Steven E. Churchill

Australopithecus sediba: A New Species of Homo-Like Australopith from South Africa

From Australopithecus to Homo Our genus Homo is thought to have evolved a little more than 2 million years ago from the earlier hominid Australopithecus. But there are few fossils that provide detailed information on this transition. Berger et al. (p. 195; see the cover) now describe two partial skeletons, including most of the skull, pelvis, and ankle, of a new species of Australopithecus that are informative. The skeletons were found in a cave in South Africa encased in sediments dated by Dirks et al. (p. 205) to about 1.8 to 1.9 million years ago. The fossils share many derived features with the earliest Homo species, including in its pelvis and smaller teeth, and imply that the transition to Homo was in stages. A new species of Australopithecus, about 1.9 million years old, shows many derived features with Homo, helping to reveal its evolution. Despite a rich African Plio-Pleistocene hominin fossil record, the ancestry of Homo and its relation to earlier australopithecines remain unresolved. Here we report on two partial skeletons with an age of 1.95 to 1.78 million years. The fossils were encased in cave deposits at the Malapa site in South Africa. The skeletons were found close together and are directly associated with craniodental remains. Together they represent a new species of Australopithecus that is probably descended from Australopithecus africanus. Combined craniodental and postcranial evidence demonstrates that this new species shares more derived features with early Homo than any other australopith species and thus might help reveal the ancestor of that genus.

Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa

Homo naledi is a previously-unknown species of extinct hominin discovered within the Dinaledi Chamber of the Rising Star cave system, Cradle of Humankind, South Africa. This species is characterized by body mass and stature similar to small-bodied human populations but a small endocranial volume similar to australopiths. Cranial morphology of H. naledi is unique, but most similar to early Homo species including Homo erectus, Homo habilis or Homo rudolfensis. While primitive, the dentition is generally small and simple in occlusal morphology. H. naledi has humanlike manipulatory adaptations of the hand and wrist. It also exhibits a humanlike foot and lower limb. These humanlike aspects are contrasted in the postcrania with a more primitive or australopith-like trunk, shoulder, pelvis and proximal femur. Representing at least 15 individuals with most skeletal elements repeated multiple times, this is the largest assemblage of a single species of hominins yet discovered in Africa. DOI: http://dx.doi.org/10.7554/eLife.09560.001

Makers of the early Aurignacian of Europe.

Despite intensive study and a number of remarkable discoveries in the last two decades of the 20th century, our understanding of the cultural and biological processes that resulted in the emergence of the Upper Paleolithic and the establishment of modern humans in Interpleniglacial Europe remains far from complete. There is active debate concerning the timing and location of the origins of the Aurignacian, the nature of the origins of Initial Upper Paleolithic industries (whether by autochthonous development or through acculturation by Aurignacian peoples), the timing of the appearance of early modern humans and the disappearance of the Neandertals, and the relationship of archeologically defined cultures to these different types of hominids. Frustrating our attempts to address these latter two questions is a general paucity of taxonomically diagnostic human fossil material from early Upper Paleolithic contexts. We undertake here a review of the human fossil record of Interpleniglacial Europe, and its archeological and chronological context, to clarify to the extent possible the nature of the relationship between hominid groups and the earliest Upper Paleolithic artifact industries, particularly the early Aurignacian. Although substantial difficulties involved in interpreting the fossil, archeological, and geochronological records of this time period prohibit making any definitive statements, a number of observations are suggested by the current data: 1) the Middle Paleolithic of Europe appears to have been made exclusively by Neandertals; 2) Initial Upper Paleolithic industries (with the exception of the Bachokirian) appear to have their roots in the late Middle Paleolithic industries of their respective regions; 3) all of the human fossils yet recovered from Initial Upper Paleolithic (except the Bachokirian) contexts for which any diagnostic morphology is present have their greatest morphological affinities with Neandertals and not early modern humans; 4) modern humans were almost certainly established in Europe by ca. 32 ky BP, with a strong possibility that they were there by ca. 36 ky BP. Claims for an appearance before 36 ky BP cannot be substantiated with currently available evidence; 5) the hypothesis that modern humans are uniquely associated with the Aurignacian cannot yet be refuted. Aurignacian-associated human fossils (including those from the Bachokirian) for which any diagnostic morphology is present have their greatest affinities with early modern Europeans and not Neandertals; and 6) Neandertals and modern humans coexisted in Europe for at least 2,000-4,000 years, and perhaps for 8,000-10,000 years or longer. The overall picture is one of an extended period of cultural contact, involving some degree of genetic exchange, between Neandertals and early modern Europeans.

Australopithecus sediba Hand Demonstrates Mosaic Evolution of Locomotor and Manipulative Abilities

The hand of Australopithecus sediba, a rare example in the hominid fossil record, shows short fingers and a long thumb consistent with improved precision gripping while retaining strength for climbing. Hand bones from a single individual with a clear taxonomic affiliation are scarce in the hominin fossil record, which has hampered understanding the evolution of manipulative abilities in hominins. Here we describe and analyze a nearly complete wrist and hand of an adult female [Malapa Hominin 2 (MH2)] Australopithecus sediba from Malapa, South Africa (1.977 million years ago). The hand presents a suite of Australopithecus-like features, such as a strong flexor apparatus associated with arboreal locomotion, and Homo-like features, such as a long thumb and short fingers associated with precision gripping and possibly stone tool production. Comparisons to other fossil hominins suggest that there were at least two distinct hand morphotypes around the Plio-Pleistocene transition. The MH2 fossils suggest that Au. sediba may represent a basal condition associated with early stone tool use and production.

The Foot and Ankle of Australopithecus sediba

Australopithecus sediba had a human-like ankle and arch but an ape-like heel and tibia, implying that while bipedal, this species was also adept at climbing trees. A well-preserved and articulated partial foot and ankle of Australopithecus sediba, including an associated complete adult distal tibia, talus, and calcaneus, have been discovered at the Malapa site, South Africa, and reported in direct association with the female paratype Malapa Hominin 2. These fossils reveal a mosaic of primitive and derived features that are distinct from those seen in other hominins. The ankle (talocrural) joint is mostly humanlike in form and inferred function, and there is some evidence for a humanlike arch and Achilles tendon. However, Au. sediba is apelike in possessing a more gracile calcaneal body and a more robust medial malleolus than expected. These observations suggest, if present models of foot function are correct, that Au. sediba may have practiced a unique form of bipedalism and some degree of arboreality. Given the combination of features in the Au. sediba foot, as well as comparisons between Au. sediba and older hominins, homoplasy is implied in the acquisition of bipedal adaptations in the hominin foot.

Australopithecus sediba at 1.977 Ma and Implications for the Origins of the Genus Homo

Further U-series dating and the magnetic stratigraphy of the hosting cave deposits show that Australopithecus sediba lived just under 2 million years ago, near or just before the emergence of Homo. Newly exposed cave sediments at the Malapa site include a flowstone layer capping the sedimentary unit containing the Australopithecus sediba fossils. Uranium-lead dating of the flowstone, combined with paleomagnetic and stratigraphic analysis of the flowstone and underlying sediments, provides a tightly constrained date of 1.977 ± 0.002 million years ago (Ma) for these fossils. This refined dating suggests that Au. sediba from Malapa predates the earliest uncontested evidence for Homo in Africa.

Geological Setting and Age of Australopithecus sediba from Southern Africa

From Australopithecus to Homo Our genus Homo is thought to have evolved a little more than 2 million years ago from the earlier hominid Australopithecus. But there are few fossils that provide detailed information on this transition. Berger et al. (p. 195; see the cover) now describe two partial skeletons, including most of the skull, pelvis, and ankle, of a new species of Australopithecus that are informative. The skeletons were found in a cave in South Africa encased in sediments dated by Dirks et al. (p. 205) to about 1.8 to 1.9 million years ago. The fossils share many derived features with the earliest Homo species, including in its pelvis and smaller teeth, and imply that the transition to Homo was in stages. A new species of Australopithecus, about 1.9 million years old, shows many derived features with Homo, helping to reveal its evolution. We describe the geological, geochronological, geomorphological, and faunal context of the Malapa site and the fossils of Australopithecus sediba. The hominins occur with a macrofauna assemblage that existed in Africa between 2.36 and 1.50 million years ago (Ma). The fossils are encased in water-laid, clastic sediments that were deposited along the lower parts of what is now a deeply eroded cave system, immediately above a flowstone layer with a U-Pb date of 2.026 ± 0.021 Ma. The flowstone has a reversed paleomagnetic signature and the overlying hominin-bearing sediments are of normal polarity, indicating deposition during the 1.95- to 1.78-Ma Olduvai Subchron. The two hominin specimens were buried together in a single debris flow that lithified soon after deposition in a phreatic environment inaccessible to scavengers.

A Partial Pelvis of Australopithecus sediba

Although it had a small brain and skull, Australopithecus sediba shows some human-like features in its reconstructed pelvis. The fossil record of the hominin pelvis reflects important evolutionary changes in locomotion and parturition. The partial pelves of two individuals of Australopithecus sediba were reconstructed from previously reported finds and new material. These remains share some features with australopiths, such as large biacetabular diameter, small sacral and coxal joints, and long pubic rami. The specimens also share derived features with Homo, including more vertically oriented and sigmoid-shaped iliac blades, greater robusticity of the iliac body, sinusoidal anterior iliac borders, shortened ischia, and more superiorly oriented pubic rami. These derived features appear in a species with a small adult brain size, suggesting that the birthing of larger-brained babies was not driving the evolution of the pelvis at this time.

A Case of Marked Bilateral Asymmetry in the Upper Limbs of an Upper Palaeolithic Male from Barma Grande (Liguria), Italy

Barma Grande 2, a male skeleton of upper palaeolithic age from Balzi Rossi (Liguria, Italy), shows a marked degree of upper limb bilateral asymmetry. Similar cases of asymmetry in palaeolithic hominid fossils have variously been attributed to high levels of behavioural asymmetry (related to handedness) or a pathologically induced alteration of upper limb skeletal remodelling processes. As in many of these cases, the skeleton from Barma Grande lacks any indications of trauma or pathology in the smaller left limb. Consideration of the morphology of the preserved upper limb elements and a comparative analysis of asymmetry in normal and pathological male palaeolithic fossils and normal recent human samples suggests that the asymmetry in Barma Grande 2 was a secondary effect of trauma or pathology in the left side. The degree and pattern of asymmetry in numerous humeral and ulnar measurements indicates an adult onset of altered loading patterns. Several possible aetiological factors are considered, with the most probable being an entrapment neuropathy, direct trauma to one or more muscles about the shoulder, or possibly glenohumeral joint instability.

The Lower Limb and Mechanics of Walking in Australopithecus sediba

The discovery of a relatively complete Australopithecus sediba adult female skeleton permits a detailed locomotor analysis in which joint systems can be integrated to form a comprehensive picture of gait kinematics in this late australopith. Here we describe the lower limb anatomy of Au. sediba and hypothesize that this species walked with a fully extended leg and with an inverted foot during the swing phase of bipedal walking. Initial contact of the lateral foot with the ground resulted in a large pronatory torque around the joints of the foot that caused extreme medial weight transfer (hyperpronation) into the toe-off phase of the gait cycle (late pronation). These bipedal mechanics are different from those often reconstructed for other australopiths and suggest that there may have been several forms of bipedalism during the Plio-Pleistocene.