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A 3.8-million-year-old hominin fossil reveals what the cranium of the oldest known Australopithecus species looked like, casting doubt on assumptions about how these ancient relatives of humans evolved. A fossil reveals what the face of Australopithecus anamensis looked like.

The discovery of 4.5-million-year-old fossils of the hominid Ardipithecus ramidus increases our knowledge of a fascinating stage of human evolution, before hominids left the forests for the open savanna. Fossil finds of this age are rare, but deposits in Gona, Ethiopia, have yielded material from at least nine individuals. Their context shows that they lived in an environment of moderate rainfall woodland, and grasslands. Comparative biomolecular studies suggest that the last common ancestor of humans and chimpanzees, our closest living relatives, lived during the Late Miocene–Early Pliocene1,2. Fossil evidence of Late Miocene–Early Pliocene hominid evolution is rare and limited to a few sites in Ethiopia3,4,5, Kenya6 and Chad7. Here we report new Early Pliocene hominid discoveries and their palaeoenvironmental context from the fossiliferous deposits of As Duma, Gona Western Margin (GWM), Afar, Ethiopia. The hominid dental anatomy (occlusal enamel thickness, absolute and relative size of the first and second lower molar crowns, and premolar crown and radicular anatomy) indicates attribution to Ardipithecus ramidus. The combined radioisotopic and palaeomagnetic data suggest an age of between 4.51 and 4.32 million years for the hominid finds at As Duma. Diverse sources of data (sedimentology, faunal composition, ecomorphological variables and stable carbon isotopic evidence from the palaeosols and fossil tooth enamel) indicate that the Early Pliocene As Duma sediments sample a moderate rainfall woodland and woodland/grassland.

Remains of earlyHomo andParanthropus have been recovered from two contemporaneous sites (Uraha and Malema) in the “Hominid Corridor” in Northern Malawi (Chiwondo Beds). Faunal dating suggests an age of 2.5–2.3 Ma for both hominids. The two specimens, a mandible attributed toHomo rudolfensis (UR 501 from Uraha), and a maxillary fragment ofParanthropus boisci. (RC 911 from Malema) known only from eastern Africa, represent the southernmost known distribution of these taxa. The biogeographic significance of these hominids from the Malawi-Rift lay in their association with the eastern African endemic animal group. Biogeographic variation in south-eastern Africa may be linked to habitat change occurring due to climate change, with maximum change occurring around 2.5 Ma.

Molecular studies suggest that the lineages leading to humans and chimpanzees diverged approximately 6.5–5.5 million years (Myr) ago, in the Late Miocene1,2,3. Hominid fossils from this interval, however, are fragmentary and of uncertain phylogenetic status, age, or both4,5,6. Here I report new hominid specimens from the Middle Awash area of Ethiopia that date to 5.2–5.8 Myr and are associated with a wooded palaeoenvironment7. These Late Miocene fossils are assigned to the hominid genus Ardipithecus and represent the earliest definitive evidence of the hominid clade. Derived dental characters are shared exclusively with all younger hominids. This indicates that the fossils probably represent a hominid taxon that postdated the divergence of lineages leading to modern chimpanzees and humans. However, the persistence of primitive dental and postcranial characters in these new fossils indicates that Ardipithecus was phylogenetically close to the common ancestor of chimpanzees and humans. These new findings raise additional questions about the claimed hominid status of Orrorin tugenensis8, recently described from Kenya and dated to ∼6 Myr9.

It is traditionally believed that human ancestors evolved in a warm and dry environment. The available evidence, however, favours the vision that it happened in a warm and wet environment. The paleo-environmental data suggest that the early australopithecinesAustralopithecus anamensis, afarensis andafricanus lived in warm, moist, and wooded landscapes such as gallery forests. In the Pleistocene, the robust australopithecinesA. robustus andboisei seem to have dwelt in more open, possibly cooler and generally dryer places, in the vicinity of shallow and relatively stagnant waters of lakesides, lagoons, marshes and riverbanks. Dental and microwear studies suggest that the australopithecines, more than Western lowland gorillas, regularly fed on aquatic herbaceous vegetation (AHV). Homo fossils, on the other hand, as suggested by the paleo-environmental data, are more frequently discovered near lakes, seas and rivers where molluscs were abundant. Shellfish could provide a dietary supplement for their omnivorous diet. This is how early hominines might have learned to use stones to crack bivalves. This subsequently could have led to stone tool use for other purposes.

Bipedalism has traditionally been regarded as the fundamental adaptation that sets hominids apart from other primates. Fossil evidence demonstrates that by 4.1 million years ago1, and perhaps earlier2, hominids exhibited adaptations to bipedal walking. At present, however, the fossil record offers little information about the origin of bipedalism, and despite nearly a century of research on existing fossils and comparative anatomy, there is still no consensus concerning the mode of locomotion that preceded bipedalism3,4,5,6,7,8,9,10. Here we present evidence that fossils attributed to Australopithecus anamensis (KNM-ER 20419)11 and A. afarensis (AL 288-1)12 retain specialized wrist morphology associated with knuckle-walking. This distal radial morphology differs from that of later hominids and non-knuckle-walking anthropoid primates, suggesting that knuckle-walking is a derived feature of the African ape and human clade. This removes key morphological evidence for a Pan–Gorilla clade, and suggests that bipedal hominids evolved from a knuckle-walking ancestor that was already partly terrestrial.

The discovery of Australopithecus anamensis fossils from strata lying between tephra dated at 4.17 and 4.12 million years ago, and from slightly higher strata not well constrained in age by overlying dated units, provoked the claim that more than one species might be represented: it was suggested that the stratigraphically higher fossils, which include the important tibia, humerus and a large, presumed male, mandible (KNM-KP 29287), might belong to a later, more derived hominid. We have recovered new fossils from Kanapoi and Allia Bay, Kenya, during field work in 1995-1997 that confirm the primitive status of Australopithecus anamensis, the earliest species of Australopithecus. Isotope dating confirms A. anamensis' intermediate age as being between those of Ardipithecus ramidus and Australopithecus afarensis. New specimens of maxilla, mandible and capitate show that this species is demonstrably more primitive than A. afarensis. A lower first deciduous molar (dm 1) is intermediate in morphology between that reported for Ardipithecus ramidus and A. afarensis. Single-crystal 40Ar-39Ar age determinations on the Kanapoi Tuff show that, except for a large mandible, all of the hominid fossils from Kanapoi are from sediments deposited between 4.17+/-0.03 and 4.07+/-0.02 million years ago.

Nine hominid dental, cranial and postcranial specimens from Kanapoi, Kenya, and 12 specimens from Allia Bay, Kenya, are described here as a new species of Australopithecus dating from between about 3.9 million and 4.2 million years ago. The mosaic of primitive and derived features shows this species to be a possible ancestor to Australopithecus afarensis and suggests that Ardipithecus ramidus is a sister species to this and all later hominids. A tibia establishes that hominids were bipedal at least half a million years before the previous earliest evidence showed.

Seventeen hominoid fossils recovered from Pliocene strata at Aramis, Middle Awash, Ethiopia make up a series comprising dental, cranial and postcranial specimens dated to around 4.4 million years ago. When compared with Australopithecus afarensis and with modern and fossil apes the Aramis fossil hominids are recognized as a new species of Australopithecus--A. ramidus sp. nov. The antiquity and primitive morphology of A. ramidus suggests that it represents a long-sought potential root species for the Hominidae.

Sedimentary deposits in the Middle Awash research area of Ethiopia's Afar depression have yielded vertebrate fossils including the most ancient hominids known. Radioisotopic dating, geochemical analysis of interbedded volcanic ashes and biochronological considerations place the hominid-bearing deposits at around 4.4 million years of age. Sedimentological, botanical and faunal evidence suggests a wooded habitat for the Aramis hominids.

The first sites with Pliocene and Pleistocene mammals west of the Rift Valley in Central Africa in northern Chad were reported in 1959 (ref. 1), and documented the presence of mixed savannah and woodland habitats. Further sites and a probable Homo erectus cranio-facial fragment were subsequently discovered. In 1993 a survey of Pliocene and Pleistocene formations in the Borkou-Ennedi-Tibesti Province of Chad (B.E.T.) led to the discovery of 17 new sites in the region of Bahr el Ghazal (classical Arabic for River of the Gazelles) near Koro Toro. One site, KT 12 (15 degrees 58'10"N, 18 degrees 52'46"E) yielded an australopithecine mandible associated with a fauna biochronologically estimated to be 3.0-3.5 Myr old. Australopithecine species described since 1925 are known from southern Africa and from sites spread along the eastern Rift Valley from Tanzania to Ethiopia (Fig. 1). This new find from Chad, which is most similar in morphology to Australopithecus afarensis, documents the presence of an early hominid a considerable distance, 2,500 km, west of the Rift Valley.

Specimens of Australopithecus boisei have been found in 2.5-Myr-old sediments west of Lake Turkana, Kenya. The primitive morphology of these early A. boisei suggests that robust and hyper-robust Australopithecus developed many of their common features in parallel and further that A. africanus is unlikely to have been ancestral to A. boisei.

Remains of 13 early hominids have been found in the Laetolil Beds in northern Tanzania, 30 miles south of Olduvai Gorge. Potassium–argon dating of the fossiliferous deposits gives an upper limit averaging 3.59 Myr and a lower limit of 3.77 Myr. An extensive mammalian fauna is associated. The fossils occur in the upper 30 m of ash-fall and aeolian tuffs whose total measured thickness is 130 m.