Adaptive foraging behaviours in the Horn of Africa during Toba supereruption.

Although modern humans left Africa multiple times over 100,000 years ago, those broadly ancestral to non-Africans dispersed less than 100,000 years ago1. Most models hold that these events occurred through green corridors created during humid periods because arid intervals constrained population movements2. Here we report an archaeological site-Shinfa-Metema 1, in the lowlands of northwest Ethiopia, with Youngest Toba Tuff cryptotephra dated to around 74,000 years ago-that provides early and rare evidence of intensive riverine-based foraging aided by the likely adoption of the bow and arrow. The diet included a wide range of terrestrial and aquatic animals. Stable oxygen isotopes from fossil mammal teeth and ostrich eggshell show that the site was occupied during a period of high seasonal aridity. The unusual abundance of fish suggests that capture occurred in the ever smaller and shallower waterholes of a seasonal river during a long dry season, revealing flexible adaptations to challenging climatic conditions during the Middle Stone Age. Adaptive foraging along dry-season waterholes would have transformed seasonal rivers into 'blue highway' corridors, potentially facilitating an out-of-Africa dispersal and suggesting that the event was not restricted to times of humid climates. The behavioural flexibility required to survive seasonally arid conditions in general, and the apparent short-term effects of the Toba supereruption in particular were probably key to the most recent dispersal and subsequent worldwide expansion of modern humans.

Olduvai's oldest Oldowan.

Previously, Olduvai Bed I excavations revealed Oldowan assemblages <1.85 Ma, mainly in the eastern gorge. New western gorge excavations locate a much older ∼2.0 Ma assemblage between the Coarse Feldspar Crystal Tuff (∼2.015 Ma) and Tuff IA (∼1.98 Ma) of Lower Bed I, predating the oldest eastern gorge DK assemblage below Tuff IB by ∼150 kyr. We characterize this newly discovered fossil and artifact assemblage, adding information on landscape and hominin resource use during the ∼2.3-2.0 Ma period, scarce in Oldowan sites. Assemblage lithics and bones, lithofacies boundaries, and phytolith samples were surveyed and mapped. Sedimentological facies analysis, tephrostratigraphic and sequence stratigraphic principles were applied to reconstruct paleoenvironments and sedimentary processes of sandy claystone (lake), sandstone (fluvial), and sandy diamictite (debris flow) as principal lithofacies. Artifacts, sized, weighed, categorized, were examined for petrography, retouch, and flake scar size. Taxonomic classifications and taphonomic descriptions of faunal remains were made, and phytoliths were categorized based on reference collections. Lithics are dominantly quartzite, mainly debitage and less frequently simple cores, retouched pieces, and percussors. Well-rounded spheroids and retouched flakes are rare. Identifiable taxa, Ceratotherium cf. simum (white rhinoceros) and Equus cf. oldowayensis (extinct zebra), accord with nearby open savanna grasslands, inferred from C3 grass, mixed and/or alternating with C4 grass-dominated phytolith assemblages. Palms, sedges, and dicots were also identified from phytoliths. Diatoms and sponge spicules imply nearby freshwater. The assemblage accumulated at the toe of a Ngorongoro Volcano-sourced fan-delta apron of stacked debris flows, fluvials, and tuffs, preserving fossil tree stumps and wooded grassland phytoliths farther upfan. It formed after the climax of Ngorongoro volcanic activity during a Paleolake Olduvai lowstand and was then buried and preserved by lacustrine clays, marking the first of two lake transgressions, signifying wetter climates. Orbital precessional lake cycles were superposed upon multimillennial (∼4.9 kyr) lake fluctuations.

Bone tools from Beds II-IV, Olduvai Gorge, Tanzania, and implications for the origins and evolution of bone technology.

The advent of bone technology in Africa is often associated with behavioral modernity that began sometime in the Middle Stone Age. Yet, small numbers of bone tools are known from Early Pleistocene sites in East and South Africa, complicating our understanding of the evolutionary significance of osseous technologies. These early bone tools vary geographically, with those in South Africa indicating use in foraging activities such as termite extraction and those in East Africa intentionally shaped in a manner similar to lithic tool manufacture, leading some to infer multiple hominin species were responsible for bone technology in these regions, with Paranthropus robustus assumed to be the maker of South African bone tools and Homo erectus responsible for those in East Africa. Here, we present on an assemblage of 52 supposed bone tools primarily from Beds III and IV, Olduvai Gorge, Tanzania, that was excavated by Mary Leakey in the late 1960s and early 1970s, but was only partially published and was never studied in detail from a taphonomic perspective. The majority of the sites from which the tools were recovered were deposited when only H. erectus is known to have existed in the region, potentially allowing a direct link between this fossil hominin and bone technology. Our analysis confirms at least six bone tools in the assemblage, the majority of which are intentionally flaked large mammal bones. However, one of the tools is a preform of the oldest barbed bone point known to exist anywhere in the world and pushes back the initial appearance of this technology by 700 kyr.

A hidden treasure of the Lower Pleistocene at Olduvai Gorge, Tanzania: The Leakey HWK EE assemblage.

HWK EE is a little-known archaeological site from the top of Lower Bed II and the basal part of Middle Bed II, Olduvai Gorge, Tanzania. The site was originally excavated in the early 1970s by Mary Leakey, but the excavations and resulting lithic and fossil assemblages were never described. Here we report for the first time on the lithic and fossil assemblages that were recovered by Mary Leakey from the site. The lithic assemblage is one of the largest of any Oldowan site and is characterized by a core-and-flake technology with simple flaking techniques and minimal reduction of cores. Retouched flake frequencies and battered tools are higher than those reported for Olduvai Bed I and Lower Bed II assemblages, but flaking schemes are poorly organized. The fossil assemblage is well-preserved, taxonomically-rich, but dominated by bovids, and includes abundant feeding traces of both hominins and carnivores. Hominins are inferred to have broken the majority of limb bones at the site for access to marrow, while both carnivores and hominins likely had access to at least some flesh. HWK EE may represent one of the last Homo habilis sites at Olduvai Gorge, and is important to understanding the behavioral and cultural mechanisms that led to the emergence of the Acheulean and Homo erectus in the region.

The paleoecology of Pleistocene birds from Middle Bed II, at Olduvai Gorge, Tanzania, and the environmental context of the Oldowan-Acheulean transition.

Fossil bird data (community composition and taphonomic profiles) are used here to infer the environmental context of the Oldowan-Acheulean transitional period at Olduvai Gorge, Tanzania. This is the first comprehensive report on the Middle Bed II avifauna and includes fossils excavated by the Olduvai Geochronology and Archaeology Project (OGAP) and recently rediscovered fossils collected by Mary Leakey. Crane, ibis, darter, owl, raptor, crow, and vulture are reported from Bed II for the first time. The presence of these taxa, absent earlier in this Bed, point to a general opening and drying of the landscape with grassland and open woodland expansion. Taxa associated with dense, emergent wetland vegetation, such as dabbling ducks and rails, are uncommon and less diverse than earlier in Bed II. This suggests more mature wetlands with clearer waters. Cormorants continue to be common, but are less diverse. Cormorants and other roosting taxa provide evidence of trees in the area. Compared to lowermost Bed II, the Middle to Upper Bed II landscape is interpreted here as more open and drier (but not necessarily more arid), with matured wetlands, scattered trees, and a greater expansion of grasslands.

Large mammal diets and paleoecology across the Oldowan-Acheulean transition at Olduvai Gorge, Tanzania from stable isotope and tooth wear analyses.

The well-dated Pleistocene sediments at Olduvai Gorge have yielded a rich record of hominin fossils, stone tools, and vertebrate faunal remains that, taken together, provide insight to hominin behavior and paleoecology. Since 2008, the Olduvai Geochronology and Archaeology Project (OGAP) has undertaken extensive excavations in Bed II that have yielded a large collection of early Pleistocene stone tools and fossils. The strata of Lower, Middle and Upper Bed II at Olduvai Gorge capture the critical transition from Oldowan to Acheulean technology and therefore provide an opportunity to explore the possible role of biotic and abiotic change during the transition. Here, we analyze newly discovered and existing fossil teeth from Bed II sites using stable isotope and tooth wear methods to investigate the diets of large mammals. We reconstruct the dietary ecology of Bed II mammals and evaluate whether vegetation or hydroclimate shifts are associated with the technological change. Combined isotope and tooth wear data suggest most mammals were C4 grazers or mixed feeders. Carbon isotope data from bulk enamel samples indicate that a large majority of Bed II large mammals analyzed had diets comprising mostly C4 vegetation (>75% of diet), whereas only a small number of individuals had either mixed C3-C4 or mostly C3 diets (<25% C4). Mesowear generally indicates an increase of the abrasiveness of the diet between intervals IIA and IIB (∼1.66 Ma), probably reflecting increased grazing. Microwear indicates more abrasive diets in interval IIA suggesting stronger seasonal differences at the time of death during this interval. This is also supported by the intratooth isotope profiles from Equus oldowayensis molars, which suggest a possible decrease in seasonality across the transition. Neither stable isotope nor tooth wear analyses indicate major vegetation or hydrological change across the Oldowan-Acheulean transition.

Dietary traits of the ungulates from the HWK EE site at Olduvai Gorge (Tanzania): Diachronic changes and seasonality.

The Oldowan site HWK EE (Olduvai Gorge, Tanzania) has yielded a large fossil and stone tool assemblage at the transition from Lower to Middle Bed II, ∼1.7 Ma. Integrated tooth wear and stable isotope analyses were performed on the three most abundant ungulate taxa from HWK EE, namely Alcelaphini, cf. Antidorcas recki (Antilopini) and Equus oldowayensis (Equini), to infer dietary traits in each taxon. Some paleodietary changes were observed for cf. A. recki and E. oldowayensis based on tooth wear at the transition from the Lemuta to the Lower Augitic Sandstone (LAS) interval within the HWK EE sequence. Stable carbon and oxygen isotope data show no significant changes in bulk diet or hydroclimate between the Lemuta and LAS intervals. The combined tooth wear and stable isotope data suggest similar paleoecological conditions across the two HWK EE intervals, but that differences in vegetation consumed among ungulates may have resulted in changes in dietary niches. Integrating tooth wear and stable isotope analyses permits the characterization of ungulate diets and habitats at HWK EE where C4 dominated and minor mixed C3 and C4 habitats were present. Our results provide a better understanding of the paleoenvironmental conditions of the Lemuta and LAS intervals. The LAS assemblage was mostly accumulated during relatively dry periods at Olduvai Gorge when grasses were not as readily available and grazing animals may have been more nutritionally-stressed than during the formation of the Lemuta assemblage. This helps to contextualize variations in hominin and carnivore feeding behavior observed from the faunal assemblages produced during the two main occupations of the site.

The carnivorous feeding behavior of early Homo at HWK EE, Bed II, Olduvai Gorge, Tanzania.

The regular consumption of large mammal carcasses, as evidenced by butchery marks on fossils recovered from Early Stone Age archaeological sites, roughly coincides with the appearance of Homo habilis. However, the significance of this niche expansion cannot be appreciated without an understanding of hominin feeding behavior and their ecological interactions with mammalian carnivores. The Olduvai Geochronology and Archaeology Project (OGAP) has recovered a large and well-preserved fossil assemblage from the HWK EE site, which was deposited just prior to the first appearance of Acheulean technology at Olduvai Gorge and likely represents one of the last H. habilis sites at Olduvai. This taphonomic analysis of the larger mammal fossil assemblage excavated from HWK EE shows evidence of multiple occupations over a long period of time, suggesting the site offered resources that were attractive to hominins. There was a water source indicated by the presence of fish, crocodiles, and hippos, and there was possible tree cover in an otherwise open habitat. The site preserves several stratigraphic intervals with large fossil and artifact assemblages within two of these intervals. Feeding traces on bone surfaces suggest hominins at the site obtained substantial amounts of flesh and marrow, particularly from smaller size group 1-2 carcasses, and exploited a wide range of taxa, including megafauna. A strong carnivore signal suggests hominins scavenged much of their animal foods during the two main stratigraphic intervals. In the later interval, lower carnivore tooth mark and hammerstone percussion mark frequencies, in addition to high epiphyseal to shaft fragment ratios, suggest hominins and carnivores did not fully exploit bone marrow and grease, which may have been acquired from nutritionally-stressed animals that died during a dry period at Olduvai. The diversity of fauna that preserve evidence of butchery suggests that the HWK EE hominins were opportunistic in their acquisition of carcass foods.

The contexts and early Acheulean archaeology of the EF-HR paleo-landscape (Olduvai Gorge, Tanzania).

Renewed fieldwork at the early Acheulean site of EF-HR (Olduvai Gorge, Tanzania) has included detailed stratigraphic studies of the sequence, extended excavations in the main site, and has placed eleven additional trenches within an area of nearly 1 km2, to sample the same stratigraphic interval as in the main trench across the broader paleo-landscape. Our new stratigraphic work suggests that EF-HR is positioned higher in the Bed II sequence than previously proposed, which has implications for the age of the site and its stratigraphic correlation to other Olduvai Middle Bed II sites. Geological research shows that the main EF-HR site was situated at the deepest part of an incised valley formed through river erosion. Archaeological excavations at the main site and nearby trenches have unearthed a large new assemblage, with more than 3000 fossils and artefacts, including a hundred handaxes in stratigraphic position. In addition, our test-trenching approach has detected conspicuous differences in the density of artefacts across the landscape, with a large cluster of archaeological material in and around the main trench, and less intense human activity at the same level in the more distant satellite trenches. All of these aspects are discussed in this paper in the light of site formation processes, behavioral contexts, and their implications for our understanding of the early Acheulean at Olduvai Gorge.

Paleoecology of the Serengeti during the Oldowan-Acheulean transition at Olduvai Gorge, Tanzania: The mammal and fish evidence.

Eight years of excavation work by the Olduvai Geochronology and Archaeology Project (OGAP) has produced a rich vertebrate fauna from several sites within Bed II, Olduvai Gorge, Tanzania. Study of these as well as recently re-organized collections from Mary Leakey's 1972 HWK EE excavations here provides a synthetic view of the faunal community of Olduvai during Middle Bed II at ∼1.7-1.4 Ma, an interval that captures the local transition from Oldowan to Acheulean technology. We expand the faunal list for this interval, name a new bovid species, clarify the evolution of several mammalian lineages, and record new local first and last appearances. Compositions of the fish and large mammal assemblages support previous indications for the dominance of open and seasonal grassland habitats at the margins of an alkaline lake. Fish diversity is low and dominated by cichlids, which indicates strongly saline conditions. The taphonomy of the fish assemblages supports reconstructions of fluctuating lake levels with mass die-offs in evaporating pools. The mammals are dominated by grazing bovids and equids. Habitats remained consistently dry and open throughout the entire Bed II sequence, with no major turnover or paleoecological changes taking place. Rather, wooded and wet habitats had already given way to drier and more open habitats by the top of Bed I, at 1.85-1.80 Ma. This ecological change is close to the age of the Oldowan-Acheulean transition in Kenya and Ethiopia, but precedes the local transition in Middle Bed II. The Middle Bed II large mammal community is much richer in species and includes a much larger number of large-bodied species (>300 kg) than the modern Serengeti. This reflects the severity of Pleistocene extinctions on African large mammals, with the loss of large species fitting a pattern typical of defaunation or 'downsizing' by human disturbance. However, trophic network (food web) analyses show that the Middle Bed II community was robust, and comparisons with the Serengeti community indicate that the fundamental structure of food webs remained intact despite Pleistocene extinctions. The presence of a generalized meat-eating hominin in the Middle Bed II community would have increased competition among carnivores and vulnerability among herbivores, but the high generality and interconnectedness of the Middle Bed II food web suggests this community was buffered against extinctions caused by trophic interactions.

New excavations at the HWK EE site: Archaeology, paleoenvironment and site formation processes during late Oldowan times at Olduvai Gorge, Tanzania.

This paper reports the results of renewed fieldwork at the HWK EE site (Olduvai Gorge, Tanzania). HWK EE is positioned across the boundary between Lower and Middle Bed II, a crucial interval for studying the emergence of the Acheulean at Olduvai Gorge. Our excavations at HWK EE have produced one of the largest collections of fossils and artefacts from any Oldowan site, distributed across several archaeological units and a large excavation surface in four separate trenches that can be stratigraphically correlated. Here we present the main stratigraphic and archaeological units and discuss site formation processes. Results show a great density of fossils and stone tools vertically through two stratigraphic intervals (Lemuta and Lower Augitic Sandstone) and laterally across an area of around 300 m2, and highlight the confluence of biotic and abiotic agents in the formation of the assemblage. The large size and diversity of the assemblage, as well as its good preservation, qualify HWK EE as a reference site for the study of the late Oldowan at Olduvai Gorge and elsewhere in Africa. In addition, the description of the stratigraphic and archaeological sequence of HWK EE presented in this paper constitutes the foundation for further studies on hominin behavior and paleoecology in Lower and Middle Bed II.

A new high-resolution 3-D quantitative method for identifying bone surface modifications with implications for the Early Stone Age archaeological record.

Bone surface modifications have become important indicators of hominin behavior and ecology at prehistoric archaeological sites. However, the method by which we identify and interpret these marks remains largely unchanged despite decades of research, relying on qualitative criteria and lacking standardization between analysts. Recently, zooarchaeologists have begun using new technologies capable of capturing 3-D data from bone surface modifications to advance our knowledge of these informative traces. However, an important step in this research has been overlooked and after years of work, we lack both a universal and replicable protocol and an understanding of the precision of these techniques. Here we propose a new standard for identifying bone surface modifications using high-resolution 3-D data and offer a systematic and replicable approach for researchers to follow. Data were collected with a white-light non-contact confocal profilometer and analyzed with Digital Surf's Mountains® software. Our data show that when methods are standardized, results between researchers are statistically indistinguishable. Multivariate analyses using the measured parameters allow discrimination between stone tool cut marks and mammalian carnivore tooth marks with 97.5% accuracy. Application of this method to fossil specimens resulted in 100% correspondence with identifications made by an experienced analyst using macroscopic observations of qualitative features of bone surface modifications. High-resolution 3-D analyses of bone surface modifications have great potential to improve the reliability and accuracy of taphonomic research, but only if our methods are replicable and precise.

In situ ∼2.0 Ma trees discovered as fossil rooted stumps, lowermost Bed I, Olduvai Gorge, Tanzania.

The discovery of fossil rooted tree stumps in lowermost Lower Bed I from the western Olduvai Basin, Tanzania, age-bracketed by the Naabi Ignimbrite (2.038 ± 0.005 Ma) and Tuff IA (1.88 ± 0.05 Ma), provides the first direct, in situ, and to date oldest evidence of living trees at Olduvai Gorge. The tree relicts occur in an interval dominated by low-viscosity mass flow and braided fluvial sediments, deposited at the toe of a largely Ngorongoro Volcano-sourced volcaniclastic fan apron that comprised a widely spaced network of ephemeral braided streams draining northward into the Olduvai Basin. Preservation of the trees occurred through their engulfment by mass flows, post-mortem mold formation resulting from differential decay of woody tissues, and subsequent fluvially-related sediment infill, calcite precipitation, and cast formation. Rhizolith preservation was triggered by the interaction of root-induced organic and inorganic processes to form rhizocretionary calcareous root casts. Phytolith analyses were carried out to complete the paleoenvironmental reconstruction. They imply a pronounced seasonality and indicate a wooded landscape with grasses, shrubs, and sedges growing nearby, comparable to the low, open riverine woodland (unit 4c) along the Garusi River and tributaries in the Laetoli area. Among the tree stump cluster were found outsized lithic clasts and those consisting of quartzite were identified as Oldowan stone tool artifacts. In the context of hominin activity, the identification of wooded grassland in association with nearby freshwater drainages and Oldowan artifacts significantly extends our paleoenvironmental purview on the basal parts of Lower Bed I, and highlights the hitherto underrated role of the yet poorly explored western Olduvai Gorge area as a potential ecologically attractive setting and habitat for early hominins.

The larger mammal fossil assemblage from JK2, Bed III, Olduvai Gorge, Tanzania: implications for the feeding behavior of Homo erectus.

Little is known about the type and amount of animal proteins consumed by Homo erectus, a species distinguished from its predecessors by more human-like brain and body proportions and its association with more advanced stone tool technology. Here I present an interpretation of the feeding behavior of African H. erectus based upon the first taphonomic analysis of the larger mammal fossil assemblage from the JK2 site, Bed III, Olduvai Gorge. Results indicate that both hominins and carnivores consumed some flesh and bone marrow at the site. A low incidence of percussion marking suggests hominins did not break all long bones in the assemblage. Relatively high carnivore tooth mark frequencies and low cut mark frequencies independently suggest that both hominins and carnivores had access to flesh, while specimens that are both tooth- and butchery-marked demonstrate occasional hominin and carnivore feeding from the same carcass. Together, the bone surface modification data suggest a mixed and possibly time-averaged taphonomic history for the assemblage with at least some carcasses accessed by hominins early in the consumption sequence and others only by carnivores. The results for the JK2 assemblage contribute to a growing literature concerning the feeding behavior of African H. erectus, a species that appears to have relied on carcass foods to meet some of the nutritional demands of its larger brain and body size.

Early Pleistocene cut marked hominin fossil from Koobi Fora, Kenya.

Identification of butchery marks on hominin fossils from the early Pleistocene is rare. Our taphonomic investigation of published hominin fossils from the Turkana region of Kenya revealed likely cut marks on KNM-ER 741, a ~ 1.45 Ma proximal hominin left tibia shaft found in the Okote Member of the Koobi Fora Formation. An impression of the marks was created with dental molding material and scanned with a Nanovea white-light confocal profilometer, and the resulting 3-D models were measured and compared with an actualistic database of 898 individual tooth, butchery, and trample marks created through controlled experiments. This comparison confirms the presence of multiple ancient cut marks that are consistent with those produced experimentally. These are to our knowledge the first (and to date only) cut marks identified on an early Pleistocene postcranial hominin fossil.

Validation of bone surface modification models for inferring fossil hominin and carnivore feeding interactions, with reapplication to FLK 22, Olduvai Gorge, Tanzania.

Resolving the issue of how Early Stone Age hominins acquired large mammal carcasses requires information on their feeding interactions with large carnivores. This ecological information and its behavioral and evolutionary implications are revealed most directly from the tooth, cut, and percussion marks on bone surfaces generated by hominin and carnivore feeding activities. This paper employs a bootstrap method, a form of random resampling with replacement, to refine published neotaphonomic models that use the assemblage-wide proportions of long bones bearing feeding traces to infer the sequences in which Plio-Pleistocene hominins and carnivores accessed flesh, marrow, and/or grease from carcasses. Results validate the sensitivity of the models for inferring hominin feeding ecology, which have been questioned on grounds shown here to be unfounded. The bootstrapped feeding trace models are applied to the late Pliocene larger mammal fossil assemblage from FLK 22 (Zinjanthropus site), Olduvai Gorge, Tanzania. High frequencies of tooth and percussion marking on long bone midshaft fragments from FLK 22 are most consistent with those feeding trace models that simulate hominin scavenging from carcasses defleshed by carnivores, while cut mark data indicate that hominins more often had access to upper forelimb flesh than upper hind limb flesh. Together, the bone surface modification data indicate that hominins typically gained secondary access to partially defleshed carnivore kills, but they also allow for the possibility of some carcasses being processed only by carnivores and only by hominins.

Cut marks on bone surfaces: influences on variation in the form of traces of ancient behaviour.

Although we know that our lineage has been producing sharp-edged tools for over 2.6 Myr, our knowledge of what they were doing with these tools is far less complete. Studies of these sharp-edged stone tools show that they were most probably used as cutting implements. However, the only substantial evidence of this is the presence of cut marks on the bones of animals found in association with stone tools in ancient deposits. Numerous studies have aimed to quantify the frequency and placement of these marks. At present there is little consensus on the meaning of these marks and how the frequency relates to specific behaviours in the past. Here we investigate the possibility that mechanical properties associated with edges of stone tools as well as the properties of bones themselves may contribute to the overall morphology of these marks and ultimately their placement in the archaeological record. Standardized tests of rock mechanics (Young's modulus and Vickers hardness) indicate that the hardness of tool edges significantly affects cut-mark morphology. In addition, we show that indentation hardness of bones also impacts the overall morphology of cut marks. Our results show that rock type and bone portions influence the shape and prevalence of cut marks on animal bones.