Bovids as indicators of Plio-Pleistocene paleoenvironments in east Africa

The environmental context of human evolutionary history in Eurasia and Africa

This review has three main aims: (1) to make specific predictions about the habitat of the hypothetical last common ancestor of the chimpanzee/bonobo-human clade; (2) to outline the major trends in environments between 8-6 Ma and the late Pleistocene; and (3) to pinpoint when, and in some cases where, human ancestors evolved to cope with the wide range of habitats they presently tolerate. Several lines of evidence indicate that arboreal environments, particularly woodlands, were important habitats for late Miocene hominids and hominins, and therefore possibly for the last common ancestor of the chimpanzee/bonobo-human clade. However, as there is no clear candidate for this last common ancestor, and because the sampling of fossils and past environments is inevitably patchy, this prediction remains a working hypothesis at best. Nonetheless, as a primate, it is expected that the last common ancestor was ecologically dependent on trees in some form. Understanding past environments is important, as palaeoenvironmental reconstructions provide the context for human morphological and behavioural evolution. Indeed, the impact of climate on the evolutionary history of our species has long been debated. Since the mid-Miocene, the Earth has been experiencing a general cooling trend accompanied by aridification, which intensified during the later Pliocene and Pleistocene. Numerous climatic fluctuations, as well as local, regional and continental geography that influenced weather patterns and vegetation, created hominin environments that were dynamic in space and time. Behavioural flexibility and cultural complexity were crucial aspects of hominin expansion into diverse environments during the Pleistocene, but the ability to exploit varied and varying habitats was established much earlier in human evolutionary history. The development of increasingly complex tool technology facilitated re-expansion into tropical forests. These environments are difficult for obligate bipeds to negotiate, but their exploitation was accomplished by archaic and/or anatomically modern humans independently in Africa and south-east Asia. Complex social behaviour and material culture also allowed modern humans to reach some of the most hostile regions of the globe, above the Arctic Circle, by the late Pleistocene. This, with colonization of the Americas and Australasia, established Homo sapiens as a truly cosmopolitan species.

New evidence for hominin carcass processing strategies at 1.5 Ma, Koobi Fora, Kenya

Reconstruction of early Pleistocene hominin carcass acquisition and processing behaviors are necessarily based at least in part on butchered fossil bones. This paper provides zooarchaeological and taphonomic analyses and behavioral interpretations of three approximately 1.5 million-year-old archaeofaunas from areas 1A and 103 in the Okote Member of the Koobi Fora Formation, northern Kenya: FwJj14A, FwJj14B, and GaJi14. These sites are all located in similar paleoenvironmental contexts, near shallow water with swampy, seasonally flooded areas, and some evidence for more wooded or gallery forest settings. Both individual specimen--and assemblage-level analyses of butchery-marked bones indicate that the hominins appear to have practiced similar butchery strategies at all of these sites, with butchery (defleshing, disarticulation, and marrow extraction) of both high- and low-ranked skeletal elements with no apparent preference for prey size, skeletal region, limb class, or limb portion. Only four tooth-marked specimens, including one likely crocodile-tooth-marked bone, are preserved in all three archaeofaunas. A paucity of limb epiphyses suggests that bone-crunching hyenids may have deleted these portions subsequent to hominin butchery. Strangely, there are no stone tools preserved with the 292 cut-marked and 27 percussion-marked faunal specimens (out of a total of 6,039 specimens), suggesting that raw material availability may have conditioned hominin lithic discard patterns at these locales. These assemblages increase our knowledge of the dietary behavior and ecology of Homo erectus, and provide support for variability in early Pleistocene hominin carcass foraging patterns.

Flaked stones and old bones: biological and cultural evolution at the dawn of technology

The appearance of Oldowan sites ca. 2.6 million years ago (Ma) may reflect one of the most important adaptive shifts in human evolution. Stone artifact manufacture, large mammal butchery, and novel transport and discard behaviors led to the accumulation of the first recognized archaeological debris. The appearance of the Oldowan sites coincides with generally cooler, drier, and more variable climatic conditions across Africa, probably resulting in a net decrease in woodland foods and an increase in large mammal biomass compared to the early and middle Pliocene. Shifts in plant food resource availability may have provided the stimulus for incorporating new foods into the diet, including meat from scavenged carcasses butchered with stone tools. Oldowan artifact form varies with clast size, shape, raw material physical properties, and flaking intensity. Oldowan hominins preferred hard raw materials with good fracture characteristics. Habitual stone transport is evident from technological analysis, and raw material sourcing to date suggests that stone was rarely moved more than 2-3 km from source. Oldowan debris accumulation was spatially redundant, reflecting recurrent visitation of attractive points on the landscape. Thin archaeological horizons from Bed I Olduvai Gorge, Tanzania, were probably formed and buried in less than 10 years and document hominin processing of multiple carcasses per year. Transport beyond simple refuging behavior is suggested by faunal density at some site levels. By 2.0 Ma, hominin rank within the predatory guild may have been moderately high, as they probably accessed meaty carcasses through hunting and confrontational scavenging, and hominin-carnivore competition appears minimal at some sites. It is likely that both Homo habilis sensu stricto and early African H. erectus made Oldowan tools. H. habilis sensu stricto was more encephalized than Australopithecus and may foreshadow H. erectus in lower limb elongation and some thermoregulatory adaptations to hot, dry climatic conditions. H. erectus was large and wide-ranging, had a high total energy expenditure, and required a high-quality diet. Reconstruction of H. erectus reproductive energetics and socioeconomic organization suggests that reproductively active females received assistance from other group members. This inference, combined with archaeological evidence for acquisition of meaty carcasses, suggests that meat would have been a shared food. This is indirectly confirmed by nutritional analysis suggesting that the combination of meat and nutritionally dense plant foods was the likely diet fueling body size increase and encephelization in Homo. Most discussion of Oldowan hominin behavior and ecology, including that presented here, is based on materials from a few sites. There is a critical need to analyze additional large, primary-context lithic and faunal assemblages to better assess temporal, geographic, and environmental variability in Oldowan behavior.

Stable isotopes of pedogenic carbonates as indicators of paleoecology in the Plio-Pleistocene (upper Bed I), western margin of the Olduvai Basin, Tanzania

Paleosol carbonates from trenches excavated as part of a landscape-scale project in Bed I of Olduvai Gorge, Tanzania, were analyzed for stable carbon and oxygen isotopic composition. The approximately 60,000-year interval ( approximately 1.845-1.785 Ma) above Tuff IB records evidence for lake and fluvial sequences, volcanic eruptions, eolian and pedogenic processes, and the development of a fluvial plain in the western margin of the basin. Significant temporal variation in the carbonate delta(18)O values records variation of local precipitation and supports the shifts in climatic conditions interpreted from the lithologic record. During this period, carbonate delta(13)C values varied between depositional facies indicating that the paleolandscape supported a local biomass of about 40-60% C(4) plants within a mosaic of grassy woodlands and wooded grasslands. The lithologic and stable isotope record in this small lake basin indicates the area was much wetter, with more woody C(3) plants, during this interval than is the semi-arid area today. The record also reflects the variation in climatic conditions (wet/dry) documented by other global climate proxies for this time.

Savanna Chimpanzees, Pan troglodytes verus, Hunt with Tools

Although tool use is known to occur in species ranging from naked mole rats [1] to owls [2], chimpanzees are the most accomplished tool users [3-5]. The modification and use of tools during hunting, however, is still considered to be a uniquely human trait among primates. Here, we report the first account of habitual tool use during vertebrate hunting by nonhumans. At the Fongoli site in Senegal, we observed ten different chimpanzees use tools to hunt prosimian prey in 22 bouts. This includes immature chimpanzees and females, members of age-sex classes not normally characterized by extensive hunting behavior. Chimpanzees made 26 different tools, and we were able to recover and analyze 12 of these. Tool construction entailed up to five steps, including trimming the tool tip to a point. Tools were used in the manner of a spear, rather than a probe or rousing tool. This new information on chimpanzee tool use has important implications for the evolution of tool use and construction for hunting in the earliest hominids, especially given our observations that females and immature chimpanzees exhibited this behavior more frequently than adult males.

Bovid postcranial ecomorphological survey of the Laetoli paleoenvironment

Here we report on a bovid postcranial ecomorphological survey of the fossil assemblages from the Plio-Pleistocene site of Laetoli, Tanzania. A global sample of extant bovids (n=205), cervids (n=14), and tragulids (n=5) from seven known habitat types constitutes the comparative data set. All long bones, carpals, tarsals, and phalanges were measured. Discriminant function analyses (DFA) were conducted in order to evaluate the ability of each element to accurately predict habitat affiliation. The baseline of chance accuracy for DFAs (i.e., the percentage of correct predictions that can be expected when habitat assignments are randomized) served as the cut-off point between good and bad habitat predictors. A total of 22 elements yielded percentages of correct classification over the baseline of accuracy, and these were extended to the Laetoli fossil assemblages. Summaries of the number of specimens predicted to belong to each habitat type were used to reconstruct the paleoenvironment. The results indicate that, at the time of the deposition of the Laetolil Beds, the area had heavy woodland-bushland cover with some lighter tree and bush cover and grass available. These results lend strong support to recent suggestions that the area was on the more wooded end of the habitat spectrum, contra initial conclusions that it represented a mosaic of more open habitats. The results also indicate that, during the deposition of the Ndolanya Beds, the environment had become more open and the grassland component of the environment had increased significantly. Light woodland-bushland and an abundance of grass cover dominated the landscape, although tracts of land with denser vegetation likely existed. This conclusion agrees with earlier suggestions that the area was a semiarid bushland.

Taphonomic bias, taxonomic bias and historical non-equivalence of faunal structure in early hominin localities

Environmental interpretation of fossil assemblages requires an accurate reconstruction of the community from which the assemblage was derived, which in turn depends on the quality of a comparative model usually based on the study of modern equivalents. The degree of inaccuracy introduced by taphonomic and other types of bias is often difficult to assess and the suitability of comparative models has rarely been addressed in this light. Here we apply a recently developed method to assess the bias present in a range of key hominin bearing localities from the Neogene of East and South Africa. The ecological structure of several of the investigated faunas can be shown to depart substantially from that of a comprehensive range of modern comparative faunas. Bias, where present, affects primarily the small mammals, which tend to be under-represented, and the large primary consumers, which tend to be over-represented. This has potentially significant implications for past and future palaeoecological reconstruction of these localities as numerous methods that are currently in use rely extensively on either the small mammals or the large primary consumers, and in particular the bovids. Understanding the nature of the bias, when present, will go some way towards improving the quality of environmental reconstructions.

Influence of Plio-Pleistocene aridification on human evolution: evidence from paleosols of the Turkana Basin, Kenya

New stable carbon isotope measurements, coupled with paleoprecipitation estimates, both from Plio-Pleistocene paleosols of the Turkana Basin, Kenya, provide a high-resolution record of aridification and increasing C4 biomass during the past 4.3 Ma. This aridification trend is marked by several punctuations at 3.58-3.35, 2.52-2, and 1.81-1.58 Ma, during which the running mean and variance of delta13C and paleoaridity estimates increase, suggesting that the proportion of C4 biomass increases in savanna mosaics during periods of heightened aridity. Increase in C4 biomass during these aridification events not only increases the proportion of open habitats, but increases the spatial neg-entropy, or heterogeneity of the ecosystem. The aridification events identified correspond to intervals of increased turnover, but more importantly, increased diversity of bovids. Although the record of hominins from the Turkana Basin lacks the temporal resolution and diversity of the bovid record, the aridification intervals identified are marked by similar increases in the diversity and turnover of hominins. These results support the hypothesis that hominins evolved in savanna mosaics that changed through time, and suggest that the evolution of bovids and hominins was driven by shifts in climatic instability and habitat variability, both diachronic and synchronic.

Partial cranium ofCercopithecoides kimeui Leakey, 1982 from Rawi Gully, southwestern Kenya

The Rawi Gully, located on the Homa Peninsula in southwestern Kenya, has produced several fossil elements of a large cercopithecid from sediments approximately 2.5 million years old (Ma). Nearly all of these elements appear to represent a single adult male individual of the colobine species Cercopithecoides kimeui Leakey, 1982. Part of the face, mandible, dentition, and several small postcranial fragments were collected by the Homa Peninsula Paleoanthropological Project (HPPP) in 1994 and 1995. This individual also appears to be represented by material collected in two previous expeditions to the site, one led by David Pilbeam in the 1970s and an earlier expedition led by L.S.B. Leakey in 1933. This specimen may extend the first appearance of C. kimeui by approximately 500 Kyr, and provides the first evidence for much of the male facial morphology in this species. Furthermore, Rawi may represent a more wooded habitat than the other occurrences of C. kimeui at Olduvai Gorge, Tanzania, and Koobi Fora, Kenya, indicating that C. kimeui may have been relatively flexible in its habitat preferences.

Examining time trends in the Oldowan technology at Beds I and II, Olduvai Gorge

The lithic analysis of the Bed I and II assemblages from Olduvai Gorge reveals both static and dynamic time trends in early hominids' technology from 1.8 to 1.2 m.y.a. The Bed I Oldowan (1.87-1.75 m.y.a.) is characterized by the least effort strategy in terms of raw material exploitation and tool production. The inclusion of new raw material, chert, for toolmaking in the following Developed Oldowan A (DOA, 1.65-1.53 m.y.a.) facilitated more distinctive and variable flaking strategies depending on the kind of raw materials. The unique characters of DOA are explainable by this raw material factor, rather than technological development of hominids. The disappearance of chert in the subsequent Developed Oldowan B and Acheulian (1.53-1.2 m.y.a.) necessitated a shift in tool production strategy more similar to that of Bed I Oldowan than DOA. However, the evidence suggests that Bed II hominids might have been more skillful toolmakers, intensive tool-users, and engaged in more active transport of stone tools than the Bed I predecessors. Koobi Fora hominids maintained a more static tool-using behavior than their Olduvai counterparts due mainly to a stable supply of raw materials. They differed from Olduvai hominids in terms of less battering of cores, consistent transport behavior, and few productions of side-struck flakes, indicating a regional variation of toolmaking and using practice. However, they shared with Olduvai hominids a temporal trend toward the production of larger flakes from larger cores after 1.6 m.y.a. Increased intake of animal resources and the expansion of ranging area of Homo ergaster would have led to the development of technological organization. Technological changes in the Oldowan industry are attested at Olduvai Gorge, Koobi Fora, and Sterkfontein, suggesting that it was a pan-African synchronous phenomenon, beginning at 1.5 m.y.a.

Paleosols, stable carbon isotopes, and paleoenvironmental interpretation of Kanapoi, Northern Kenya

This study uses the interpretation of paleosol features at Kanapoi, Kenya (4.2-3.4 Ma) to reconstruct the ecosystem occupied by Australopithecus anamensis. The paleosols at Kanapoi provide a unique and fortuitous opportunity, in that the bulk of the hominid specimens derive from paleosols, providing direct evidence of the environment that the Kanapoi hominids occupied. Seven named types of paleosols are recognized at Kanapoi, each representing a trace fossil of the local ecosystem during soil formation. The hominid-bearing Dite paleosols provide evidence that A. anamensis inhabited areas of semi-arid, seasonal climate regimes with mean annual precipitation ranging from about 350-600 mm. The in situ hominid collections from Dite paleosols show that A. anamensis at least occasionally occupied relatively open low tree-shrub savanna vegetation formed in well drained settings, and may have preferred these conditions over other poorly drained soils. The relatively open conditions of Dite paleosols existed within a spatially variable ecosystem, characterized by a mosaic of environments, ranging from forb-dominated edaphic grassland to gallery woodland, providing a larger view of the mixed ecosystem in which A. anamensis lived. Synthesis of paleoenvironmental indicators of A. anamensis at Kanapoi and Allia Bay, Kenya suggests that as early as 4 Ma hominids thrived in varied ecosystems.

Environmental hypotheses of hominin evolution

The study of human evolution has long sought to explain major adaptations and trends that led to the origin of Homo sapiens. Environmental scenarios have played a pivotal role in this endeavor. They represent statements or, more commonly, assumptions concerning the adaptive context in which key hominin traits emerged. In many cases, however, these scenarios are based on very little if any data about the past settings in which early hominins lived. Several environmental hypotheses of human evolution are presented in this paper. Explicit test expectations are laid out, and a preliminary assessment of the hypotheses is made by examining the environmental records of Olduvai, Turkana, Olorgesailie, Zhoukoudian, Combe Grenal, and other hominin localities. Habitat-specific hypotheses have prevailed in almost all previous accounts of human adaptive history. The rise of African dry savanna is often cited as the critical event behind the development of terrestrial bipedality, stone toolmaking, and encephalized brains, among other traits. This savanna hypothesis has been countered recently by the woodland/forest hypothesis, which claims that Pliocene hominins had evolved in and were primarily attracted to closed habitats. The ideas that human evolution was fostered by cold habitats in higher latitudes or by seasonal variations in tropical and temperate zones also have their proponents. An alternative view, the variability selection hypothesis, states that large disparities in environmental conditions were responsible for important episodes of adaptive evolution. The resulting adaptations enhanced behavioral versatility and ultimately ecological diversity in the human lineage. Global environmental records for the late Cenozoic and specific records at hominin sites show the following: 1) early human habitats were subject to large-scale remodeling over time; 2) the evidence for environmental instability does not support habitat-specific explanations of key adaptive changes; 3) the range of environmental change over time was more extensive and the tempo far more prolonged than allowed by the seasonality hypothesis; and 4) the variability selection hypothesis is strongly supported by the persistence of hominins through long sequences of environmental remodeling and the origin of important adaptations in periods of wide habitat diversity. Early bipedality, stone transport, diversification of artifact contexts, encephalization, and enhanced cognitive and social functioning all may reflect adaptations to environmental novelty and highly varying selective contexts.