_REF] and faunal enamel datasets [START_REF] Blumenthal | Aridity and hominin environments[END_REF][START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF][START_REF] Manthi | Diets of mammalian fossil fauna from Kanapoi, northwestern Kenya[END_REF][START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF] this study), including those of hominins (Cerling et al., 2013b).

Australopithecus anamensis occurs in the OTD during 4.3-3.9 Ma [START_REF] Leakey | New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya[END_REF][START_REF] Leakey | New specimens and confirmation of an early age for Australopithecus anamensis[END_REF]Ward et al., 1999b[START_REF] Ward | Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya[END_REF], thus we use this species and time period as a comparison to gauge potentially new or different selective pressures on stone tool making behaviors preserved in the 3.4-3.0 Ma.

Although a comparison of all middle Pliocene eastern African hominin sites is a useful endeavor for looking at large-scale environmental shifts (e.g., [START_REF] Drapeau | The Omo Mursi formation: A window into the east African Pliocene[END_REF][START_REF] Kingston | Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: implications for early hominin paleoecology[END_REF][START_REF] Trauth | Late Cenozoic moisture history of East Africa[END_REF], we focus our study on records in the OTD to minimize confounding influences on the proxy records used here as a function of the idiosyncratic geography and hydrology of different rift basins. Eastern African basins are variable with respect to temperature [START_REF] Gebrechorkos | Long-term trends in rainfall and temperature using high-resolution climate datasets in East Africa[END_REF], elevation [START_REF] Junginger | Hydrological constraints of paleo-Lake Suguta in the northern Kenya rift during the African humid period (15-5 ka BP)[END_REF], and meteoric water sources [START_REF] Levin | Isotopic composition of waters from Ethiopia and Kenya: insights into moisture sources for eastern Africa[END_REF], which in turn influence vegetation  13 C and  18 O values [START_REF] Blumenthal | Stable isotopic variation in tropical forest plants and applications in primatology[END_REF][START_REF] Sparks | Leaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects[END_REF][START_REF] Tieszen | Natural variation in the carbon isotope values of plants: Implications for archaeology, ecology, and paleoecology[END_REF][START_REF] Yakir | Natural deuterium and oxygen-18 enrichment in leaf water of cotton plants grown under wet and dry conditions: evidence for water compartmentation and its dynamics[END_REF] and soil pore and surface water  18 O values [START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF][START_REF] Dansgaard | Stable isotopes in precipitation[END_REF][START_REF] Gat | Oxygen and hydrogen isotopes in the hydrologic cycle[END_REF][START_REF] Kim | Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates[END_REF].

Taphonomic and collector biases are also concerns when reconstructing vegetation structures with isotopic analyses of faunal remains (e.g., [START_REF] Robinson | Late Pliocene environmental change during the transition from Australopithecus to Homo[END_REF]. Moreover, considering modern mammalian speciosity in Kenya alone (309 species, Musila et al., 2019) and herbivore ungulate population sizes (e.g., 10 2 -10 4 ; [START_REF] Georgiadis | Conserving Wildlife in African Landscapes: Kenya's Ewaso Ecosystem[END_REF], the number of enamel samples analyzed to date from eastern African fossil locales is miniscule. Due to preservation in and around the LOM3 site, we contribute a modest faunal enamel sample in order to make better comparisons to the early Pliocene OTD databases, but emphasize that larger sample sizes from many species are warranted to gauge the environmental context of Pliocene hominins.

The Omo-Turkana Depression

The OTD, also known as the Turkana Depression and the Turkana-Omo rift, extends across northern Kenya and southern Ethiopia (Figure 2) and lies within the eastern branch of the East African Rift System (EARS) between the Kenyan and Ethiopian domes [START_REF] Ebinger | Rift deflection, migration, and propagation: linkage of the Ethiopian and Eastern rifts, Africa[END_REF][START_REF] Knapp | Accommodation of East African rifting across the Turkana Depression[END_REF]. The OTD is composed of several structural sub-basins within the tectonic complex associated with the development of the EARS [START_REF] Feibel | A geological history of the Turkana Basin[END_REF]. The OTD presently contains one of the largest rift lakes, Lake Turkana, with an area of 7000 km 2 and a catchment surface of 130,800 km 2 [START_REF] Unep ; University Of Kalmar | Molar microwear textures and the diets of Australopithecus anamensis and Australopithecus afarensis[END_REF][START_REF] Frostick | The East African Rift basins[END_REF]. Today, the lake is a saline-alkaline and closed-basin lake that receives over 90% of its water from rainfall over the Ethiopian Highlands, via the Omo River, with minor inputs from the Turkwel and Kerio river systems [START_REF] Yuretich | Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana), eastern Rift Valley, Kenya[END_REF][START_REF] Cerling | A Mass-balance approach to basin sedimentation: constraints on the recent history of the Turkana Basin[END_REF]. The general climate within the rift is arid to semi-arid and receives 250-500 mm of rainfall annually [START_REF] Nicholson | A review of climate dynamics and climate variability in Eastern Africa[END_REF]. Air temperatures are hot, averaging 29C today, and have been estimated to 30-40C during the Plio-Pleistocene [START_REF] Passey | Hightemperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates[END_REF]. The Omo River meander belt and delta, now situated in the Omo Valley and northern Turkana Basin respectively, has gallery forests along its banks with depths of about ~1 km, which transition to open grasslands along the floodplains [START_REF] Carr | Patterns of vegetation along the Omo River in southwest Ethiopia[END_REF].

Mudflats and grasslands surround present day Lake Turkana, and woody vegetation is largely restricted to the deltaic systems and thin margins of perennial and ephemeral tributaries [START_REF] Mbaluka | Vegetation of the Koobi Fora region northeast of Lake Turkana, Marsabit County, Northern Kenya[END_REF].

Holocene and late Pleistocene lacustrine proxy records demonstrate that Lake Turkana's water levels and temperatures were responsive to regional-scale climatic events such as the African Humid Period and precession-paced variations in solar insolation [START_REF] Halfman | High-resolution record of cyclic climatic change during the past 4 ka from Lake Turkana, Kenya[END_REF][START_REF] Forman | Variations in water level for Lake Turkana in the past 8500 years near Mt. Porr, Kenya and the transition from the African humid period to holocene aridity[END_REF][START_REF] Garcin | East African mid-Holocene wet-dry transition recorded in palaeo-shorelines of Lake Turkana, northern Kenya Rift[END_REF][START_REF] Bloszies | Water level history for Lake Turkana, Kenya in the past 15,000 years and a variable transition from the African humid period to holocene aridity[END_REF][START_REF] Morrissey | Late Quaternary TEX86 paleotemperatures from the world's largest desert lake, Lake Turkana, Kenya[END_REF][START_REF] Morrissey | Paleohydrology of Lake Turkana and its influence on the Nile River System[END_REF][START_REF] Nutz | Stepwise drying of Lake Turkana at the end of the African humid period: a forced regression modulated by solar activity variations?[END_REF]. A rich archaeological record of early pastoralists and fishing peoples suggests a dynamic livelihood on these changing lakeshores [START_REF] Ashley | Paleoenvironmental context of archaeological sites, implications for subsistence strategies under Holocene climate change, northern Kenya[END_REF][START_REF] Hildebrand | A monumental cemetery built by eastern Africa's first herders near lake Turkana, Kenya[END_REF][START_REF] Lahr | Inter-group violence among early Holocene hunter-gatherers of West Turkana, Kenya[END_REF][START_REF] Owen | Palaeolimnology and archaeology of Holocene deposits north-east of Lake Turkana, Kenya[END_REF][START_REF] Prendergast | Fishing in a fluctuating landscapes: terminal Pleistocene and early Holocene subsistence strategies in the Lake Turkana Basin, Kenya[END_REF][START_REF] Wright | Lakeside view: sociocultural responses to changing water levels of Lake Turkana[END_REF]).

Plio-Pleistocene sedimentary strata in the OTD, designated as the Omo Group (de Heinzelin, 1983), begins circa 4.3 Ma when the region shifts from the volcanic activity of mostly flood basalts to a depositional regime dominated by sedimentary clastic detritus [START_REF] Brown | Geochronology of the Turkana Depression of Northern Kenya and Southern Ethiopia[END_REF][START_REF] Bruhn | Pliocene volcano-tectonics and paleogeography of the Turkana Basin, Kenya and Ethiopia[END_REF]. Omo Group Plio-Pleistocene environmental proxy records include those from the Kanapoi and Lothagam formations in the southwestern portion of the Turkana Basin, the Mursi Formation located in the Omo River Valley, the Koobi Fora Formation exposed along the northeast and southeast side of the Turkana Basin, and the Nachukui Formation along the northwest and southwest basin [START_REF] Feibel | Palaeoenvironmental context for the late Neogene of the Turkana Basin[END_REF]Feibel, 2003aFeibel, , 2003b;;[START_REF] Harris | Geology and paleontology of Plio-Pleistocene localities west of Lake Turkana, Kenya[END_REF]. Pliocene mudstones, sandstones, and conglomerates comprise the bulk of the Nachukui Formation's Lomekwi Member, which preserves the LOM3 site [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF].

The Omo Group formations have been dated and correlated through a combination of tephrastratigraphy, radioisotopic ages, and paleomagnetic stratigraphy (Brown andFeibel, 1986, 1991;[START_REF] Feibel | Stratigraphic context of fossil hominids from the Omo Group deposits; northern Turkana Basin, Kenya and Ethiopia[END_REF][START_REF] Hillhouse | Magnetostratigraphy of the Koobi Fora Formation, Lake Turkana, Kenya[END_REF][START_REF] Lepre | Early Pleistocene lake formation and hominin origins in the Turkana-Omo rift[END_REF]Lepre andKent, 2010, 2015;[START_REF] Lepre | An earlier origin for the Acheulian[END_REF][START_REF] Mcdougall | K-Ar and 40 Ar/ 39 Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya[END_REF][START_REF] Mcdougall | K-Ar and 40 Ar/ 39 Ar dating of the hominid-bearing Pliocene-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya[END_REF]Brown, 2006, 2008;[START_REF] Mcdougall | New single crystal 40 Ar/ 39 Ar ages improve time scale for deposition of the Omo Group, Omo-Turkana Basin, East Africa[END_REF], which has afforded a high degree of temporal resolution for several long records of terrestrial environments (Figure 5).

Late Pliocene and early to middle Pleistocene sedimentary records indicate the

presence of large lakes. The longest in duration is Lake Lorenyang [START_REF] Feibel | A terrestrial auxillary stratotype point and section for the Plio Pleistocene boundary in the Turkana Basin, East Africa[END_REF] beginning at 2.14 Ma [START_REF] Lepre | Early Pleistocene lake formation and hominin origins in the Turkana-Omo rift[END_REF] and extending to ~1.5 Ma [START_REF] Feibel | A geological history of the Turkana Basin[END_REF] and likely caused by the damning of the basin with the eruption of Mt. Kulal during the early Pleistocene [START_REF] Bruhn | Pliocene volcano-tectonics and paleogeography of the Turkana Basin, Kenya and Ethiopia[END_REF]. Fluctuations in lake levels during the early Pleistocene may have been influenced by orbitally-paced variations in water delivery to the Omo River source region in the Ethiopian Highlands [START_REF] Boës | Lake-level changes and hominin occupations in the arid Turkana basin during volcanic closure of the Omo River outflows to the Indian Ocean[END_REF][START_REF] Joordens | An astronomically-tuned climate framework for hominins in the Turkana Basin[END_REF][START_REF] Lepre | Plio-Pleistocene facies environments from the KBS Member, Koobi Fora Formation: implications for climate controls on the development of lake-margin hominin habitats in the northeast Turkana Basin (northwest Kenya)[END_REF][START_REF] Nutz | Orbitally-driven evolution of Lake Turkana (Turkana Depression, Kenya, EARS) between 1.95 and 1.72 Ma: a sequence stratigraphy perspective[END_REF]. The increasing abundance of C4 grasslands in the basin during the early Pleistocene may have been partially influenced by paleogeographic variations as the basin transitioned from fluvial-to lacustrine-dominated depositional systems [START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF][START_REF] Quinn | Paleogeographic variations of pedogenic carbonate  13 C values from Koobi Fora, Kenya: Implications for floral compositions of Plio-Pleistocene hominin environments[END_REF][START_REF] Wynn | Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols from the Turkana Basin, Kenya[END_REF].

Early to middle Pliocene depositional environments in the OTD are largely characterized as fluvial and dominated by the proto-Omo, -Kerio and -Turkwel rivers [START_REF] Feibel | A geological history of the Turkana Basin[END_REF]. Prior to the eruption of Mt. Kulal, the hydrological configuration is

reconstructed as an open system with an outlet to the Indian Ocean via the Turkana River [START_REF] Feibel | Freshwater stingrays from the Plio-Pleistocene of the Turkana Basin, Kenya and Ethiopia[END_REF]. Short-term lakes of various sizes punctuated the Pliocene fluvial landscapes as a function of accommodation space and sedimentation in the half-graben basin. The estimated largest and deepest lake, Lake Lonyumun, reached an aerial expanse of 9000 km 2 circa 4.1 Ma; another lake, Lake Lokochot, was likely shallower but nearly reached a comparable aerial extent circa 3.5 Ma [START_REF] Feibel | A terrestrial auxillary stratotype point and section for the Plio Pleistocene boundary in the Turkana Basin, East Africa[END_REF]. These large lakes show evidence for deltaic sediments at the interface of emptying perennial rivers such as the proto-Omo, -Kerio, and -Turkwel, similar to the modern deltas at the north and south ends of Lake Turkana. Adjacent to the LOM3 site, the local paleogeography of the northwest Turkana Basin included alluvial fans and other transverse drainages that likely intersected at times with axial depositional systems, such as the proto-Omo River and small ephemeral lakes [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Harris | Geology and paleontology of Plio-Pleistocene localities west of Lake Turkana, Kenya[END_REF][START_REF] Leakey | New hominin genus from eastern Africa shows diverse middle Pliocene lineages[END_REF].

The OTD preserves one of the most complete fossil and archaeological records of Plio-Pleistocene hominin evolution. Sediments capture several key evolutionary landmarks in the hominin lineage including the first appearance datum (FAD) of genus Australopithecus [START_REF] Leakey | New four-million-year-old hominid species from Kanapoi and Allia Bay, Kenya[END_REF], FAD and the last appearance datum (LAD) of K. platyops [START_REF] Leakey | New hominin genus from eastern Africa shows diverse middle Pliocene lineages[END_REF], an early Homo sp. candidate FAD [START_REF] Prat | First occurrence of early Homo in the Nachukui Formation (West Turkana, Kenya) at 2.3-2.4 Myr[END_REF], H. rudolfensis FAD [START_REF] Leakey | Evidence for an advanced Plio-Pleistocene hominid from East Rudolf, Kenya[END_REF], H. habilis FAD and LAD [START_REF] Spoor | Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya[END_REF][START_REF] Wood | Koobi Fora Research Project Volume 4: Hominid Cranial Remains[END_REF], the earliest Acheulean industry [START_REF] Lepre | An earlier origin for the Acheulian[END_REF], numerous Pleistocene sites preserving Oldowan industry diversity through time [START_REF] Braun | Early hominin diet included diverse terrestrial and aquatic animals 1.95 Ma in East Turkana, Kenya[END_REF][START_REF] Delagnes | Late Pliocene hominid knapping skills: the case of Lokalalei 2C, West Turkana, Kenya[END_REF]Isaac, 1997;[START_REF] Harmand | Economic behaviors and cognitive capacities of early hominins between 2.34 Ma and 0.70 Ma in West Turkana, Kenya[END_REF]Harmand, , 2009a,b;,b;[START_REF] Mana | Temporal and stratigraphic framework for paleoanthropology site within East-Central Area 130, Koobi Fora, Kenya[END_REF][START_REF] Roche | Early hominid stone tool production and technical skill 2.34 myr ago in West Turkana, Kenya[END_REF][START_REF] Roche | Les sites archéologiques plio-pléistocènes de la formation de Nachukui, Ouest-Turkana, Kenya: bilan synthétique[END_REF], and the focus of this study, the earliest lithic technology, known as the Lomekwian [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF].

Isotopic proxy records of terrestrial African environments

Pedogenic carbonate  13 C and  18 O analyses

For nearly forty years, African terrestrial vegetation communities have been reconstructed with measurements of  13 CPC and  18 OPC values preserved in paleosols [START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF](Cerling, , 1992a;;[START_REF] Cerling | An isotopic study of a fluviolacustrine sequence: the Plio-Pleistocene Koobi Fora sequence, East Africa[END_REF]. Thorough descriptions of these methods are available elsewhere (e.g., Cerling et al., 2011a;[START_REF] Koch | Isotopic reconstruction of past continental environments[END_REF][START_REF] Levin | Environment and climate of early human evolution[END_REF]; here we briefly describe the methods and data interpretation used in this study.

Globally C3 and C4 plants show normal and non-overlapping distributions [START_REF] Sage | C4 plant biology[END_REF]. C3 plants (trees, shrubs, few sedges, and cool growing season grasses) utilize the Calvin-Benson three-carbon photosynthetic pathway and discriminate against the heavier of the two stable isotopes of carbon, 13 C. C4 plants (most sedges and warm season growing grasses) utilize the Hatch-Slack four carbon photosynthetic pathway and allow some incorporation of 13 C. In eastern Africa, C3 dicots from closed and open canopy forests have lower δ 13 C values (-31.4 ± 0.5 ‰, -27.8 ± 0.3 ‰, respectively) than those of savanna and bushlands (-27.0 ± 0.2 ‰; Cerling et al., 2003c). Mesic C4 grasses (NADP subpathway) have an average δ 13 C value of -11.8 ± 0.2‰; xeric C4 grasses utilizing the NAD and PCK subpathways have average δ 13 C values of -13.1 ± 0.3‰ and -12.9 ± 0.1‰, respectively (Cerling et al., 2003c) in eastern African savanna environments. Vegetation δ 13 C values are inferred from those measured in pedogenic carbonate nodules (Cerling et al., 1992b;[START_REF] Cerling | Stable carbon and oxygen isotopes in soil carbonate[END_REF]. Pedogenic carbonates at depths greater than 30 cm in soils with relatively high respiration rates incorporate CO2 of decaying organic matter derived from surface vegetation during soil development with an offset of 14-17‰ [START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF](Cerling, , 1992b;;[START_REF] Cerling | Stable carbon and oxygen isotopes in soil carbonate[END_REF][START_REF] Quade | Systematic variations in the carbon and oxygen isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin, United States[END_REF]. Thus the  13 CPC values preserved in paleosols are used to quantitatively measure the relative amounts of C3 and C4 vegetation on the land surface during soil carbonate formation (Cerling, 1992a). Pedogenic carbonates form in warm, dry periods and have been shown to overestimate C4 contributions to paleovegetation (Breeker et al., 2009). Cerling et al. (2011b)  18 OPC values are controlled by soil pore water  18 O values, temperaturedependent isotopic fractionation during carbonate formation, evaporation rates and soil moisture [START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF][START_REF] Kim | Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates[END_REF]. At depths greater than 30 cm, soil pore water  18 O values track those of meteoric water [START_REF] Amundson | The relationship between the oxygen isotopi composition of soil CO2 and soil water[END_REF][START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF][START_REF] Cerling | Stable carbon and oxygen isotopes in soil carbonate[END_REF]. Soil pore water  18 O values are controlled by several factors including 1) evaporation relative to precipitation (i.e., aridity) [START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF][START_REF] Cerling | Stable carbon and oxygen isotopes in soil carbonate[END_REF], 2) rainfall source  18 OMW values [START_REF] Dansgaard | Stable isotopes in precipitation[END_REF][START_REF] Ferretti | Partitioning evapotranspiration fluxes from a Colorado grassland using stable isotopes: seasonal variations and ecosystem implications of elevated atmospheric CO2[END_REF][START_REF] Gazis | A stable isotope study of soil water: evidence for mixing and preferential flow paths[END_REF], 3) air and soil temperatures [START_REF] Breecker | Seasonal bias in the formation and stable isotopic composition of pedogenic carbonate in modern soils from central New Mexico, USA[END_REF][START_REF] Cerling | The stable isotopic composition of modern soil carbonate and its relationship to climate[END_REF][START_REF] Kim | Equilibrium and nonequilibrium oxygen isotope effects in synthetic carbonates[END_REF], or 4) some combination of these factors.

Advances in mass spectrometry have reduced the amount of carbonate required to obtain reliable  13 CPC and  18 OPC results, thus it is common for a single pedogenic nodule to be drilled and analyzed multiple times (e.g., database of [START_REF] Levin | Compilation of East Africa soil carbonate stable isotope data[END_REF]. Pedogenic nodules form by accretion [START_REF] Retallack | Pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols[END_REF][START_REF] Wieder | Micromorphological fabrics and developmental stages of carbonate nodular forms related to soil characteristics[END_REF] and therefore record several episodes of environmental conditions depending on nodule growth rate and size. Sampling one nodule multiple times for  13 CPC and  18 OPC values potentially reveals variability in vegetation structure and soil water during nodule formation. By no means is this the ideal gauge of environmental variability; however, it has been an underutilized dataset in paleoenvironmental research in eastern African hominin localities that reflects aspects of past vegetation structure and climatic conditions not visible with measures of bulk carbonate of one pedogenic nodule.

Faunal  13 CEC and  18 OEC analyses

Faunal enamel carbonate δ 13 CEC and δ 18 OEC values record δ 13 C values of whole diet and  18 O values of body water, respectively, and thus have been used for some time to reconstruct continental paleoenvironments (e.g., [START_REF] Koch | Isotopic reconstruction of past continental environments[END_REF]. Fossil faunal δ 13 CEC values have been extensively used to estimate vegetation structures at Plio-Pleistocene hominin locales relative to modern eastern African environments [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF][START_REF] Drapeau | The Omo Mursi formation: A window into the east African Pliocene[END_REF][START_REF] Kingston | Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: implications for early hominin paleoecology[END_REF][START_REF] Manthi | Diets of mammalian fossil fauna from Kanapoi, northwestern Kenya[END_REF][START_REF] Wynn | Dietary flexibility of Australopithecus afarensis in the face of paleoecological change during the middle Pliocene: Faunal evidence from Hadar, Ethiopia[END_REF][START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF]. Based on δ 13 CEC values of herbivorous ungulate taxa, specimens are assigned to one of three dietary groups: C3-browsers, C3-C4-mixed feeders, and C4-grazers [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF]. Relative percentages of the three feeding categories are used to characterize the faunal community composition (e.g., [START_REF] Manthi | Diets of mammalian fossil fauna from Kanapoi, northwestern Kenya[END_REF][START_REF] Robinson | Late Pliocene environmental change during the transition from Australopithecus to Homo[END_REF]. δ 13 CEC values of carnivores reflect the feeding category(ies) of their prey, and of omnivores, a mixture of plant and prey δ 13 C values [START_REF] Lee-Thorp | Stable carbon isotope ratio differences between bone collagen and bone apatite, and their relationship to diet[END_REF]. δ 13 CEC-based dietary reconstructions and by extension environmental reconstructions are not without methodological assumptions. [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF] proposed the following δ 13 CEC values to delineate the three feeding categories of the Artiodactyla, Perissodactyla and Proboscidea (APP) orders: C3-browsers,  13 CEC <-8‰; C4 grazers,  13 CEC >-1‰; C3-C4-mixed feeders,  13 CEC between -8 and -1‰, which were based on the large modern  13 CEC database of APP taxa, a pre-industrial CO2  13 C offset of 1.5‰, a ε*enamel-diet value of 14‰. In fossil organisms, the ε*enamel-diet value is based on modern analogs. The use the ε*enamel-diet value of +14‰ (e.g., Cerling et al., 2013b, Manthi et al., 2020), is based on large ruminant and nonruminant mammals [START_REF] Cerling | Carbon isotope fractionation between diet and bioapatite in ungulate mammals and implications for ecological and paleoecological studies[END_REF]Cerling et al., 2003a;[START_REF] Passey | Carbon isotopic fractionation between diet, breath, and bioapatite in different mammals[END_REF]. Alternatively, Kellner and Schoeninger (2007) suggested that the ε*enamel-diet value for a non-fermenting-gut omnivore is +10‰; however, this ε*enamel-diet value may be more applicable to members of genus Homo than Pliocene hominins [START_REF] Schoeninger | Stable isotope analyses and the evolution of human diets[END_REF]. An ε*enamel-diet value of +13‰ [START_REF] Cerling | Stable isotope ecology in the Ituri Forest[END_REF][START_REF] Passey | Carbon isotopic fractionation between diet, breath, and bioapatite in different mammals[END_REF] has been utilized to characterize extant primates (e.g., [START_REF] Loudon | The stable isotope ecology of Pan in Uganda and beyond[END_REF]Smith et al., 2010). [START_REF] Tejada-Lara | Body mass predicts isotope enrichment in herbivorous mammals[END_REF] suggested the ε*enamel-diet value of mammalian herbivores is predicted by body mass and gut physiology, producing ε*enamel-diet values ranges between +9 and +16‰. The use of specific ε*enamel-diet values can impact dietary reconstructions [START_REF] Quinn | Isotopic equifinality and rethinking the diet of Australopithecus anamensis[END_REF].

Notably, global CO2  13 C values have changed through time [START_REF] Tipple | Carbon isotope ratio of Cenozoic CO2: A comparative evaluation of available geochemical proxies[END_REF], and thus through-time analyses of faunal  13 CEC values must consider specific CO2  13 C offsets when differentiating diet categories [START_REF] Uno | Late Miocene to Pliocene carbon isotope record of differential diet change among east African herbivores[END_REF][START_REF] Garrett | Stable isotope paleoecology of late Pleistocene Middle Stone Age humans from equatorial East Africa, Lake Victoria basin, Kenya[END_REF][START_REF] Faith | Plio-Pleistocene decline of African megaherbivores: No evidence for ancient hominin impacts[END_REF]. Vegetation structure also impacts dietary reconstructions by altering the end members of C3 and C4 diets [START_REF] Garrett | Stable isotope paleoecology of late Pleistocene Middle Stone Age humans from equatorial East Africa, Lake Victoria basin, Kenya[END_REF]. Moreover, [START_REF] Robinson | Late Pliocene environmental change during the transition from Australopithecus to Homo[END_REF] have suggested that the methods of [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF] artificially skew δ 13 CEC-based interpretations of feeding categories and general community structures because they do not account for differential species abundances present in collector-and taphonomicbiased faunal assemblages.

δ 18 O values are preserved in both the carbonate (subscript EC) and phosphate (subscript EP) phases of enamel and have been shown to resist diagenetic alteration [START_REF] Wang | A model of fossil tooth and bone diagenesis: implications for paleodiet reconstruction from stable isotopes[END_REF]; consequently faunal δ 18 OEP/EC analyses have been widely employed for environmental reconstructions in deep time [START_REF] Ayliffe | Oxygen isotope composition of the bone phosphate of Australian kangaroos: potential as a palaeoenvironmental recorder[END_REF][START_REF] Ayliffe | The preservation of glacial-interglacial climatic signatures in the oxygen isotopes of elephant skeletal phosphate[END_REF][START_REF] Bryant | Oxygen isotopic composition of fossil horse tooth phosphate as a record of continental paleoclimate[END_REF][START_REF] Bryant | Biologic and climatic signals in the oxygen isotopic composition of Eocene-Oligocene equid enamel phosphate[END_REF][START_REF] Kingston | Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: implications for early hominin paleoecology[END_REF][START_REF] Koch | Oxygen isotope variation in the tusks of extinct proboscideans: a measure of season of death and seasonality[END_REF][START_REF] Wang | Stable isotopic variations in modern herbivore tooth enamel, plants and water on the Tibetan Plateau: implication for paleoclimate and paleoelevation reconstructions[END_REF][START_REF] Wang | A 25 m.y. isotopic record of paleodiet and environmental change from fossil mammals and paleosols from the NE margin of the Tibetan Plateau[END_REF][START_REF] Wynn | Dietary flexibility of Australopithecus afarensis in the face of paleoecological change during the middle Pliocene: Faunal evidence from Hadar, Ethiopia[END_REF]. Mammalian  18 OEC values are derived primarily from drinking water, atmospheric O2, and water consumed in food [START_REF] Bryant | A model of oxygen isotope fractionation in body water of large mammals[END_REF][START_REF] Kohn | Predicting animal d18O: accounting for diet and physiological adaptation[END_REF][START_REF] Kohn | Stable isotope compositions of biological apatites[END_REF][START_REF] Podlesak | Turnover of oxygen and hydrogen isotopes in the body water, CO2, hair, and enamel of a small mammal[END_REF].

In eastern African savanna environments, obligate drinkers have been shown to have lower but overlapping  18 OEP/EC values than non-obligate drinkers [START_REF] Bocherens | Isotopic biogeochemistry ( 13 C, 18 O) of mammal enamel from African Pleistocene hominid sites: implications for the preservation of paleoclimatic signals[END_REF]Cerling et al., 2003b). Eastern African C3-browsing and C3-C4-mixed-feeding herbivore ungulates were found to have lower bone phosphate  18 O values than C4grazing ungulates [START_REF] Kohn | Herbivore tooth oxygen isotope compositions: effects of diet and physiology[END_REF]. As recently reviewed by [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF], C3-browsers obtain substantial amounts of water from C3 vegetation. C4 grasses offer little water content and therefore C4-grazers typically obtain water from surface water sources, which generally have lower  18 O values than that of C3 vegetation. Sponheimer andLee-Thorp (1999b, 2001) suggested that trophic level is correlated with  18 OEC values in a modern sample of the Morea Estate mammals and fossil fauna from Swartkrans and Equus Cave (South Africa). Few studies have aimed to further test this prediction in modern eastern African ecosystems. One jackal from Turkana (Cerling et al., 2003b) yielded a lower  18 OEC value than measured herbivores but comparable values as elephant and black rhino. Lake Turkana hippos (Cerling et al., 2003b), but not those inhabiting rivers [START_REF] Blumenthal | Aridity and hominin environments[END_REF], yielded higher  18 OEC values than the one isotopically analyzed jackal. At Tsavo, lions (n=9) yielded lower  18 OEC values than several herbivorous species, but showed no difference from those of elephant and black rhino [START_REF] Cerling | Stable isotope ecology of the common hippopotamus[END_REF].

Several studies in African forested environments have shown that  18 OEC values of forest-living fauna vary with canopy height. For example, high canopy primates achieve higher  18 OEC values than low canopy or ground dwellers in closed environments [START_REF] Cerling | Stable isotope ecology in the Ituri Forest[END_REF][START_REF] Krigbaum | Stable isotope canopy effects for sympatric monkeys at Tai Forest, Cote d'Ivoire[END_REF][START_REF] Nelson | Chimpanzee fauna isotopes provide new interpretations of fossil ape and hominin ecologies[END_REF]. This is because high canopy leaves experience higher evapotranspiration than vegetation at low canopy and ground levels in a forest [START_REF] Sternberg | Oxygen isotope ratio stratification in a tropical moist forest[END_REF], and those different vegetation  18 O values are transferred to consumers [START_REF] Carter | Oxygen isotope ratios in primate bone carbonate reflect amount of leaves and vertical stratification in the diet[END_REF][START_REF] Nelson | Chimpanzee fauna isotopes provide new interpretations of fossil ape and hominin ecologies[END_REF]. Additionally, woodland-and grassland-dwellers achieve higher δ 18 OEC values than those living in forests (Cerling et al., 2003a). This can result from drinking open habitat evaporative surface waters (Cerling et al., 2003a), but is also influenced by ingesting open habitat vegetation, which experiences higher rates of evapotranspiration [START_REF] Belsky | The effects of trees on their physical, chemical, and biological environments in a semi-arid savanna in Kenya[END_REF].

The number of variables controlling faunal  18 OEC values impacts the method's usefulness for assessing environmental change. For example, water dependency and behavioral variables [START_REF] Kohn | Herbivore tooth oxygen isotope compositions: effects of diet and physiology[END_REF] also influence  18 OEC values, which act to reduce  18 OEC distinctions between herbivore feeding categories [START_REF] Bocherens | Isotopic biogeochemistry ( 13 C, 18 O) of mammal enamel from African Pleistocene hominid sites: implications for the preservation of paleoclimatic signals[END_REF]Cerling et al., 2003b). Moreover, [START_REF] Kohn | Predicting animal d18O: accounting for diet and physiological adaptation[END_REF], [START_REF] Kohn | Herbivore tooth oxygen isotope compositions: effects of diet and physiology[END_REF][START_REF] Schoeninger | Tooth oxygen isotope ratios as paleoclimate monitors in arid ecosystems[END_REF] emphasize that behavioral variables unknown, and perhaps unknowable, in fossil species such as activity times, night feeding, and shade-seeking behaviors can influence  18 OEC values.

Several studies from eastern African hominin sites have shown that analyzing  13 CEC and  18 OEC values of mammalian taxa from discrete stratigraphic intervals captures a large range of available resources via different dietary behaviors practiced by various species [START_REF] Du | Stable carbon isotopes from paleosol carbonate and herbivore enamel document differing paleovegetation signals in the eastern African Plio-Pleistocene[END_REF][START_REF] Kingston | Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: implications for early hominin paleoecology[END_REF][START_REF] Lehmann | Stable isotopic composition of fossil mammal teeth and environmental change in southwestern South Africa during the Pliocene and Pleistocene[END_REF][START_REF] Patterson | Comparative isotopic evidence from East Turkana supports a dietary shift within the genus Homo[END_REF][START_REF] Robinson | Paleoenvironmental change in late Middle Pleistocene-Holocene Kibish Formation, southern Ethiopia: evidence from ungulate isotopic ecology[END_REF][START_REF] Wynn | Dietary flexibility of Australopithecus afarensis in the face of paleoecological change during the middle Pliocene: Faunal evidence from Hadar, Ethiopia[END_REF]. If diagenetic alteration of tooth enamel is negligible,  13 CEC and  18 OEC analyses of individual specimens record diet and water intake over the duration of enamel formation (10 0 -10 1 years), which represents a small amount of time in contrast to the formation times of pedogenic carbonates (10 1 -10 3 years). Spatially, however, faunal  13 CEC and  18 OEC values record information from across the range of animal movements, which vary by species and span 10 1 -10 2 km 2 [START_REF] Estes | The Behavior Guide to African Mammals[END_REF]. In a similar approach to measuring intranodular variability, faunal isotopic variation measured in several specimens from individual taxonomic groups serves as a proxy of dietary selectivity or flexibility (methods of [START_REF] Groenewald | Dietary resource partitioning among three coeval proboscidean taxa (Anacus capensis, Mammuthus subplanifrons, Loxodonta cookie) from the South African Early Pliocene locality of Langebaanweg E Quarry[END_REF]. Combining the two distinct yet complementary proxy records from paleosols and faunal specimens makes visible different aspects of the local and regional environments.

Materials and Methods

Pliocene OTD  13 CPC and  18 OPC analyses

We present new  18 OPC values (nodules=23, analyses=47) from LOM3 (see sampling described in [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF]; Supplementary Table 1). We combine  13 CPC and  18 OPC data and compare across two 400-kyr intervals, 4.3-3.9 Ma (n=70) and 3.4-3.0 Ma (n=77). Published δ 13 CPC and δ 18 OPC values from the OTD are drawn from the Kanapoi Formation (data from [START_REF] Quinn | Revisiting the pedogenic carbonate isotopes and paleoenvironmental interpretation of Kanapoi[END_REF][START_REF] Wynn | Paleosols, stable carbon isotopes and paleoenvironmental interpretation of Kanapoi, Northern Kenya[END_REF], the Koobi Fora Formation (data from [START_REF] Cerling | An isotopic study of a fluviolacustrine sequence: the Plio-Pleistocene Koobi Fora sequence, East Africa[END_REF][START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF], and the Nachukui Formation (data from [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF][START_REF] Wynn | Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols from the Turkana Basin, Kenya[END_REF]. OTD δ 13 CPC and δ 18 OPC data used in this study have been compiled into a publicly available database [START_REF] Levin | Environment and climate of early human evolution[END_REF]. WC values were estimated after methods of Cerling et al. (2011).

We utilized standard variation of intranodular  13 CPC and  18 OPC values from these two intervals, 4.3-3.9 Ma (n=38) and 3.4-3.0 Ma (n=33), to represent environmental variability (data from [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF][START_REF] Quinn | Revisiting the pedogenic carbonate isotopes and paleoenvironmental interpretation of Kanapoi[END_REF] this study).

Pliocene OTD faunal δ 13 CEC and δ 18 OEC analyses

We add faunal δ 13 CEC and δ 18 OEC values (taxa =8, n =28) from LOM3 to published OTD faunal δ 13 CEC and δ 18 OEC values. We limited enamel sampling to APP specimens identifiable at taxonomic levels of tribe or lower. We also analyzed crocodylians from the site for δ 18 OEC values to proxy meteoric water δ 18 O values (e.g., [START_REF] Amiot | Oxygen isotope fractionation between crocodilian phosphate and water[END_REF]. We combine δ 13 CEC and δ 18 OEC data of specimens collected from five geologic formations in the Omo Group: Kanapoi Formation (data from Cerling et al., 2013b;Manthi et al., 2020 and references therein), Koobi Fora Formation (data from Blumenthal et al., 2017 and references therein;Cerling et al., 2013b, 2015 andreferences therein;[START_REF] Schoeninger | Paleoenvironment of Australopithecus anamensis at Allia bay, East Turkana, Kenya: Evidence from mammalian herbivore enamel stable isotopes[END_REF], Shungura Formation (data from [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF][START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF], Mursi Formation (data from [START_REF] Drapeau | The Omo Mursi formation: A window into the east African Pliocene[END_REF], and the Nachukui Formation (data from Blumenthal et al., 2017 and references therein;Cerling et al., 2013b, 2015 andreferences therein) and compare data of taxonomic groups. Fourteen ungulate herbivore taxa (4.3-3.9 Ma, n=212; 3.4-3.0 Ma, n=197) were utilized for assessing change in δ 13 CEC values, and 8 ungulate herbivore taxa and Crocodylia (4.3-3.9 Ma, n=134; 3.4-3.0 Ma, n=139) for comparing δ 18 OEC values (Table 1). After [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF] we distinguish two Rhinocerotidae taxa based on bimodal δ 13 CEC values, Rhinocerotidae (G) and Rhinocerotidae (B), to denote grazing (cf. Ceratotherium) and browsing (cf. Diceros) forms, but without the proper morphological indicators for taxonomic designations.

Given the small sample sizes of individual taxa and the lack of representation by numerous species in the sample, we use pairwise non-parametric comparisons of δ 13 CEC and δ 18 OEC values grouped by taxon present in both time intervals. We assess variability in food and water inputs by comparing interquartile ranges of δ 13 CEC and δ 18 OEC values of each taxonomic group separated into the two temporal bins. After [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF], we place the taxonomic and temporal groups along the spectrum of δ 13 CEC values designated as separate feeding categories. However, we also consider canopy effects on vegetation  13 C values [START_REF] Kohn | Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate[END_REF] and separate the C3 browser category into "forest" C3 browsers and "woodland" C3 browsers. We define forest C3-browsers when  13 CEC values drop below -14‰. This is based on the average plant  2010), the time periods compared in this study vary only by 0.2‰ and therefore we do not change our feeding category demarcations through time. We also compare feeding categories through time by combining δ 13 CEC values and gauge change in mean δ 13 CEC values and variance (similar to methods of [START_REF] Sealy | Climate and ecology of the palaeo-Agulhas Plain from stable carbon and oxygen isotopes in bovid tooth enamel from Nelson Bay Cave, South Africa[END_REF].

Mass Spectrometry

Pedogenic nodules were cross-sectioned and sampled for microcrystalline calcite with a 0.5 mm carbide drill bit (Foredom Series), avoiding surface and sparry calcite. We subsampled nodules twice and averaged the subsample values. Enamel samples from LOM3 were also sampled with a rotary drill affixed with a diamond drill bit (Foredom Series). We eroded the outer layer of enamel prior to collecting enamel powders, avoiding discolorations/inclusions and taking advantage of natural breaks. Enamel samples were pretreated with a buffered acetic acid, pH = 5.0 (after Cerling et al., 2013b;[START_REF] Koch | The effects of sample treatment and diagenesis on the isotopic integrity of carbonate in biogenic hydroxylapatite[END_REF]. All analyses were conducted at the Stable Isotope Laboratory at Rutgers University on a FISIONS Optima Mass Spectrometer with an attached Multi-Prep device. All samples were reacted in 100% phosphoric acid at 90°C for 13 minutes.

Values for  18 OPC are reported in delta notation (per mil, ‰) versus V-PDB through analysis of NBS-19; error for  18 OPC is <0.05‰. Enamel samples that yielded >10% carbonate content were excluded from the study due to the likelihood of the presence of non-structural carbonate (methods after Cerling et al., 2013b[START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF].

Results

Pliocene OTD δ 13 CPC and δ 18 OPC values

Summary data for the compiled Pliocene OTD δ 13 CPC and δ 18 OPC values for the two time intervals are shown in Figures 678.  13 CPC values show a slight (0.9‰) but statistically significant (t test, p<0.05) increase from the 4.3-3.9 Ma interval to the 3.4-3.0 Ma interval. Using the methods of Cerling et al. (2011b), the average δ 13 CPC increase of 0.9‰ equates to an 8% decrease in WC. The dominant vegetation structural category is reconstructed as grassy woodlands/ bushland/shrubland in both time periods. δ 18 OPC values show a change by only 0.1‰ between the 4.3-3.9 Ma and the 3.4-3.0 Ma intervals, and this is not statistically significant (Figure 7). Intranodular δ 13 CPC and δ 18 OPC standard deviations and interquartile ranges are slightly higher in the 3.0-3.4 Ma interval than in the 4.3-3.9 Ma interval, averaging 0.1 and 0.3 respectively (Figure 8), and both are statistically significant (t test, p<0.05).

Pliocene OTD faunal  13 CEC and  18 OEC values

Pliocene OTD faunal  13 CEC and  18 OEC values are listed in Table 1, and box plots are illustrated in figures 9 and 10. Pairwise comparisons of  13 CEC values for each of the 14 herbivore ungulate taxonomic groups examined across the two time intervals yielded statistical significance for three taxa (Mann-Whitney U, p<0.05, Figure 9). Antilopini, Aepycerotini and Notochoerus shift within the C3-C4 mixed feeding category toward more C4-feeding. None of the herbivorous species yielded statistically significant changes in interquartile range (f test, p=0.10) in the 3.4-3.0 Ma interval. Figure 11 plots probability densities of all herbivore ungulate  13 CEC values separated into the three feeding categories (C3-browsers, C3-C4-mixed feeders, C4-grazers). The position and shape of the plots illustrate a slight directional shift toward more C4 consumption especially within the C3-C4-mixed-feeding category. C3-C4-mixed feeders show significant changes in mean and median  13 CEC values (t test, p<0.000; Mann-Whitney U, p<0.01) and variance (f test, p=0.04).

Pairwise comparisons of  18 OEC values for each of the 10 herbivore ungulate taxonomic groups examined across the two time intervals yielded statistical significance for only one taxa (Mann-Whitney U, p<0.05, Figure 10). Alcelaphini yielded a statistically significant decrease in  18 OEC values (Mann-Whitney U, p<0.01). Giraffidae and Aepycerotini yielded lower  18 OEC mean values, but do not achieve statistical significance (Mann-Whitney U, p<0.08). Pairwise  18 OEC interquartile ranges vary widely, but none were significantly different from one another (f test, p>0.05).

Discussion

Environmental change during the origins of stone tools

Environmental proxy data from the Pliocene OTD confirms the modest directional shift to higher abundances of C4 vegetation through time. Average WC decreased by 8%, and three herbivore ungulate taxa showed increases in C4 consumption.

Modern species opportunistically shift their diets in response to resource availability (e.g., [START_REF] Cromsigt | Resource partitioning among savanna grazers mediated by local heterogeneity: an experimental approach[END_REF][START_REF] Du Toit | Large herbivores and savanna heterogeneity[END_REF][START_REF] Illius | Resource heterogeneity and ungulate population dynamics[END_REF][START_REF] Kartzinel | DNA metabarcoding illuminates dietary niche partitioning by African large herbivores[END_REF][START_REF] Owen-Smith | Distinctive features of the nutritional ecology of browsing versus grazing ruminants[END_REF] and therefore can track changes in vegetation structure. Three Pliocene fossil taxa consuming more C4 plants may indicate a higher abundance of C4 grasslands during the 3.4-3.0 Ma interval in general agreement with WC estimates.

However, faunal  13 CEC values are also a reflection of species interactions [START_REF] Buschke | Simple mechanistic models can partially explain local but not range-wide co-occurrence of African mammals[END_REF] and resource partitioning of C3 and C4 vegetation by herbivore competitors (e.g., [START_REF] Codron | Influence of competition on niche dynamics of syntopic grazing ungulates: contrasting the predictions of habitat selection models using stable isotope analysis[END_REF][START_REF] Davis | The influence of evolutionary history and body size on partitioning of habitat resources by mammalian herbivores in southeastern Australia[END_REF]Feranac et al., 2007;[START_REF] Malindie | Carbon isotope differences between grassland and savanna herbivores reveal environmentally driven rather than phylogenetically conserved niches[END_REF], which makes quantifying C4 grass abundance with faunal  13 CEC values a difficult task.

As shown by [START_REF] Uno | Late Miocene to Pliocene carbon isotope record of differential diet change among east African herbivores[END_REF], Patterson and others (2017, 2019), and Negash et al. (2020) in different portions of the OTD, dietary shifts by herbivore ungulates toward greater C4 consumption were highly variable in tempo and mode. Moreover, throughtime faunal  13 CEC data show that the dietary niches of many individual taxa were relatively constant during the Pliocene and did not undergo significant transitions into the C4 biome until after 3 Ma [START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF]. Although none achieve statistical significance, Aepycerotini and Notochoerus may have become more dietarily selective during the middle Pliocene, whereas Tragelaphini, like hominins, may have become more opportunistic (e.g., [START_REF] Groenewald | Dietary resource partitioning among three coeval proboscidean taxa (Anacus capensis, Mammuthus subplanifrons, Loxodonta cookie) from the South African Early Pliocene locality of Langebaanweg E Quarry[END_REF] (Figure 9). Based on the combined ungulate herbivore  13 CEC values, it appears that the C3-C4 mixed feeding niche contracted over this study interval (Figure 11), which may have been due to reductions in C3 resource availability and/or increased competition.

Unchanging  18 OEC values in taxa whose modern descendants tend to be water dependent (e.g., Hippopotamidae, Crocodylia, Deinotheriidae, Elephantidae) suggests that meteoric water  18 O values did not significantly change over the two time intervals.

Hippopotamidae  18 OEC values reflect the  18 O values of the water in which they swim and thus have been suggested as faithful records of meteroric water  18 O values [START_REF] Cerling | Stable isotope ecology of the common hippopotamus[END_REF][START_REF] Levin | A stable isotope aridity index for terrestrial environments[END_REF]. Crocodylia  18 OEC values, which also mirror meteoric water  18 O values [START_REF] Amiot | Oxygen isotope fractionation between crocodilian phosphate and water[END_REF], are comparable to those of Hippopotamidae. OTD Deinotheriidae and Elephantidae, perhaps similar to elephants today [START_REF] Estes | The Behavior Guide to African Mammals[END_REF], likely spent a significant portion of time in surface waters as well. Notably, Elephantidae  18 OEC values are significantly higher than those of Deinotheriidae, which may reflect differences in diet (e.g., [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF], gut physiology [START_REF] Hempson | A continent-wide assessment of the form and intensity of large mammal herbivory in Africa[END_REF], or body size [START_REF] Bryant | A model of oxygen isotope fractionation in body water of large mammals[END_REF][START_REF] Longinelli | Oxygen isotopes in mammal bone phosphate: a new tool for paleohydrological and paleoclimatological research?[END_REF][START_REF] Luz | Fractionation of oxygen isotopes between mammalian bone-phosphate and environmental drinking water[END_REF], but neither revealed change in pairwise comparisons.

Of the taxa that yielded lower  18 OEC values in the 3.4-3.0 Ma interval, (e.g., Giraffidae, Alcelaphini, Aepycerotini), only Giraffidae was categorized as evaporationsensitive (ES) in the  18 OEC-based aridity index of [START_REF] Levin | A stable isotope aridity index for terrestrial environments[END_REF] and refined by [START_REF] Blumenthal | Aridity and hominin environments[END_REF]. But notably, the decrease in Giraffidae  18 OEC values was not statistically significant. As mentioned, [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF] cautions that  18 OEC-based water deficit estimates based on herbivore ungulate species with changing diets, such as Tragelaphini, may mimic changes in water deficits as more C4-grazing triggers more surface water drinking. The relatively lower  18 OEC values of Aepycerotini may have been a by-product of higher C4 consumption (e.g., [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF]. The drop in  18 OEC values in Alcelaphini is accompanied by a slight decrease in  13 CEC values, which is opposite of that predicted for changes in drinking behavior. Modern Alcelaphini taxa vary in water dependency and tolerance for arid habitats and are not identified as ES taxa [START_REF] Levin | A stable isotope aridity index for terrestrial environments[END_REF]. According to [START_REF] Estes | The Behavior Guide to African Mammals[END_REF], few species live in arid areas without access to water; those that do (e.g., hartebeests in the Kalahari) dig up roots and tubers or feed on melons to access water. As a C4-grazer, OTD Alcelaphini likely obtained their water from surface sources due to the low water content in grasses. Reduced  18 OEC values imply that surface drinking water sources shifted to lower  18 O values.

Alcelaphini  18 OEC values may have tracked changes in local water sources such as floodplain lakes and small distributaries that are more susceptible to evaporative fluctuations than large rivers.

Using the criterion of [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF], Giraffidae, which does not change its diet over the two time intervals, is ostensibly a good candidate for accurate water deficit calculations. Although our pairwise comparison did not yield statistical significance, Blumenthal et al.'s (2017) aridity index based on the Hippopotamidae-Giraffidae equation yielded water deficits from Kanapoi (~4.1 Ma, 1315±994 mm/year), Allia Bay (3.9 Ma, 500±1315 mm/year) and the Lomekwi member (3.4 Ma, 1±892 mm/year), demonstrating increasingly more humid conditions across 4.3-3.0 Ma study interval.

Whether this potential humidity trend was caused by local or regional climatic forcing is unknown. [START_REF] Donges | Nonlinear detection of paleoclimate-variability transitions possibly related to human evolution[END_REF] found that the interval of 3.33-3.28 Ma shows evidence for decreasing sea-surface temperatures, lower surface air temperatures and changes in the evaporation-precipitation balance. They suggest that under these conditions, eastern Africa underwent a significant humid period, pointing to the Awash Basin's lacustrine phase as evidence for increased rainfall. As mentioned, the OTD does not show evidence for a lacustrine phase during LOM3 times; however, Giraffidae  18 OEC values may indicate slightly more humid conditions. Clearly additional enamel samples of Giraffidae are needed from the early and middle Pliocene in the OTD to further test this finding.

It is noteworthy that OTD  18 OEC values appear to track changes in humidity that were not recorded in  18 OPC values, which may indicate that changes in humidity were minor. However,  18 OPC values have several influences, which may have acted in concert to blur environmental signals. [START_REF] Koch | Isotopic reconstruction of past continental environments[END_REF] suggested that increased  18 OPC values can be a by-product of grassland expansion. Relatively more C4 vegetation in grass form results in increased light stress and less soil moisture relative to soils formed under tree cover.

As a result, a decrease in soil moisture caused by a reduction in woody cover could have counteracted the increase in precipitation on  18 OPC values during the middle Pliocene.

Increasing standard deviations of  13 CPC and  18 OPC values within nodules suggest that variations in vegetation structure and climatic conditions marginally increased across the two time intervals. While intranodular isotopic variability is not a seasonal proxy, higher environmental variability on the order of nodule formation times (100s-1000s years) may indicate more fluctuating environments during the 3.4-3.0 Ma interval. This pattern of increasing variability is not predicted based on the African dust records [START_REF] Potts | Alternating high and low climate variability: the context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF] and may suggest other controls on climatic variability in the region during the middle Pliocene such as sea-surface temperature [START_REF] Fedorov | Patterns and mechanisms of early Pliocene warmth[END_REF] or the Indian Ocean Dipole [START_REF] Marchant | The Indian Ocean dipole? The unsung driver of climatic variability in East Africa[END_REF]. The increase in standard deviation, however, is ≤0.3‰ for both isotopic systems, which is negligible with respect to significant changes in OTD environments. For example, variations in  13 CPC values of 0.3‰ would not result in significant changes in vegetation structure with the paleoshade proxy (Cerling et al., 2011b). Although HSPDP did not target the Pliocene sediments in the OTD, increase sampling density throughout the Pliocene sequence and additional climate proxy records on seasonal and other short-term time scales (e.g., [START_REF] Ascari | Fossil herbivores and crocodiles as paleoclimatic indicators of environmental shifts from bed I to Bed II times of the Olduvai Gorge, Tanzania. Palaeogeog, Palaeoclim[END_REF][START_REF] Blumenthal | Isotopic records of climate seasonality in equid teeth[END_REF][START_REF] Macho | Palaeoenvironmental reconstruction: evidence for seasonality at Allia Bay, Kenya, at 3.9 million years[END_REF] are warranted to further test if environmental variability increased through time by amplitudes impacting environmental conditions.

We interpret subtle changes in C4 abundances and potentially a slight increase in humidity in the OTD for the middle Pliocene compared to the early Pliocene, which provides support for decoupling the link between aridity and C4 grassland spread (e.g., [START_REF] Blumenthal | Aridity and hominin environments[END_REF][START_REF] Levin | Environment and climate of early human evolution[END_REF][START_REF] Polissar | Synchronous rise of African C4 ecosystems 10 million years ago in the absence of aridification[END_REF]. The increasing trend of eastern African C4 vegetation appears related to the long-term decrease in atmospheric pCO2 [START_REF] Faith | Plio-Pleistocene decline of African megaherbivores: No evidence for ancient hominin impacts[END_REF][START_REF] Polissar | Synchronous rise of African C4 ecosystems 10 million years ago in the absence of aridification[END_REF]. Observably, vegetation communities are not passive responders to changing environmental conditions, but rather compete with one another for substrate, water, and sunlight [START_REF] Bond | What limits trees in C4 grasslands and savannas?[END_REF][START_REF] Bond | The importance of low atmospheric CO2 and fire in promoting the spread of grasslands and savannas[END_REF][START_REF] Ludwig | The influence of savanna trees on nutrient, water and light availability and the understory vegetation[END_REF]. In the OTD, the hydrological regime was dominated by the large meander belt of the proto-Omo River and punctuated with lake phases, which in turn provided changing conditions of water availability, substrate, and sedimentation. Avulsing river and fluctuating lake margin landscapes may have offered a competitive advantage for C4 vegetation during the Pliocene without an increase in aridity per se.

Pliocene hominin dietary niche expansion and stone tool origins

Modern savannas support a higher secondary productivity than woodlands and forests (data compiled in [START_REF] Leonard | Metabolic correlates of hominid brain expansion[END_REF]. Reasons for this relationship are multifaceted and involve the interplay of vegetation structure, effective rainfall, feeding behaviors, and fire, among others [START_REF] Hempson | A continent-wide assessment of the form and intensity of large mammal herbivory in Africa[END_REF][START_REF] Sankaran | Determinants of woody cover in African savannas[END_REF]. The Pliocene OTD C3-C4 feeding niche sustained a high number of herbivore ungulate species [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF][START_REF] Manthi | Diets of mammalian fossil fauna from Kanapoi, northwestern Kenya[END_REF][START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF] similar to other eastern African Pliocene hominin locales [START_REF] Kingston | Isotopic dietary reconstructions of Pliocene herbivores at Laetoli: implications for early hominin paleoecology[END_REF][START_REF] Robinson | Late Pliocene environmental change during the transition from Australopithecus to Homo[END_REF].

The move by many eastern African mammalian species into open woodlands and grasslands was variable [START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF][START_REF] Ségalen | Timing of C4 grass expansion across sub-Saharan Africa[END_REF][START_REF] Uno | Late Miocene to Pliocene carbon isotope record of differential diet change among east African herbivores[END_REF] but perhaps represents responses by some herbivores to reduce competition (e.g., [START_REF] Kartzinel | DNA metabarcoding illuminates dietary niche partitioning by African large herbivores[END_REF][START_REF] Malindie | Carbon isotope differences between grassland and savanna herbivores reveal environmentally driven rather than phylogenetically conserved niches[END_REF]. OTD hominins' diets generally followed the trend of increasing C4 resources locally, which implies opportunistic foraging with changing food abundances. Recently, however, [START_REF] Wynn | Isotopic evidence for the timing of the dietary shift toward C4 foods in eastern African Paranthropus[END_REF] interpreted that the varied timing of dietary shifts by hominins, non-hominin primates, and herbivore ungulates in the OTD toward C4 consumption signals behavioral changes rather than the tracking of C4 resource abundances per se. The increasing database of OTD faunal  13 CEC values supports the hypothesis that hominins experienced a "protracted" adaptation to the widespread presence of C4 resources available by the late Miocene and early Pliocene [START_REF] Ségalen | Timing of C4 grass expansion across sub-Saharan Africa[END_REF].

Biotic competition for the broad C3-C4 feeding niche may have been one of the selective pressures for stone tool making behaviors. As viewed in the temporal bins, K. platyops shared its general diet shift toward a greater reliance on C4 plants with three ungulate herbivore C3-C4 mixed feeders in the OTD (Figure 9). Several ungulate herbivores and Theropithecus, the large-bodied baboon, had occupied the C4 biome for some time (Cerling et al., 2013a;[START_REF] Uno | Late Miocene to Pliocene carbon isotope record of differential diet change among east African herbivores[END_REF]. As many researchers have suggested, stone tools may have offered enhanced efficiency for processing vegetation (e.g., [START_REF] Brain | Cultural and taphonomic comparisons of hominids from Swartkrans and Sterkfontein[END_REF][START_REF] Dominguez-Rodrigo | Woodworking activities by early humans: A plant residue analysis on Acheulean stone tools from Peninj (Tanzania)[END_REF][START_REF] Goren-Inbar | Nuts, nut cracking, and pitted stones at Gesher Benot Ya`aqov, Israel[END_REF][START_REF] Hardy | The importance of dietary carbohydrate in human evolution[END_REF][START_REF] Keeley | Microwear polishes on early stone tools from Koobi Fora, Kenya[END_REF][START_REF] Panger | Older than the Oldowan? rethinking the emergence of hominin tool use[END_REF][START_REF] Schick | Making Silent Stones Speak[END_REF][START_REF] Sept | Plant foods and early hominids at site FxJj 50, Koobi Fora, Kenya[END_REF][START_REF] Sept | Beyond bones: archaeological sites, early hominid subsistence, and the costs and benefits of exploiting wild plant foods in east African riverine landscapes[END_REF].

Notably, the combined ungulate herbivore  13 CEC data show evidence for dietary niche contraction within the C3-C4 mixed feeding niche in the 3.4-3.0 Ma interval. In contrast, hominins expand dietary breadth from a woodland C3-browser to a broad C3-C4-mixed feeder, possibly filling the dietary niche left vacated by some of the C3-C4-mixed feeders, especially Antilopini, Aepycerotini, and Notochoerus (Figures 9 and11).

Hominins, like humans and primates today, are thought to have been obligate drinkers [START_REF] Grine | Dental microwear and stable isotopes inform the paleoecology of extinct hominins[END_REF][START_REF] Lee-Thorp | Stable isotopes in fossil hominin tooth enamel suggest a fundamental dietary shift in the Pliocene[END_REF] and thus the average 2‰ decrease in  18 OEC values between A. anamensis and K. platyops suggest drinking behaviors changed. In the OTD, [START_REF] Quinn | Influence of Plio-Pleistocene basin hydrology on the Turkana hominin enamel carbonate  18 O values[END_REF] suggested that the presence of Lake Lonyumun during A. anamensis' times might have provided an 18 O-enriched source of drinking water resulting in higher  18 OEC values for A. anamensis. In contrast, K. platyops was imbibing primarily from the relatively unevaporated proto-Omo River and tributaries due to the OTD's hydrological configuration in the middle Pliocene. The lack of significant changes in Hippopotamidae  18 OEC values through time in the OTD [START_REF] Harris | Stable isotope ecology of fossil hippopotamids from the Lake Turkana Basin of East Africa[END_REF] and also shown here with other water dependent taxa (Crocodylia, Deinotheridae, Elephantidae) implies that  18 O values of meteoric waters did not significantly shift over the two time intervals (Figure 10) and thus changing water sources appears a less likely scenario for the middle Pliocene. It appears that humidity increased over the study interval, which may have influenced the change in hominin  18 OEC values if a substantial amount of ingested water was derived from plant resources similar to other waterindependent taxa (Figure 10).

Dietary changes could have lowered K. playtops'  18 OEC values such as feeding on ground level food resources in closed woodland settings (e.g., [START_REF] Nelson | Chimpanzee fauna isotopes provide new interpretations of fossil ape and hominin ecologies[END_REF] or on wetland sedges and underground storage organs (e.g., [START_REF] Lee-Thorp | The demise of "Nutcracker Man[END_REF]. Changes in feeding and activity times could have also impacted K. playtops'  18 OEC values (e.g., [START_REF] Kohn | Predicting animal d18O: accounting for diet and physiological adaptation[END_REF][START_REF] Schoeninger | Tooth oxygen isotope ratios as paleoclimate monitors in arid ecosystems[END_REF]. Additionally, lower  18 OEC values indicate that K. platyops may have increased its water dependency by drinking a higher proportion of surface waters than ingesting water contained in vegetation. The relationship between lower  18 OEC values and increasing C4 grass consumption as shown for Tragelaphini by [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF] and here with Aepycerotini aligns with the dietary increase of C4 resources by hominins.

Prior to the LOM3 publication [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF], [START_REF] Macho | An ecological and behavioural approach to hominin evolution during the Pliocene[END_REF] proposed that the increase in C4 food consumption by Pliocene hominins was in part selected by an increase in predation as evident by an increasing speciosity of members of the Order Carnivora in the OTD and other eastern African hominin locales circa 3.6 Ma (data from Werdelin and Lewis;[START_REF] Retallack | Pedogenic carbonate proxies for amount and seasonality of precipitation in paleosols[END_REF], 2013). More frequent terrestriality practiced by habitual bipeds in the context of increased predation risk was thought to have been a prime mover of diet change. Likewise, predation pressure could be viewed as an influence for the origins of stone tool making. However, Werdelin clarified the increase in speciosity during the middle Pliocene was caused by the "Laetoli effect" (commentary section of [START_REF] Thompson | Origins of the human predatory pattern[END_REF], a by-product due to a high preservation rate at one site.

Moreover, Faurby and others (2020) recently demonstrated that Plio-Pleistocene Carnivora, specifically large-bodied species, across eastern Africa underwent a reduction in abundance, which they argue was influenced by competition with hominins (but see [START_REF] Faith | Plio-Pleistocene decline of African megaherbivores: No evidence for ancient hominin impacts[END_REF]. This decreasing trend in Carnivora began as early as 4 Ma, and notably, much of these data derive from the OTD. [START_REF] Faurby | Brain expansion in early hominins predicts carnivore extinctions in East Africa[END_REF] assert that carnivores were outcompeted for animal food resources by hominins who engaged in scavenging of carcasses, stealing from carnivore kills (kleptoparasitism), or hunting behaviors. The authors suggest that hunting is a less likely explanation until hominins achieve body size and morphology for endurance hunting like that observed in genus Homo (e.g., [START_REF] Bramble | Endurance running and the evolution of Homo[END_REF] and/or the use of stone tools.

Tool-related activities by Homo [START_REF] Blumenschine | Percussion marks on bone surfaces as a new diagnostic of hominid behaviour[END_REF][START_REF] Braun | Early hominin diet included diverse terrestrial and aquatic animals 1.95 Ma in East Turkana, Kenya[END_REF][START_REF] Bunn | Archaeological evidence for meat-eating by Plio-Pleistocene hominids from Koobi Fora and Olduvai Gorge[END_REF][START_REF] Bunn | Early Pleistocene hominid foraging strategies along the ancestral Omo River at Koobi Fora, Kenya[END_REF][START_REF] Bunn | Systematic butchery by Plio-Pleistocene hominids at Olduvai Gorge, Tanzania[END_REF][START_REF] Dominguez-Rodrigo | Cutmarked bones from Pliocene archaeological sites at Gona, afar, Ethiopia: Implications for the function of the world's oldest stone tools[END_REF][START_REF] Pobiner | New evidence for hominin carcass processing strategies at 1.5Ma, Koobi Fora, Kenya[END_REF][START_REF] Potts | Cutmarks made by stone tools on bones from Olduvai Gorge, Tanzania[END_REF] and Australopithecus [START_REF] De Heinzelin | Environment and behavior of 2.5-million-year-old Bouri hominids[END_REF][START_REF] Mcpherron | Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia[END_REF] have been associated with animal carcass nutrient acquisition. [START_REF] Thompson | Origins of the human predatory pattern[END_REF] proposed that the Human Predatory Pattern (HPP) involving the scavenging of animals of equal or greater body size arose in Pliocene hominins with percussion-based marrow extraction of animal carcasses, rather than flaketool assisted meat procurement like that interpreted for early Homo. The timing in the decrease of carnivore speciosity in the OTD is suggestive of direct competition between carcass-scavenging hominins and sympatric predators beginning with A. anamensis and increasing throughout the Plio-Pleistocene [START_REF] Faurby | Brain expansion in early hominins predicts carnivore extinctions in East Africa[END_REF]. Recent 44 Ca/ 42 Ca analysis, typically a proxy used to distinguish herbivory from carnivory, did not distinguish A. anamensis from K. platyops, and both species have 44 Ca/ 42 Ca values overlapping those of Homo sp., Theropithecus, and some eastern African carnivores (Martin et al., 2020). Indeed, discerning different dietary components of omnivorous primates requires multiple lines of evidence; we suggest that an increase in faunivory by K. platyops is consistent with the decrease of  18 OEC values relative to those of A.

anamensis (e.g., Sponheimer andLee-Thorp, 1999b, 2001). Although many variables could have influenced Pliocene hominin  18 OEC values [START_REF] Faith | Paleodietary change and its implications for aridity indices derived from 18 O of herbivore enamel[END_REF][START_REF] Lee-Thorp | Stable isotopes in fossil hominin tooth enamel suggest a fundamental dietary shift in the Pliocene[END_REF][START_REF] Quinn | Influence of Plio-Pleistocene basin hydrology on the Turkana hominin enamel carbonate  18 O values[END_REF], we cannot discount the possibility that faunivory via scavenging behaviors was one of several potential  18 OEC-lowering factors.

Temporally discrete and sizable environmental events between 3-2 Ma are well documented in eastern Africa [START_REF] Alemseged | Fossils from Mille-Logya, Afar, Ethiopia, elucidate the link between Pliocene environmental changes and Homo origins[END_REF][START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF][START_REF] Robinson | Late Pliocene environmental change during the transition from Australopithecus to Homo[END_REF][START_REF] Wynn | Dietary flexibility of Australopithecus afarensis in the face of paleoecological change during the middle Pliocene: Faunal evidence from Hadar, Ethiopia[END_REF], but the adaptive morphological transitions from genus Australopithecus to Homo during this time appear less punctuated [START_REF] Antón | Evolution of early Homo: an integrated biological perspective[END_REF][START_REF] Kimbel | From Australopithecus to Homo: the transition that wasn[END_REF] than once characterized [START_REF] Mchenry | Australopithecus to Homo: transformations in body and mind[END_REF].

Brain size and hand morphology did not cross a significant grade-level threshold with the first appearance of genus Homo, rather these adaptations were rooted in Australopithecus [START_REF] Kimbel | From Australopithecus to Homo: the transition that wasn[END_REF]. Dietary niche expansion may have also occurred as a gradual increase in C4 food consumption beginning with A. anamensis. The initial dietary interpretations of A. anamensis concluded that the earliest biped consumed 0-10% of C4 foods (Cerling et al., 2013b), similar to that of modern savanna chimps [START_REF] Loudon | The stable isotope ecology of Pan in Uganda and beyond[END_REF]. However, unlike savanna chimps, A. anamensis' dentognathic morphology was adapted for processing hard/tough foods [START_REF] Grine | Dental microwear and stable isotopes inform the paleoecology of extinct hominins[END_REF][START_REF] Macho | Australopithecus anamensis: A finite element approach to studying the functional adaptations of extinct hominins[END_REF]Ward et al., 1999b[START_REF] Ward | Morphology of Australopithecus anamensis from Kanapoi and Allia Bay, Kenya[END_REF][START_REF] White | Asa Issie, Aramis and the origin of Australopithecus[END_REF]. Although microwear data contrast those of dentognathic morphology and imply soft foods were regularly consumed [START_REF] Grine | Dental microwear and stable isotopes inform the paleoecology of extinct hominins[END_REF], A. anamensis' dental microwear patterns are similar to those of the C3-C4 mixed feeding A. afarensis [START_REF] Grine | Was the early Pliocene hominin 'Australopithecus' anamensis a hard object feeder?[END_REF][START_REF] Unep ; University Of Kalmar | Molar microwear textures and the diets of Australopithecus anamensis and Australopithecus afarensis[END_REF]. [START_REF] Quinn | Isotopic equifinality and rethinking the diet of Australopithecus anamensis[END_REF] In a common thread, we suggest a gradual and stepwise but not necessarily abrupt behavioral shift underpins the earliest known stone tool making behaviors as represented by LOM3. Tools are used by extant chimpanzees to forage [START_REF] Boesch | Tool use and tool making in wild chimpanzees[END_REF][START_REF] Haslam | On the tool use behavior of the bonobo-chimpanzee last common ancestor, and the origins of hominine stone tool use[END_REF][START_REF] Haslam | Primate archaeology[END_REF][START_REF] Marchant | Percussive technology: chimpanzee baobab smashing and the evolutionary modeling of hominid knapping[END_REF]; thus tool-assisted foraging may be a shared ape-human behavioral trait also present in the earliest hominins [START_REF] Toth | The Oldowan: the tool making of early hominins and chimpanzees compared[END_REF]. As [START_REF] Thompson | Origins of the human predatory pattern[END_REF] proposed, the use of unmodified stone for carcass marrow extraction might have been practiced throughout the early Pliocene without visible archaeological traces. Stone tool making [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF] and use [START_REF] Mcpherron | Evidence for stone-tool-assisted consumption of animal tissues before 3.39 million years ago at Dikika, Ethiopia[END_REF] became archaeologically visible when hominins began to knap stones deliberately to produce sharp flakes. Although this step does not meet [START_REF] Shea | Occasional, obligatory, and habitual stone tool use in hominin evolution[END_REF] criteria of obligatory and repeated-form stone tool making as seen in late Pleistocene Homo, LOM3 artifacts may mark the beginnings of habitual tool-assisted foraging behaviors for the purposes of extracting nutrients from a wide range of vegetal resources as well as animal carcasses. Clearly, additional lines of evidence including more archaeological occurrences are needed to test this hypothesis. Nevertheless, we interpret that mounting evidence suggests that hominins may have expanded their dietary niche in the early Pliocene with an incremental increase in C4 food consumption and carcass scavenging potentially with unmodified stone tools. The origins of stone tool making behaviors enabled hominins to increasingly outcompete other C3-C4-mixed feeders (Figures 9,11) and perhaps carnivores. We assert that the earliest tool making was an important step in human evolution but constitutes one of several adaptations for enhancing foraging efficiency as Pliocene hominins incrementally expanded their dietary niche into the C3-C4 biome and gradually encroached on the carnivore guild.

Figure 12 illustrates a (food-centric) feedback loop hypothesizing the adaptive relationships between the environment, dietary niche expansion, morphological adaptations and the origins of stone tool making. The construction of the model was influenced by several contributions to paleoanthropology [START_REF] Aiello | The expensive-tissue hypothesis[END_REF][START_REF] Bromage | Biogeographic and climatic basis for a narrative of early hominid evolution[END_REF][START_REF] Faurby | Brain expansion in early hominins predicts carnivore extinctions in East Africa[END_REF][START_REF] Kimbel | From Australopithecus to Homo: the transition that wasn[END_REF][START_REF] Kingston | Shifting adaptive landscapes: progress and challenges in reconstructing early hominid environments[END_REF][START_REF] Leonard | Comparative primate energetics and hominid evolution[END_REF][START_REF] Leonard | Metabolic correlates of hominid brain expansion[END_REF]Shipman and Walker, 1995;[START_REF] Thompson | Origins of the human predatory pattern[END_REF]. Gradual but consistent environmental change and biotic competition served as selective factors for hominins to expand the dietary niche to include foraging C3-C4 plants and carcass scavenging. Incremental changes in foraging across habitats with different and more variable food properties (e.g., hardness, caloric and macronutrient contents) and procurement requirements (e.g., stone percussion) influenced a mosaic suite of morphological changes to the body and brain including but not limited to habitual bipedal locomotion, a masticatory apparatus for hard, tough, and abrasive foods, hand morphology for object manipulation, and an increased brain size.

(Un)modified stone-assisted percussion-based, food-getting behaviors were also influenced by food properties and procurement requirements. Moreover, changes in morphology and stone tool making behaviors, in turn, enhanced foraging efficiency and continued to increase hominins' capacity for dietary niche expansion. Gradual environmental changes in the early Pliocene and occurring throughout the middle Pliocene did not necessarily evoke abrupt changes to hominin morphology and diet, but provided consistent changes to resource abundances, shaping biotic competition.

Likewise, stone tool-making behaviors involving rudimentary but deliberate core and flake technology do not represent the start of a new dietary niche, but the result of a longterm and stepwise adaptive zone throughout the Pliocene.

Conclusions

OTD environments shifted incrementally toward more open biomes but with slightly more humid conditions across the early and middle Pliocene. Some aspects of the environment became marginally more variable as indicated by intranodular  13 CPC and  18 OPC values, but these changes are modest. Three herbivore ungulate taxa accompanied hominins in their increasing consumption of C4 resources over the study interval.

Notably, the C3-C4 feeding niche contracted through time, which was then occupied by broad C3-C4-mixed feeding hominins. No climatic changes are indicated with  18 OPC values from the early to middle Pliocene, but faunal  18 OEC values signal a slight increase in humidity. The drop in hominin  18 OEC values suggests an increase in water dependency as a function of increasing C4 consumption, but also may have tracked the vegetation  18 O decrease and/or the consumption of animal products. The gradual increase in C4 vegetation detected across this time interval is consistent with the longterm decrease in pCO2; however, the evidence for a slight uptick in humidity suggests that C4 grassland spread was not accompanied by aridification. To date, OTD proxy records do not indicate the types of habitat changes predicted by variability or pulsed selection hypotheses per se (e.g., [START_REF] Maslin | East African climate pulses and early human evolution[END_REF][START_REF] Potts | Alternating high and low climate variability: the context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF]Schultz and Maslin, 2013), but rather point to gradual, long-term environmental shifts. Making stone tools to more efficiently and effectively access foods provided the earliest knappers with a competitive edge relative to the established C3-C4 feeding guild members including carnivores. We assert that hominin dietary niche expansion in the Pliocene, involving a wide range of plant foods and possibly carcass scavenging, was not a punctuated event but rather a series of incremental steps as hominins became increasingly more reliant on tool-assisted foraging, which culminated in stone tool making origins.

Table and Figure Captions

Table 1. OTD fossil mammalian mean  13 CEC and  18 OEC values  (‰) and interquartile ranges (data from [START_REF] Blumenthal | Aridity and hominin environments[END_REF]Cerling et al., 2013b[START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF][START_REF] Drapeau | The Omo Mursi formation: A window into the east African Pliocene[END_REF][START_REF] Manthi | Diets of mammalian fossil fauna from Kanapoi, northwestern Kenya[END_REF][START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF]this study). Figure 1. The LOM3 archaeological site and artifacts Photos courtesy of the Mission Préhistorique au Kenya (MPK)/West Turkana Archaeological Project (WTAP). Figure 2. A: regional map with location of OTD (after [START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF]; B: OTD map with locations of the Omo Group Formations mentioned in this study (after Feibel, 2011). Figure 3. A: Generalized eastern African Plio-Pleistocene hominin phylogeny characterized with three main lineages and representative fossil specimens from the OTD. *denotes debate of specimen designation in the OTD. No complete cranial specimens of A. afarensis or A. anamensis are preserved in the OTD [START_REF] Kimbel | Identification of a partial cranium of Australopithecus afarensis, from the Koobi Fora Formation, Kenya[END_REF]Ward et al., 1999); those pictured are from Ethiopia. Permissions of hominin fossil photos: courtesy of Human Origins Program, Smithsonian Institution (Homo, Paranthropus, A. afarensis); A. anamensis by Dale Omori courtesy of Cleveland Museum of Natural History; K. platyops by Fred Spoor copyright National Museums of Kenya. B: OTD hominin  13 CEC values (data from Cerling et al., 2013b;[START_REF] Wynn | Isotopic evidence for the timing of the dietary shift toward C4 foods in eastern African Paranthropus[END_REF]. C: Early Stone Age record in the OTD represented by single artifacts from assemblage; first appearance (gray box) and known duration (gray arrow); ^denotes older Oldowan site occurrences in Ethiopia; artifact photos courtesy of the Mission Préhistorique au Kenya (MPK)/West Turkana Archaeological Project (WTAP). Figure 4. Global records of environmental change: A: pCO2 [START_REF] Stap | CO2 over the past 5 million years: continuous simulation and new d11B-based proxy data[END_REF], B: marine benthic  18 O values (L04 stack: [START_REF] Lisiecki | A Pliocene-Pleistocene stack of 57 globally distributed benthic  18 O records[END_REF], C: African dust flux from ODP 721/722 [START_REF] Demenocal | Plio-Pleistocene African climate[END_REF][START_REF] Demenocal | African climate change and faunal evolution during the Pliocene-Pleistocene[END_REF]; shaded bars are proposed intervals of high climatic variability [START_REF] Potts | Alternating high and low climate variability: the context of natural selection and speciation in Plio-Pleistocene hominin evolution[END_REF]; D: Curve depicts ~100-kyr eccentricity cycle and hence a proxy of Plio-Pleistocene tropical insolation and monsoonal rainfall intensity; shaded bars indicate eastern African lake phases that correlate to the peaks in the 405-kyr cycle [START_REF] Trauth | Late Cenozoic moisture history of East Africa[END_REF]. Figure 5. Plio-Pleistocene OTD environmental records: A: Exponentially-smoothed (alpha = 0.1)  13 CPC values (‰) estimates (data from [START_REF] Cerling | An isotopic study of a fluviolacustrine sequence: the Plio-Pleistocene Koobi Fora sequence, East Africa[END_REF][START_REF] Harmand | Before the Oldowan: 3.3 Ma Stone Tools from Lomekwi 3, West Turkana, Kenya[END_REF][START_REF] Levin | Paleosol carbonates from the Omo Group: isotopic records of local and regional environmental change in East Africa[END_REF][START_REF] Quinn | Paleogeographic variations of pedogenic carbonate  13 C values from Koobi Fora, Kenya: Implications for floral compositions of Plio-Pleistocene hominin environments[END_REF]Quinn et al., , 2013;;[START_REF] Quinn | Revisiting the pedogenic carbonate isotopes and paleoenvironmental interpretation of Kanapoi[END_REF]Patterson et al., 2019;[START_REF] Wynn | Paleosols, stable carbon isotopes and paleoenvironmental interpretation of Kanapoi, Northern Kenya[END_REF][START_REF] Wynn | Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols from the Turkana Basin, Kenya[END_REF]; B: relative abundance of APP taxa (methods after [START_REF] Cerling | Dietary changes of large herbivores in the Turkana Basin, Kenya from 4 to 1 Ma[END_REF]data from Blumenthal et al., 2017 and references therein;Cerling et al., 2015 and references therein;[START_REF] Negash | Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia[END_REF][START_REF] Patterson | Comparative isotopic evidence from East Turkana supports a dietary shift within the genus Homo[END_REF]; C: faunal abundances of arid-adapted taxa [START_REF] Bobe | The expansion of grassland ecosystems in Africa in relation to mammalian evolution and the origin of the genus Homo[END_REF]; D: pedogenic carbonate clumped isotope temperatures (C) [START_REF] Passey | Hightemperature environments of human evolution in East Africa based on bond ordering in paleosol carbonates[END_REF]; E:  18 OEC-based water deficit estimates (mm/year) [START_REF] Blumenthal | Aridity and hominin environments[END_REF]. Figure 6. A: Individual data points; B: box and whisker plots of two time intervals (4.3-3.9 Ma and 3.4-3.0 Ma) and all  13 CPC values from the OTD (‰). Large black box denotes 25 th and 75 th percentiles; black line shows median value; gray line equals mean value; whiskers denote 1.5* interquartile range. Figure 7. A: Individual data points; B: box and whisker plots of two time intervals (4.3-3.9 Ma and 3.4-3.0 Ma) and all  18 OPC values from the OTD (‰). Large black box denotes 25 th and 75 th percentiles; black line shows median value; gray line equals mean value; whiskers denote 1.5* interquartile range. Figure 8. Box and whisker plots of two time intervals (A: 4.3-3.9 Ma and B: 3.4-3.0 Ma) superimposed on all intranodular  13 CPC and  18 OPC standard deviations from the OTD (‰). Large black box denotes 25 th and 75 th percentiles; black line shows median value; gray line equals mean value; whiskers denote 1.5* interquartile range. Figure 9. Box and whisker plots separated in two time intervals (4.3-3.9 Ma=orange, 3.4-3.0 Ma=blue) of  13 CEC values of ungulate herbivore taxonomic groups and hominins from the OTD (‰). Large black box denotes 25 th and 75 th percentiles; black line shows median value; gray line equals mean value; whiskers denote 1.5* interquartile range. Asterisk denotes significant differences in pairwise comparisons (Mann-Whitney U, p<0.05). Figure 10. Box and whisker plots separated in two time intervals (4.3-3.9 Ma=orange, 3.4-3.0 Ma=blue) of  18 OEC of ungulate herbivore taxonomic groups, Crocodylia and hominins from the OTD (‰). Large black box denotes 25 th and 75 th percentiles; black line shows median value; gray line equals mean value; whiskers denote 1.5* interquartile range. Asterisk denotes significant differences in pairwise comparisons (Mann-Whitney U, p<0.05). Figure 11. Probably density function of all ungulate herbivore  13 CEC values separated into C3-browsers, C3-C4 mixed feeders, C4-grazers (methods described in text). A: 4.3-3.9 Ma, B: 3.4-3.0 Ma. Hominin  13 CEC ranges are shown with bar. Figure 12. Hypothesized feedback relationships between gradual environmental shifts, dietary niche expansion, morphological adaptations and stone tool making behaviors.

  derived a regression equation to calculate the fraction of woody canopy cover (WC) from paleosol carbonate  13 CPC values in order to categorize structure based on UNESCO classifications of African vegetation. Cerling et al. (2015) describe the classification as follows: 1) grassland: WC = 0-10%, 2) open wooded grassland: WC = 10-20%, 3) wooded grassland: WC = 20-40%, 4) grassy woodland/bushland/shrubland: WC = 40-60%, 5) woodland/bushland/shrubland: WC = 60-80%, 6) forest/closed forest: WC = 80-100%.

  13 C value of five lowland tropical closed canopy forests in Africa (-30.6‰) (data from: Loango National Park, Côte d'Ivoire, -30.5±3.5‰, Oelze et al., 2014; Tai Natural Park, Côte d'Ivoire, -30.8±4.1‰, Fahy et al., 2013; Ituri Forest, DRC, -31.2±2.8‰, Cerling et al., 2004; Salonga National Park, DRC, -29±2.7‰, Oelze et al., 2011; Kakamega Forest, Kenya, -31.4±0.5‰, Cerling et al, 2003c) and a correction for the Pliocene atmospheric CO2  13 C offset of 2.3‰ (Tipple et al., 2010). We use the term "woodland" for C3-feeders exploiting the more open C3 biome resources;  13 CEC values range from -8‰ to -14‰. Based on Tipple et al. (

  reanalyzed the  13 CEC-based C4 consumption percentage of A. anamensis in the OTD informed by Pliocene CO2  13 C values and vegetation structural categories and suggested that C4 resources may have been more regular components in the earliest habitual biped (i.e., ~20% C4 diet), constituting an stepwise increase from a purely C3 diet to a C3-C4 mixed diet. Moreover, Blumenthal et al. (2020) has found high intra-and inter-annual  13 CEC values with laser ablation techniques in specimens of A. anamensis and K. platyops, suggesting seasonal fluctuations in resource use. Thus hominins' dietary forays into the C3-C4 biome may be characterized as incremental increases in dietary breadth throughout the early Pliocene, reaching the broad C3-C4 mixed diet in the middle Pliocene.

Acknowledgments

We thank the office of the President of Kenya, the Ministry of Education, Science and Technology, the National Council for Science and Technology (NCST/RCD/12B/012/25) and the Kenya National Museums, especially Frederick Kyalo Manthi, for fieldwork and sampling collection permissions. We thank the Mission Préhistorique au Kenya/West Turkana Archaeological Project, funded by the French Ministry of Foreign Affairs (Nu681/DGM/ATT/RECH, Nu986/DGM/DPR/PRG), and the French National Research Agency (ANR-ARCHOR-12-CULT-0006; ANR-HOMTECH-17-CE27-0005). The generous and ongoing support of the Turkana Basin Institute facilitated this research project and aided with fieldwork. We thank Richard Mortlock and James Wright in the Geochemistry Laboratory, Department of Earth and Planetary Sciences, Rutgers University for mass spectrometry. Laboratory and student educational support was provided by the National Science Foundation to RLQ (NSF-CAREER, BCS-145527). The US Embassy in Nairobi provided support to JEL for travel and logistics. Numerous researchers provided methodological tools, invaluable datasets and environmental insights for the OTD, which made inferences such as these possible, especially Thure Cerling.