Earliest Olduvai hominins exploited unstable environments ~ 2 million years ago

Rapid environmental change is a catalyst for human evolution, driving dietary innovations, habitat diversification, and dispersal. However, there is a dearth of information to assess hominin adaptions to changing physiography during key evolutionary stages such as the early Pleistocene. Here we report a multiproxy dataset from Ewass Oldupa, in the Western Plio-Pleistocene rift basin of Olduvai Gorge (now Oldupai), Tanzania, to address this lacuna and offer an ecological perspective on human adaptability two million years ago. Oldupai’s earliest hominins sequentially inhabited the floodplains of sinuous channels, then river-influenced contexts, which now comprises the oldest palaeolake setting documented regionally. Early Oldowan tools reveal a homogenous technology to utilise diverse, rapidly changing environments that ranged from fern meadows to woodland mosaics, naturally burned landscapes, to lakeside woodland/palm groves as well as hyper-xeric steppes. Hominins periodically used emerging landscapes and disturbance biomes multiple times over 235,000 years, thus predating by more than 180,000 years the earliest known hominins and Oldowan industries from the Eastern side of the basin.

Palaeoenvironmental reconstruction from sites 2.6 to 1.9 Ma has relied on indirect approximations of past vegetation from fauna and/or stable isotopes [13][14][15][16][17]19,20 , denoting variably open grassland and forest mosaics in fluvial settings often from restricted stratigraphic intervals 21,22 . Our research reconciles the earliest Oldowan stone tools from Oldupai Gorge, a key complex for the study of hominin lifeways 23,24 , with multiproxy datasets in direct association with stratified archaeological and fossil assemblages, recording episodic exploitation of the same place in varied geomorphic contexts and sedimentary deposits. This dataset stands as a model of cross-disciplinary research to clarify the environmental context of early Oldowan sites. Here we examine hominin behavior in association with faunal and plant communities and provide evidence of vegetation physiognomy and cover from phytolith analysis and palynology, isotopic nalkane values from plant waxes, stable isotopes from enamel, and regional fires from microcharcoal concentrations.
The geo-archaeological record through the Ewass Oldupa sequence shows evidence of human occupation 2.0-1.9 Ma through rapidly shifting depositional environments that comprised meandering streams cutting across a volcaniclastic fan, followed by mass flow events, several transgression/regression lacustrine cycles, large and small prograding fluvial systems, with intercalated volcanism. Importantly, this record of hominin habitats predates, to our knowledge, the oldest fossiliferous and tool bearing deposits from the Eastern basin's locality of 'Douglas Korongo' (DK), which traditionally represented the earliest Oldowan presence in the region at~1.848 ± 0.003 Ma 24,30,31 .
Excavations at Ewass Oldupa recovered 1373 fossil specimens and 565 stone artefacts (Supplementary Table 1) showing consistent technological features from immediately post-Naabi to the base of Bed II. All knapping stages are present (Supplementary Table 1). Lithics derive from tabular, medium grained, grayish/whitish quartzite slabs that are clustered (7/m 2 ) in discrete archaeo-stratigraphic assemblages (7/40 m 2 ) (Figs. 3 and 4). Flakes are the dominant product (>60%), with spheroids and percussive materials being uncommon. There is no differential management of knapped surfaces, thus the technological systems do not follow hierarchical reduction strategies. Unidirectional and multidirectional knapping of cores was maintained within the constraints of natural shapes and angles for striking. Of the seven exploitation methods registered in 58 cores, multipolar-multifacial (number of cores = 19, median number of extractions = 9), unipolar-longitudinal (n = 15, median = 4), and orthogonal-bifacial (n = 13, median = 5) dominate ( Supplementary Fig. 2c). Intensity of lithic reduction is inferred from the minimum number of extractions per core, which ranges from two to 16 removals. Overall, the simplicity of Ewass Oldupa's technical repertoire is shared with other early Oldowan assemblages 6,[10][11][12]14,15 to include characteristics such as freehand hard hammer percussion and multi-facial reduction. Principal Component Analysis (PCA) ( Supplementary Fig. 2a, b, Supplementary Fig. 3) conveys that Ewass Oldupa's Oldowan shares many technical attributes with Kanjera South 14 , Fejej 11 , and to some extent Frida Leakey Korongo Zinj (FLK Zinj) 24 . Significantly, several aspects of Ewass Oldupa's stone industries (unifacial and bipolar cores, number of extractions, flake size, angular fragments, and scarcity of percussion tools) best match the techno-typological profile seen in older Oldowan sites (Gona 10 and Gona 12) 15 . In addition, PCA brings out a mosaic of early and classic Oldowan traits in Ewass Oldupa's lithics, which fall between the oldest Oldowan > 2 Ma and younger assemblages ≤1.85 Ma, where spheroids abound. Likewise, this PCA, inclusive of 18 assemblages and comprising 11 technical variables 15 , demonstrates the outlier character of Lomekwi and its complete lack of affinity with Ewass Oldupa (Supplementary Figs. 2,3,  Supplementary Table 2).
Comparison of geochemical fingerprinting of artefacts and regional rock outcrops ( Supplementary Fig. 4) manifests hominins engaged in sourcing up to 12 km across the basin 32,33 while also exploiting quartzite locally, 400 m to the south (Naisiusiu). Macroscopically, four quartzite types of distinct colour, grain size, textural, and mineral composition were the preferred raw materials. Analysis of these varieties relative to reduction stage ( Supplementary Fig. 2d) indicates that Oldowan groups targeted specific quartzite types for flakes, while other variants were for retouched tools. Spheroids and choppers were made of quartzite and ignimbrite, accessible from riverbeds.
Early Oldowan ecology at~2 Ma. The earliest Oldowan presence at Ewass Oldupa, dating to 2.03 Ma, is in a post-eruptive landscape, occurring on an expansive ignimbrite flow 34 that buried the hydrological network of the southeast portion of the basin 18    Major rock types potentially available for hominin exploitation throughout the region are also marked. b Single stratigraphic section from Ewass Oldupa, subdivided into two segments: lower (i) and upper (ii). Ngorongoro formation: the succession starts with the Naabi, a green-gray, quartz-trachyte welded tuff dated to 2.038 ± 0.005 Ma 25 . A thin red diamictite overlies the Naabi and separates it from a series of fining-upward sandstone-dominated units composed of mafic tephra (Fig. 2b) that is capped by siltstone. Ngorongoro formation, CFCT compositional zone: the strata are composed of reddishbrown sandstone, volcanic detritus, and tephra/siliciclastic diamictite 28 . (i) Lower Bed I-six units that upward fine from pebble-cobble conglomerate to cross-stratified sandstone are attributed to meandering river channels, below Tuff IA. Subsequently, a fine-grained silty sandstone that fines into waxy, green-brown claystone with carbonate nodules was deposited in a lake setting. Lastly, interbedded sandstone, siltstone, claystone, and tuffaceous beds record fluctuating environments from lacustrine to shallow lakeshore to floodplain with small fluvial channels. (ii) Upper Bed I-starts with Tuff IB, overlying waxy, green-brown claystone (Fig. 2H), and then Tuff IC. Capping the succession is a beige, weakly stratified sandstone that is geochemically consistent with Tuff 1D 22,25,26 . Above this, the Ng'eju Tuff is encountered in a succession of fine-grained floodplain deposits. Additional stratigraphic information is presented in Fig. 5. sinuous meandering river flowing northwest, as revealed in trench no. 7: the Naabi ignimbrite was the result of a high energy, catastrophic volcanic eruption and associated debris flow that drastically reshaped the landscape. After this, the volcanic events were less impactful and stable environments such as river channels and floodplains were able to develop. In a palaeogeographic setting where the quarzitic basement cropped out from the extruded pyroclastic flow and a fresh water channel ran through the distal part of a volcaniclastic fan, hominins periodically used the inselberg's foothill, within short distance of both materials for tools as well as water sources from the floodplain. The floral context is established by phytoliths (Figs. 5a, 6 and Supplementary Fig. 5), in which several fern types [35][36][37] dominate assemblages (Fig. 6a, d, e), suggesting the existence of a fern meadow with minor woody growth and grasses; thus, pioneering bracken ferns facilitated the re-establishment of woody and grassy communities after a decrease in destructive volcanic events 38 . During deposition of the CFCTcz (2.015 ± 0.006 Ma) 25 Ma, rhizoliths in fine-grained units represent fluvial floodplain palaeosols in trench no.5 (Figs. 1, 2c, and 5). Mass flows did not allow prolonged quiescence, and laharic inundation buried synchronous woodland landscapes and Oldowan remains consisting of 12 lithics excavated 1 km west by other teams 39 . Microcharcoals from wildfires peaked (16,830 particles/cm 3 ) (range through stratigraphic sequence = 72-16,830/cm 3 , median = 418/cm 3 ) concomitantly with grass opal increase (Supplementary Fig. 5a, b). Great diversity in phytolith morphotypes denotes markedly uneven communities and quick shifts in plant dominance ( Supplementary Fig. 5d, e).
Three additional Oldowan assemblages (~2 Ma) were found at the contact between the Ngorongoro Formation and Bed I, in association with a sinuous channel flowing east, and preserved 4.6 m below Tuff IA in trench no. 5 (Figs. 1, 2f, g, 3, and 5). The reconstructed environment is a short woodland mosaic as indicated by multiple proxies. For instance, plant wax isotopes, using the weighted average δ 13 C of C 27 -C 33 for each individual sample and plotted using a sine-squared mixing model with endmember values of −30‰ (pure C 3 ) and −19‰ (pure C 4 ), reveal a depositional context of mosaics (Fig. 7a). One sample illustrates a C 3 -dominated environment likely in response to increased rainfall (δ 13 C: −27.9‰), whereas the other samples exhibit δ 13 C values more indicative of mixed environments, ranging from δ 13 C −22.6‰ to δ 13 C −24.6‰. This δ 13 C distribution for Ewass Oldupa is analogous to East African Acacia-Commiphora woodlands today. Another line of evidence indicative of woodland mosaics is stable isotopes from dental enamel: the entire analysed assemblage represents pre-Tuff IA times (δ 13 C: −7.2‰ to 1.6‰), pointing to herbivores consuming a mixture of C 3 and C 4 plants (Fig. 8). Although δ 18 O measurements can be influenced by a number of factors including precipitation, temperature, rainfall source, and physiology, the range obtained (δ 18 O: −4.7‰ to −0.2‰) is consistent with animals obtaining their water from a similar source and no clear distinctions between taxa are obvious (Fig. 8, Supplementary Fig. 6, and Supplementary Table 3). The range for δ 18 O could not be resolved to obligate or non-obligate drinking status. The enamel isotopic profile from Ewass Oldupa, in lowermost Bed I, resembles that from the younger woodlands in Bed II (Fig. 8), but contrasts with the open grassland profile prevailing in penecontemporaneous localities from western Kenya. Additional supporting evidence for a reconstructed palaeoenvironment of woodland/grassland mosaics is shown by phytoliths ( Table 2, Figs. 5, 6, and Supplementary Fig. 5b), characterized by fluctuating woody dicot and grass outputs.
A fifth Oldowan assemblage includes stone tools and bones 4 m below Tuff IA (~2 Ma) on a transgressive lakeshore representing the earliest and furthest western expansion of palaeolake Oldupai,   Table 2), representing proximity of fresh water and preferred hominin sources of raw materials. Further, the Oldowan stone tools from this horizon (Fig. 4e, f) are the earliest lithic evidence deposited in a waxy clay from a saline/alkaline lacustrine environment anywhere.      Table 4). Phytoliths show woody dicots and grasses booming and busting cyclically 46 , while grassland encroachment on woodlands occurred twice (Supplementary Fig. 5b) (Figs. 1 and 7a, d-f) in a stratigraphic position equivalent to that of the oldest Homo habilis in the Eastern basin 30 (OH 24). The succession of Oldowan occupations ends with artefacts from a lakeshore context sandwiched between Tuff IB and IC (1.848/1.832 Ma), in a position and sedimentary environment that on the Western lakeshore is a counterpart to Level 22 Zinj 23,24,41,47 , while still 2 m below OH 65 21 (Fig. 1), as uncovered in trench no. 6. Pollen grains express a treeless, grassless, hyper-xeric steppe dominated by Asteraceae in cohort with Lamiaceae/Convolvulaceae and Rutaceae scrub (Fig. 6o) immediately underneath the Ng'eju Tuff, correlative with a mudstone/calcareous horizon over OH 65 22,48 .

Discussion
Ewass Oldupa is a high-resolution, multi-episode, early Oldowan stratified site that precedes and straddles Bed I. This site holds Oldupai's oldest evidence for hominin cultural remains associated with habitats from post volcanic disturbance and thus early examples of adaptation to major geomorphic and ecological transformations. The earliest Oldowan tools appear immediately after the deposition of the Naabi ignimbrite, and they are associated with a stacked fluvial series composed of mafic tephra (Fig. 1). Therein, cross-stratified sandstone and an overall upward fining capped by siltstone shows that humans provisioned near meandering rivers. This episodic exploitation of fluvial environments precedes that of the unstable surfaces characteristic of younger CFCT horizons 39 . Large lithic and faunal assemblages from the contact between the topmost CFCT and lowermost Bed I, together with a wide range of vegetation proxies, mark the onset of lacustrine occupation in the basin, which precedes the emplacement of Tuff IA. The Oldowan toolkits from this time intersect earlier and contemporaneous technologies from Ethiopia and Kenya, while advancing tool types such as the spheroid commonly seen in younger assemblages 24 . If these stone tools drove provisioning amidst unpredictable environments, it was via a generalist strategy 49,50 for a variety of tasks that did not include defleshing animal carcasses, as shown by a lack of human modified fauna.
In conclusion, the earliest Oldupai hominins occupied heterogeneous unstable environments. Sedimentary, chemical, and vegetation data indicate that early Oldowan hominins pioneered a rapid occupation of geo-settings undergoing drastic changes in hydrological resource distribution and structure, and supporting uneven floras. The evidence uncovered is of periodic, recurrent land use across a subset of environments punctuated with times when there is no evidence of hominin activity. Our work reveals early Oldowan stone tools in diverse physical environments, with indications that these environments both changed significantly over space and time. In addition, Oldowan groups utilised disturbance environments-a finding that is unique for this period and depicts complex behavior among early Pleistocene hominins. The ability to exploit diverse biomes enabled hominins to expand beyond Africa. This behavioral flexibility and adaptive suite materialized in the absence of controlled fire, amid large predators, and can be interpreted as a predictor of the invasiveness that facilitated early Homo's global dispersal.

Methods
Biomarkers. Dry, homogenized sediment was PSE extracted (Büchi Speed Extractor E-916) in three 10 min cycles in 9:1 (v:v) DCM:MeOH at 100°C and 103 bar/1500 psi. Neutral, non-polar hydrocarbons were separated from total lipid extracts by Aminopropyl column chromatography using 2:1 DCM:IPOH. Normal (n-) alkanes were then separated from the neutral fraction using silver nitrateinfused silica gel column chromatography using Hexane. Gas chromatography (GC) analysis was performed using an 7890B (Agilent) and HP-5 column (Agilent; 30 m length, 0.25 mm i.d., and 0.25 µm film) with a 5977 A MSD. Samples were injected at 250°C in splitless mode and the oven temperature programmed from 60°C (1 min hold) to 150°C at 10°C/minute, then to 320°C at 6°C/minute (10 min hold). The Mass Spectrometry (MS) source was operated at 230°C with 70 eV and a fullscan rate of m/z 50-550. Plant wax n-alkanes were identified by comparing mass spectra and retention time with an external standard mixture (C 21 -C 40 ).
Isotope-ratio MS Compound-specific δ 13 C values were analysed by 7890B GC (Agilent) with a HP-5 capillary column (Agilent) operated in split mode coupled to    a Elementar Isoprime AnthrovisION Mass Spectrometer by an Elementar GC5 furnace system interface operating at 850°C. Oven temperature was programmed from 60°C (1 min hold) to 150°C at 10°C/minute, then to 320°C at 6°C/minute (10 min hold). Helium was the carrier gas with a constant flow of 1.1 mL/min. Accuracy of δ 13 C values were evaluated against an international standard laboratory mixture (Indiana B4, Arndt Schimmelmann, University of Indiana) injected at the beginning and end of each sample set. The standard deviation of the n-alkane working standard was ≤0.5‰. Isotopic ratios are expressed as δ 13 C values in per mil relative to the Vienna Pee Dee Belemnite (VPDB) standard.
Excavation. Location of materials was recorded on a Leica TS 09 during excavation. Materials were placed within an X-Y Cartesian system, with Z-values also recorded, prior to being individually labelled and bagged. Sediments from each trench were dry-sieved (0.5 cm mesh). Identification and XYZ coordinates were exported to a database. This field database was the matrix for techno-typological classification, taxonomy, taphonomy, and spatial analysis. 3D-registered data was XY-plotted using ArcMap10.7, with XZ and YZ projections performed N-S and E-W. Kernel density analysis (search radius = 25 cm) was applied to XY data.
Fauna. Elements >2 cm were studied. For taxonomic identification, we consulted the Smithsonian Institution's digital archive of mammals from East Africa 53 , an osteological manual 54 , an established museum reference collection directly available to us at IPHES (Tarragona, Spain), as well as specialised faunal literature on Early Pleistocene fossils 55 . Surface modifications were noted for all specimens with cortex. The specimens were then studied under a stereomicroscope (Optech HZ, 10-60x). Skeletal profiles were created using the values for the number of identified specimens (NISP). Size class classification [56][57][58] was applied where no higher order classification could be made. Minimum number of elements (MNE) was calculated based on NISP, with frequency, portions, size, and side under analysis being noted 59 . MNE portions were used to standardise minimum animal units (% MAU) 60 . A correlation coefficient for MAU percentage and bone mineral density 61 , considering the MNE of each portion of all elements among ungulates. The age of a specimen was stated where possible, with "unknown" used otherwise. Fracture patterns for long and flat bones were noted 62 .
Mineral geochemistry. Composition of phenocrysts was determined by electron microprobe analysis. Preliminary petrographic analysis of polished thin sections used cross-polarizing, petrographic microscope to identify phenocryst abundance, sorting, degree of alteration, and suitability for analysis. Samples were coated with carbon and analyzed using the JEOL JXA-8200 electron microprobe. Electron microprobe beam was operated at a current of 8 μA and a voltage of 15 kV and a beam diameter of 11 mm. A combination of brightness from BSE images and EDS was used to identify potential mineral grains to analyze. Where possible, a minimum of 10 grains were analyzed per sample per mineral (30-point/sample). Microprobe calibration standards for feldspar grains were orthoclase and albite, standards for augite/clinopyroxene grains were Cr-augite and hornblende, and ilmenite was used to calibrate for oxide grains. Effort was made to analyze grains that were contained within glass or within pumiceous material. Mineral grains were analysed at random to prevent bias. Wt% totals for feldspar and pyroxene analyses less than 96% or greater than 102% were excluded from the final data, as were titanomagnetite grains with totals less than 90%.
System microscopy was at 40x (Olympus BX41, Motic BA410E). Minimum of 200 phytoliths were counted where possible, or until sample extinction. The referential included several African ecoregions 46,[63][64][65][66][67][68] , and past work at Oldupai Gorge 43,69 . Archaeological samples were compared to a baseline of 29 plant species and 35 soils from Acacia-Commiphora woodland mosaics from the study area, using general and diagnostic approaches to reconstruct physiognomy and plant cover rank through their phytolith signal 46 . Classification nomenclature followed ref. 70.
Pollen and microcharcoal. Protocol was adapted from ref. 71. Sample (2 mL) was mixed with 3% sodium hexametaphosphate (NaPO 3 ) 6 in waterbath (90°C) for 30 min and mechanically agitated over several days. Lycopodium spore tables were added to calculate pollen and microcharcoal concentrations. Samples settled and decanted (<2 µm), followed by sieving (>125 µm). Separation of pollen and charcoal was done using gentle mechanical agitation over several days followed by heavy liquid separation using lithium heteropolytungstate at specific gravity 2.0. Acetolysis was achieved through 9:1 mixture of C 4 H 6 O 3 to H 2 SO 4 and 10% HCl. The samples were mounted on microscope slide with glycerol (cover: 22 × 40 mm = inspected area). System microscopy was done at 25x, 40x, 63x. Minimum of 350 pollen grains and 200 microcharcoal were counted.
Stable carbon and oxygen isotope analysis of faunal dental enamel. Specimens selected all had excellent preservation conditions, and no weathered or fragmented dental specimens were considered. Teeth were cleaned using air-abrasion. Sample of enamel powder (6 mg) was taken from buccal edge using diamond-tipped burr drill. It was washed in 1.5% NaClO for 60 min, rinsed, then centrifuged. It was then washed with acetic acid (0.1 M) for 10 min and rinsed. Residue was lyophilised for 24 h, then set to react with H 3 PO 4 and gases were measured for stable carbon and oxygen ratios using GasBench II (Thermo) coupled with Delta V Advantage MS (Thermo). Values were compared against International Standards: IAEA-603 (δ 13  Measurement error of MERCK standard c was ±0.1‰ for δ 13 C and ±0.2‰ for δ 18 O. Overall precision of measurements through repeat extracts from an inhouse bovid tooth enamel standard c was ±0.2‰ for δ 13 C and ±0.3‰. Obligate or non-obligate drinking status was inferred from the range of δ 18 O as per references [72][73][74] . Stone tools. All specimens were measured and weighed, with attributes recorded in database. Technological classes such as core, core fragment, flake, broken flake, flake fragment with a platform, percussive material, retouched piece, and tools were distinguished. Knapping methods were classified according to faciality (noting the number of surfaces exploited) and polarity (direction of exploitation from a striking platform). The following core reduction methods were observed: (i) multipolar-multifacial: cores with four or more surfaces exploited in at least four different directions; (ii) unipolar-longitudinal: cores with a single surface exploited, unidirectionally; (iii) orthogonal-bifacial: cores with two opposed surfaces knapped, unidirectionally; (iv) centripetal-bifacial: cores with two exploitation surfaces organized by a horizontal plane, each side with at least three centripetal removals; (v) centripetal-unifacial: cores with one surface exploited and at least three centripetal extractions; (vi) bipolar-on-anvil: cores with evidence of placement on passive surface; (vii) bipolar-unifacial: cores with one surface exploited bidirectionally. Multivariate ordination utilized variables from ref. 15 through Palaeotological Statistics software. We normalized variables. The first three components of the PCA explain 80.1% of the variance.