Paleoenvironments represented by the sediments of the Early Pliocene Mursi Formation, Omo Valley, Ethiopia.

While our understanding of human origins has been enriched by extensive efforts to reconstruct the ancient environmental context of early hominins using information from hominin-bearing localities, comparatively little effort has been focused on contemporaneous fossil localities with abundant vertebrate fossils, but lacking hominins. We report here on new paleoenvironmental reconstructions of the Mursi Formation, Ethiopia, from which strata dated to >4 Ma preserve an abundant vertebrate fossil record lacking any known hominins, despite being part of a contiguous sedimentary basin known for its rich hominin fossil record. We combine new stratigraphic and sedimentological observations with data from paleosols preserved in the sedimentary sequence, along with isotopic data from pedogenic carbonate, paleosol organic matter, and sulfur minerals preserved in the sediments (gypsum, native sulfur). Paleosol features and carbon isotopic composition of fossil organic matter and pedogenic carbonate complement data from the mammalian fauna, the sum of which provide evidence of closed woodland to forest vegetation. Sedimentological data indicate that these wooded terrestrial habitats occurred near aquatic settings characterized by stagnant shallow waters of a freshwater lake, providing a reconstruction of unique habitats in contrast with hominin localities >4 Ma.

The environments of Australopithecus anamensis at Allia Bay, Kenya: A multiproxy analysis of early Pliocene Bovidae.

Australopithecus anamensis, among the earliest fully bipedal hominin species, lived in eastern Africa around 4 Ma. Much of what is currently known about the paleoecology of A. anamensis comes from the type locality, Kanapoi, Kenya. Here, we extend knowledge of the range of environments occupied by A. anamensis by presenting the first multiproxy paleoecological analysis focusing on Bovidae excavated from another important locality where A. anamensis was recovered, locality 261-1 (ca. 3.97 Ma) at Allia Bay, East Turkana, Kenya. Paleoenvironments are reconstructed using astragalar ecomorphology, mesowear, hypsodonty index, and oxygen and carbon isotopes from dental enamel. We compare our results to those obtained from Kanapoi. Our results show that the bovid community composition is similar between the two fossil assemblages. Allia Bay and Kanapoi bovid astragalar ecomorphology spans the spectrum of modern morphologies indicative of grassland, woodland, and even forest-adapted forms. Dietary reconstructions based on stable isotopes, mesowear, and hypsodonty reveal that these bovids' diet encompassed the full C3 to C4 dietary spectrum and overlap in the two data sets. Our results allow us to confidently extend our reconstructions of the paleoenvironments of A. anamensis at Kanapoi to Allia Bay, where this pivotal hominin species is associated with heterogeneous settings including habitats with varying degrees of tree cover, including grasslands, bushlands, and woodlands.

Dietary trends in herbivores from the Shungura Formation, southwestern Ethiopia.

Diet provides critical information about the ecology and environment of herbivores. Hence, understanding the dietary strategies of fossil herbivores and the associated temporal changes is one aspect of inferring paleoenvironmental conditions. Here, we present carbon isotope data from more than 1,050 fossil teeth that record the dietary patterns of nine herbivore families in the late Pliocene and early Pleistocene (3.6 to 1.05 Ma) from the Shungura Formation, a hominin-bearing site in southwestern Ethiopia. An increasing trend toward C4 herbivory has been observed with attendant reductions in the proportions of browsers and mixed feeders through time. A high proportion of mixed feeders has been observed prior to 2.9 Ma followed by a decrease in the proportion of mixed feeders and an increase in grazers between 2.7 and 1.9 Ma, and a further increase in the proportion of grazers after 1.9 Ma. The collective herbivore fauna shows two major change points in carbon isotope values at ∼2.7 and ∼2.0 Ma. While hominin fossils from the sequence older than 2.7 Ma are attributed to Australopithecus, the shift at ∼2.7 Ma indicating the expansion of C4 grasses on the landscape was concurrent with the first appearance of Paranthropus The link between the increased C4 herbivory and more open landscapes suggests that Australopithecus lived in more wooded landscapes compared to later hominins such as Paranthropus and Homo, and has implications for key morphological and behavioral adaptations in our lineage.

Isotopic evidence for the timing of the dietary shift toward C4 foods in eastern African Paranthropus.

New approaches to the study of early hominin diets have refreshed interest in how and when our diets diverged from those of other African apes. A trend toward significant consumption of C4 foods in hominins after this divergence has emerged as a landmark event in human evolution, with direct evidence provided by stable carbon isotope studies. In this study, we report on detailed carbon isotopic evidence from the hominin fossil record of the Shungura and Usno Formations, Lower Omo Valley, Ethiopia, which elucidates the patterns of C4 dietary utilization in the robust hominin Paranthropus The results show that the most important shift toward C4 foods occurred at ∼2.37 Ma, within the temporal range of the earliest known member of the genus, Paranthropus aethiopicus, and that this shift was not unique to Paranthropus but occurred in all hominins from this fossil sequence. This uptake of C4 foods by hominins occurred during a period marked by an overall trend toward increased C4 grazing by cooccurring mammalian taxa from the same sequence. However, the timing and geographic patterns of hominin diets in this region differ from those observed elsewhere in the same basin, where environmental controls on the underlying availability of various food sources were likely quite different. These results highlight the complexities of dietary responses by hominins to changes in the availability of food resources.

Net landscape carbon balance of a tropical savanna: Relative importance of fire and aquatic export in offsetting terrestrial production.

The magnitude of the terrestrial carbon (C) sink may be overestimated globally due to the difficulty of accounting for all C losses across heterogeneous landscapes. More complete assessments of net landscape C balances (NLCB) are needed that integrate both emissions by fire and transfer to aquatic systems, two key loss pathways of terrestrial C. These pathways can be particularly significant in the wet-dry tropics, where fire plays a fundamental part in ecosystems and where intense rainfall and seasonal flooding can result in considerable aquatic C export (ΣFaq ). Here, we determined the NLCB of a lowland catchment (~140 km2 ) in tropical Australia over 2 years by evaluating net terrestrial productivity (NEP), fire-related C emissions and ΣFaq (comprising both downstream transport and gaseous evasion) for the two main landscape components, that is, savanna woodland and seasonal wetlands. We found that the catchment was a large C sink (NLCB 334 Mg C km-2  year-1 ), and that savanna and wetland areas contributed 84% and 16% to this sink, respectively. Annually, fire emissions (-56 Mg C km-2  year-1 ) and ΣFaq (-28 Mg C km-2  year-1 ) reduced NEP by 13% and 7%, respectively. Savanna burning shifted the catchment to a net C source for several months during the dry season, while ΣFaq significantly offset NEP during the wet season, with a disproportionate contribution by single major monsoonal events-up to 39% of annual ΣFaq was exported in one event. We hypothesize that wetter and hotter conditions in the wet-dry tropics in the future will increase ΣFaq and fire emissions, potentially further reducing the current C sink in the region. More long-term studies are needed to upscale this first NLCB estimate to less productive, yet hydrologically dynamic regions of the wet-dry tropics where our result indicating a significant C sink may not hold.

Land transformation in tropical savannas preferentially decomposes newly added biomass, whether C3 or C4 derived.

As tropical savannas are undergoing rapid conversion to other land uses, native C3 -C4 vegetation mixtures are often transformed to C3 - or C4 -dominant systems, resulting in poorly understood changes to the soil carbon (C) cycle. Conventional models of the soil C cycle are based on assumptions that more labile components of the heterogenous soil organic C (SOC) pool decompose at faster rates. Meanwhile, previous work has suggested that the C4 -derived component of SOC is more labile than C3 -derived SOC. Here we report on long-term (18 months) soil incubations from native and transformed tropical savannas of northern Australia. We test the hypothesis that, regardless of the type of land conversion, the C4 component of SOC will be preferentially decomposed. We measured changes in the SOC and pyrogenic carbon (PyC) pools, as well as the carbon isotope composition of SOC, PyC and respired CO2 , from 63 soil cores collected intact from different land use change scenarios. Our results show that land use change had no consistent effect on the size of the SOC pool, but strong effects on SOC decomposition rates, with slower decomposition rates at C4 -invaded sites. While we confirm that native savanna soils preferentially decomposed C4 -derived SOC, we also show that transformed savanna soils preferentially decomposed the newly added pool of labile SOC, regardless of whether it was C4 -derived (grass) or C3 -derived (forestry) biomass. Furthermore, we provide evidence that in these fire-prone landscapes, the nature of the PyC pool can shed light on past vegetation composition: while the PyC pool in C4 -dominant sites was mainly derived from C3 biomass, PyC in C3-dominant sites and native savannas was mainly derived from C4 biomass. We develop a framework to systematically assess the effects of recent land use change vs. prior vegetation composition.

Fossils from Mille-Logya, Afar, Ethiopia, elucidate the link between Pliocene environmental changes and Homo origins.

Several hypotheses posit a link between the origin of Homo and climatic and environmental shifts between 3 and 2.5 Ma. Here we report on new results that shed light on the interplay between tectonics, basin migration and faunal change on the one hand and the fate of Australopithecus afarensis and the evolution of Homo on the other. Fieldwork at the new Mille-Logya site in the Afar, Ethiopia, dated to between 2.914 and 2.443 Ma, provides geological evidence for the northeast migration of the Hadar Basin, extending the record of this lacustrine basin to Mille-Logya. We have identified three new fossiliferous units, suggesting in situ faunal change within this interval. While the fauna in the older unit is comparable to that at Hadar and Dikika, the younger units contain species that indicate more open conditions along with remains of Homo. This suggests that Homo either emerged from Australopithecus during this interval or dispersed into the region as part of a fauna adapted to more open habitats.

Evidence of long-term NAO influence on East-Central Europe winter precipitation from a guano-derived δ15N record.

Currently there is a scarcity of paleo-records related to the North Atlantic Oscillation (NAO), particularly in East-Central Europe (ECE). Here we report δ15N analysis of guano from a cave in NW Romania with the intent of reconstructing past variation in ECE hydroclimate and examine NAO impacts on winter precipitation. We argue that the δ15N values of guano indicate that the nitrogen cycle is hydrologically controlled and the δ15N values likely reflect winter precipitation related to nitrogen mineralization prior to the growing season. Drier conditions indicated by δ15N values at AD 1848-1852 and AD 1880-1930 correspond to the positive phase of the NAO. The increased frequency of negative phases of the NAO between AD 1940-1975 is contemporaneous with higher δ15N values (wetter conditions). A 4‰ decrease in δ15N values at the end of the 1970's corresponds to a strong reduction in precipitation associated with a shift from negative to positive phase of the NAO. Using the relationship between NAO index and δ15N values in guano for the instrumental period, we reconstructed NAO-like phases back to AD 1650. Our results advocate that δ15N values of guano offer a proxy of the NAO conditions in the more distant past, helping assess its predictability.

Late Pliocene environmental change during the transition from Australopithecus to Homo.

It has long been hypothesized that the transition from Australopithecus to Homo in eastern Africa was linked to the spread of open and arid environments near the Plio-Pleistocene boundary, but data for the latest Pliocene are scarce. Here we present new stable carbon isotope data from the late Pliocene mammalian fauna from Ledi-Geraru, in the lower Awash Valley (LAV), Ethiopia, and mammalian community analyses from the LAV and Turkana Basin. These data, combined with pedogenic carbonate stable isotopes, indicate that the two regions were largely similar through the Plio-Pleistocene, but that important environmental differences existed during the emergence of Homo around 2.8 million years ago. The mid-Pliocene to late Pliocene interval in the LAV was characterized by increasingly C4-dominated, arid and seasonal environments. The early Homo mandible LD 350-1 has a carbon isotope value similar to that of earlier Australopithecus from the LAV, possibly indicating that the emergence of Homo from Australopithecus did not involve a dietary shift. Late Pliocene LAV environments contrast with contemporaneous environments in the Turkana Basin, which were more woody and mesic. These findings have important implications for the environmental conditions surrounding the emergence of Homo, as well as recent hypotheses regarding Plio-Pleistocene environmental change in eastern Africa.

Holocene winter climate variability in Central and Eastern Europe.

Among abundant reconstructions of Holocene climate in Europe, only a handful has addressed winter conditions, and most of these are restricted in length and/or resolution. Here we present a record of late autumn through early winter air temperature and moisture source changes in East-Central Europe for the Holocene, based on stable isotopic analysis of an ice core recovered from a cave in the Romanian Carpathian Mountains. During the past 10,000 years, reconstructed temperature changes followed insolation, with a minimum in the early Holocene, followed by gradual and continuous increase towards the mid-to-late-Holocene peak (between 4-2 kcal BP), and finally by a decrease after 0.8 kcal BP towards a minimum during the Little Ice Age (AD 1300-1850). Reconstructed early Holocene atmospheric circulation patterns were similar to those characteristics of the negative phase of the North Atlantic Oscillation (NAO), while in the late Holocene they resembled those prevailing in the positive NAO phase. The transition between the two regimes occurred abruptly at around 4.7 kcal BP. Remarkably, the widespread cooling at 8.2 kcal BP is not seen very well as a temperature change, but as a shift in moisture source, suggesting weaker westerlies and increased Mediterranean cyclones penetrating northward at this time.

Dietary flexibility of Australopithecus afarensis in the face of paleoecological change during the middle Pliocene: Faunal evidence from Hadar, Ethiopia.

One approach to understanding the context of changes in hominin paleodiets is to examine the paleodiets and paleohabitats of contemporaneous mammalian taxa. Recent carbon isotopic studies suggest that the middle Pliocene was marked by a major shift in hominin diets, characterized by a significant increase in C4 foods in Australopithecus-grade species, including Australopithecus afarensis. To contextualize previous isotopic studies of A. afarensis, we employed stable isotopes to examine paleodiets of the mammalian fauna contemporaneous with A. afarensis at Hadar, Ethiopia. We used these data to inform our understanding of paleoenvironmental change through the deposition of the Hadar Formation. While the majority of the taxa in the Hadar fauna were C4 grazers, most show little change in the intensity of C4 food consumption over the 0.5 million-year interval sampled. Two taxa (equids and bovins) do show increases in C4 consumption through the Hadar Formation and into the younger, overlying Busidima Formation. Changes in the distributions of C4-feeders, C3-feeders and mixed-C3/C4-feeders in the sampled intervals are consistent with evidence of dietary reconstructions based on ecomorphology, and with habitats reconstructed using community structure analyses. Meanwhile, A. afarensis is one of many mammalian taxa whose C4 consumption does not show directional change over the intervals sampled. In combination with a wide range of carbon and oxygen isotopic composition for A. afarensis as compared to the other large mammal taxa, these results suggest that the C3/C4 dietary flexibility of A. afarensis was relatively unusual among most of its mammalian cohort.

Digital data collection in paleoanthropology.

Understanding patterns of human evolution across space and time requires synthesizing data collected by independent research teams, and this effort is part of a larger trend to develop cyber infrastructure and e-science initiatives. At present, paleoanthropology cannot easily answer basic questions about the total number of fossils and artifacts that have been discovered, or exactly how those items were collected. In this paper, we examine the methodological challenges to data integration, with the hope that mitigating the technical obstacles will further promote data sharing. At a minimum, data integration efforts must document what data exist and how the data were collected (discovery), after which we can begin standardizing data collection practices with the aim of achieving combined analyses (synthesis). This paper outlines a digital data collection system for paleoanthropology. We review the relevant data management principles for a general audience and supplement this with technical details drawn from over 15 years of paleontological and archeological field experience in Africa and Europe. The system outlined here emphasizes free open-source software (FOSS) solutions that work on multiple computer platforms; it builds on recent advances in open-source geospatial software and mobile computing.

Paleodietary reconstruction using stable isotopes and abundance analysis of bovids from the Shungura Formation of South Omo, Ethiopia.

Preservation of the stable carbon isotopic composition of fossil tooth enamel enables us to estimate the relative proportion of C3 versus C4 vegetation in an animal's diet, which, combined with analysis of faunal abundance, may provide complementary methods of paleoenvironmental reconstruction. To this end, we analyzed stable carbon isotopic composition (δ(13)C values) of tooth enamel from four bovid tribes (Tragelaphini, Aepycerotini, Reduncini, and Alcelaphini) derived from six members of the Shungura Formation (Members B, C, D, F, G, and L; ages from ca. 2.90-1.05 Ma (millions of years ago) in the Lower Omo Valley of southwestern Ethiopia. The bovids show a wide range of δ(13)C values within taxa and stratigraphic members, as well as temporal changes in the feeding strategies of taxa analyzed throughout the middle to late Pliocene and early Pleistocene. Such variation suggests that the use of actualistic approaches for paleoenvironmental reconstruction may not always be warranted. Alcelaphini was the only taxon analyzed that retained a consistent dietary preference throughout the sequence, with entirely C4-dominated diets. Reduncini had a mixed C3/C4 to C4-dominated diet prior to 2.4 Ma, after which this taxon shifted to a largely C4-dominated diet. Aepycerotini generally showed a mixed C3/C4 diet, with a period of increased C4 diet from 2.5 to 2.3 Ma. Tragelaphini showed a range of mixed C3/C4 diets, with a median value that was briefly nearer the C4 end member from 2.9 to 2.4 Ma but was otherwise towards the C3 end member. These isotopic results, combined with relative abundance data for these bovids, imply that the environment of the Lower Omo Valley consisted of a mosaic of closed woodlands, with riverine forests and open grasslands. However, our data also signify that the overall environment gradually became more open, and that C4 grasses became more dominant. Finally, these results help document the range and extent of environments and potential diets that were available to the four hominin species encountered in the Shungura sequence.

Taphonomy of fossils from the hominin-bearing deposits at Dikika, Ethiopia.

Two fossil specimens from the DIK-55 locality in the Hadar Formation at Dikika, Ethiopia, are contemporaneous with the earliest documented stone tools, and they collectively bear twelve marks interpreted to be characteristic of stone tool butchery damage. An alternative interpretation of the marks has been that they were caused by trampling animals and do not provide evidence of stone tool use or large ungulate exploitation by Australopithecus-grade hominins. Thus, resolving which agents created marks on fossils in deposits from Dikika is an essential step in understanding the ecological and taphonomic contexts of the hominin-bearing deposits in this region and establishing their relevance for investigations of the earliest stone tool use. This paper presents results of microscopic scrutiny of all non-hominin fossils collected from the Hadar Formation at Dikika, including additional fossils from DIK-55, and describes in detail seven assemblages from sieved surface sediment samples. The study is the first taphonomic description of Pliocene fossil assemblages from open-air deposits in Africa that were collected without using only methods that emphasize the selective retention of taxonomically-informative specimens. The sieved assemblages show distinctive differences in faunal representation and taphonomic modifications that suggest they sample a range of depositional environments in the Pliocene Hadar Lake Basin, and have implications for how landscape-based taphonomy can be used to infer past microhabitats. The surface modification data show that no marks on any other fossils resemble in size or shape those on the two specimens from DIK-55 that were interpreted to bear stone tool inflicted damage. A large sample of marks from the sieved collections has characteristics that match modern trampling damage, but these marks are significantly smaller than those on the DIK-55 specimens and have different suites of characteristics. Most are not visible without magnification. The data show that the DIK-55 marks are outliers amongst bone surface damage in the Dikika area, and that trampling is not the most parsimonious interpretation of their origin.

The Omo Mursi Formation: a window into the East African Pliocene.

Dating to more than four million years ago (Ma), the Mursi Formation is among the oldest of the Plio-Pleistocene Omo Group deposits in the lower Omo Valley of southwestern Ethiopia. The sedimentary sequence is exposed along a strip ∼35 km by 4 km, but it has received relatively little attention due to the difficult access to this area. Although expeditions to the lower Omo Valley between 1968 and 1973 focused primarily on the Usno and Shungura Formations, survey of the Mursi Formation produced a faunal collection of about 250 specimens deriving exclusively from the Yellow Sands area at the southern extent of the exposures. In 2009, we reinitiated an investigation of the formation by focusing on the most northern exposures, and a new fossil site, Cholo, was identified. Cholo is depositionally similar to the lowermost exposures at the Yellow Sands, although no stratigraphic correlation between the two localities has yet been made. The fossiliferous sediments at Cholo are capped by a prominent vitric tuff that is compositionally distinct from any other known tephra preserved in East African rift basins, including the only known vitric tuff at the Yellow Sands. The faunal assemblage of the Yellow Sands area presents interesting characteristics: the fossils generally show little weathering and include a large proportion of suids (44% of the mammalian fauna) and a small proportion of bovids (14%) compared with other Pliocene African sites. The sample is also unusual in the high frequency of deinotheres (7%). Taxon-specific stable carbon isotopic composition of the Mursi mammals tends to show generally higher proportions of C3 diets compared with other Pliocene sites in East Africa and Chad. This and the particular faunal proportions suggest that the environments represented by the Mursi Formation were more closed than those of other Pliocene sites.

Baseline monitoring of the western Arctic Ocean estimates 20% of Canadian basin surface waters are undersaturated with respect to aragonite.

Marine surface waters are being acidified due to uptake of anthropogenic carbon dioxide, resulting in surface ocean areas of undersaturation with respect to carbonate minerals, including aragonite. In the Arctic Ocean, acidification is expected to occur at an accelerated rate with respect to the global oceans, but a paucity of baseline data has limited our understanding of the extent of Arctic undersaturation and of regional variations in rates and causes. The lack of data has also hindered refinement of models aimed at projecting future trends of ocean acidification. Here, based on more than 34,000 data records collected in 2010 and 2011, we establish a baseline of inorganic carbon data (pH, total alkalinity, dissolved inorganic carbon, partial pressure of carbon dioxide, and aragonite saturation index) for the western Arctic Ocean. This data set documents aragonite undersaturation in ≈ 20% of the surface waters of the combined Canada and Makarov basins, an area characterized by recent acceleration of sea ice loss. Conservative tracer studies using stable oxygen isotopic data from 307 sites show that while the entire surface of this area receives abundant freshwater from meteoric sources, freshwater from sea ice melt is most closely linked to the areas of carbonate mineral undersaturation. These data link the Arctic Ocean's largest area of aragonite undersaturation to sea ice melt and atmospheric CO2 absorption in areas of low buffering capacity. Some relatively supersaturated areas can be linked to localized biological activity. Collectively, these observations can be used to project trends of ocean acidification in higher latitude marine surface waters where inorganic carbon chemistry is largely influenced by sea ice meltwater.

Diet of Australopithecus afarensis from the Pliocene Hadar Formation, Ethiopia.

The enhanced dietary flexibility of early hominins to include consumption of C4/crassulacean acid metabolism (CAM) foods (i.e., foods derived from grasses, sedges, and succulents common in tropical savannas and deserts) likely represents a significant ecological and behavioral distinction from both extant great apes and the last common ancestor that we shared with great apes. Here, we use stable carbon isotopic data from 20 samples of Australopithecus afarensis from Hadar and Dikika, Ethiopia (>3.4-2.9 Ma) to show that this species consumed a diet with significant C4/CAM foods, differing from its putative ancestor Au. anamensis. Furthermore, there is no temporal trend in the amount of C4/CAM food consumption over the age of the samples analyzed, and the amount of C4/CAM food intake was highly variable, even within a single narrow stratigraphic interval. As such, Au. afarensis was a key participant in the C4/CAM dietary expansion by early australopiths of the middle Pliocene. The middle Pliocene expansion of the eastern African australopith diet to include savanna-based foods represents a shift to use of plant food resources that were already abundant in hominin environments for at least 1 million y and sets the stage for dietary differentiation and niche specialization by subsequent hominin taxa.

Dietary and paleoenvironmental reconstruction using stable isotopes of herbivore tooth enamel from middle Pliocene Dikika, Ethiopia: implication for Australopithecus afarensis habitat and food resources.

Carbon and oxygen isotopes of mammalian tooth enamel were used to reconstruct paleoenvironments of Australopithecus afarensis from the middle Pliocene locality of Dikika, Ethiopia. Isotopic analyses were conducted on 210 mammalian herbivore teeth from 15 different taxa collected from the Basal Member (~3.8-3.42 Ma) and Sidi Hakoma Member (3.42-3.24 Ma) of the Hadar Formation. The isotopic analyses aim specifically at reconstructing shifts in the relative abundance of C(4) grasses in mammalian diets, and more generally at paleoclimate factors such as aridity and seasonality, as well as habitat structure. Carbon isotopic data suggest a wide range of foraging strategies, characterized by mixed C(3)/C(4) to C(4)-dominated diets in wooded grasslands to open woodlands. Weighted average C(4) dietary proportions range between 60% and 86% in the Basal Member and 49% and 74% in the Sidi Hakoma Member. Paleoclimatic conditions based on the reconstructed mean annual water deficit from the δ(18)O(enamel) values indicate a wetter climate as compared to either the early Pliocene or the Pleistocene nearby. The middle Pliocene habitat structure at Dikika could be as diverse as open grassland and wooded grassland, and woodland to forest in the Sidi Hakoma Member while wooded grassland, woodland to grassland are evident in the Basal Member. All habitats except closed woodland and forest are persistent through both members; however, the relative proportion of individual habitats changed through time. These changes could have put the fauna in competition for preferred habitats and food resources, which could have forced migration, adaptation to other resources and/or extinction. Thus, the existence of A. afarensis throughout the middle Pliocene indicates either this species might have adapted to a wide range of habitats, or its preferred habitat was not affected by the observed environmental changes.

Woody cover and hominin environments in the past 6 million years.

The role of African savannahs in the evolution of early hominins has been debated for nearly a century. Resolution of this issue has been hindered by difficulty in quantifying the fraction of woody cover in the fossil record. Here we show that the fraction of woody cover in tropical ecosystems can be quantified using stable carbon isotopes in soils. Furthermore, we use fossil soils from hominin sites in the Awash and Omo-Turkana basins in eastern Africa to reconstruct the fraction of woody cover since the Late Miocene epoch (about 7 million years ago). (13)C/(12)C ratio data from 1,300 palaeosols at or adjacent to hominin sites dating to at least 6 million years ago show that woody cover was predominantly less than ∼40% at most sites. These data point to the prevalence of open environments at the majority of hominin fossil sites in eastern Africa over the past 6 million years.