G. Philip Rightmire

A Complete Skull from Dmanisi, Georgia, and the Evolutionary Biology of Early Homo

A Heady Find In the past two decades, excavations at the archaeological site at Dmanisi, Georgia, have revealed hominin fossils from the earliest Pleistocene, soon after the genus Homo first dispersed beyond Africa. Lordkipanidze et al. (p. 326; see the cover) now describe a fossil cranium from the site. Combined with mandibular remains that had been found earlier, this find completes the first entire hominin skull from this period. An early Pleistocene adult skull illuminates the evolution and morphology of the first hominins outside Africa. The site of Dmanisi, Georgia, has yielded an impressive sample of hominid cranial and postcranial remains, documenting the presence of Homo outside Africa around 1.8 million years ago. Here we report on a new cranium from Dmanisi (D4500) that, together with its mandible (D2600), represents the worlds first completely preserved adult hominid skull from the early Pleistocene. D4500/D2600 combines a small braincase (546 cubic centimeters) with a large prognathic face and exhibits close morphological affinities with the earliest known Homo fossils from Africa. The Dmanisi sample, which now comprises five crania, provides direct evidence for wide morphological variation within and among early Homo paleodemes. This implies the existence of a single evolving lineage of early Homo, with phylogeographic continuity across continents.

Human evolution in the Middle Pleistocene: The role ofHomo heidelbergensis

For paleoanthropologists working in the Middle Pleistocene, these are interesting times. New discoveries of artifacts and human fossils have been reported from western Europe, so that it now looks as though this continent was populated 800,000 years ago, if not earlier. One of the fossils, from Ceprano in Italy, is described as Homo erectus. Whether this ancient species ever reached Europe has been repeatedly questioned, but the Ceprano cranium is complete enough to provide some hard evidence. Other finds from Spain are even more spectacular. The Sima de los Huesos (‘‘Pit of Bones’’) in the Sierra de Atapuerca has yielded a wealth of skeletons that are best interpreted as early Neanderthals, perhaps close to 300,000 years in age. Older but unfortunately more fragmentary remains, also from Atapuerca, display no Neanderthal features and are claimed as representatives of a new species. Homo antecessor will require close study. These European discoveries focus fresh attention on the evidence accumulating from Africa and Asia. Human bones are known from the earlier Middle Pleistocene of Africa at localities such as Bodo in Ethiopia and Broken Hill in Zambia. The crania show anatomical features that distinguish them from Homo erectus. In the Far East, the people at Dali and other sites are also more advanced than Homo erectus, but their affinities to groups in the West are uncertain. This Middle Pleistocene record, still sparse but increasingly well dated, raises important questions. One concerns the fate of Homo erectus in different regions of the Old World. Another is how many distinct species should be recognized among the descendants of this ancient lineage. It is apparent that the traditional approach of lumping diverse humans together as ‘‘archaic’’ Homo sapiens will no longer work. The picture is highly complex, and several taxa probably are needed to accommodate the fossils. Evolutionary relationships among these populations must be clarified, but pose some major problems. I will address only a subset of these topics pertaining mainly to earlier Middle Pleistocene hominids.

Comparative studies of Late Pleistocene human remains from Klasies River Mouth, South Africa

Abstract The main site at Klasies River Mouth was excavated in 1967–1968, and quantities of shell, animal bones and some human remains were recovered in association with a Middle Stone Age industry. More recent investigation of the caves has clarified aspects of stratigraphy and dating. It is well established that the site was first inhabited during the Last Interglaciation. Two main stages of deposition leading to formation of the (older) LBS and (younger) SAS members can be recognized, and the site was abandoned about 60,000 years ago, perhaps as a consequence of changes in habitat conditions due to climatic forcing. Most of the human bones collected in 1967–1968, and several additional specimens found since 1984, come from the SAS deposits. Two recently excavated fragments of maxillae are from the LBS member. We report results of comprehensive studies of the Klasies River Mouth teeth, mandibles, upper jaws and other facial pieces, and postcranial material. Measurements and anatomical comparisons confirm that the Klasies Middle Stone Age people differ from Neanderthals and other archaic humans. Mandibles tend to be deeper at the front than posteriorly, and a chin is relatively prominent. Internal symphyseal buttresses are not expressed, and there is no retromolar space. An impression that the morphology is modern extends also to the face and to the parts of the post-cranial skeleton that are preserved. This robust but essentially modern anatomy is coupled with strong sexual dimorphism. A question that remains to be explored is how the Klasies folk compare to contemporary populations present at Qafzeh Cave and Es Skhul. It is apparent that there are some differences between the South African assemblage and those from western Asia. These differences must be checked further, but all three sites may document an early dispersal of modern humanity, perhaps from an African source.

Locomotor anatomy and biomechanics of the Dmanisi hominins

The Dmanisi hominins inhabited a northern temperate habitat in the southern Caucasus, approximately 1.8 million years ago. This is the oldest population of hominins known outside of Africa. Understanding the set of anatomical and behavioral traits that equipped this population to exploit their seasonal habitat successfully may shed light on the selection pressures shaping early members of the genus Homo and the ecological strategies that permitted the expansion of their range outside of the African subtropics. The abundant stone tools at the site, as well as taphonomic evidence for butchery, suggest that the Dmanisi hominins were active hunters or scavengers. In this study, we examine the locomotor mechanics of the Dmanisi hind limb to test the hypothesis that the inclusion of meat in the diet is associated with an increase in walking and running economy and endurance. Using comparative data from modern humans, chimpanzees, and gorillas, as well as other fossil hominins, we show that the Dmanisi hind limb was functionally similar to modern humans, with a longitudinal plantar arch, increased limb length, and human-like ankle morphology. Other aspects of the foot, specifically metatarsal morphology and tibial torsion, are less derived and similar to earlier hominins. These results are consistent with hypotheses linking hunting and scavenging to improved walking and running performance in early Homo. Primitive retentions in the Dmanisi foot suggest that locomotor evolution continued through the early Pleistocene.

Patterns in the evolution of Homo erectus

Skull and tooth dimensions of Homo erectus fossils from Africa, Europe and Asia appear to fluctuate rather than to change progressively during an interval of 1.3 Myr prior to the start of the Upper Pleistocene. No significant trends can be observed. This suggests a period of stasis in human evolution, after which Homo sapiens may have evolved rapidly from archaic mid-Pleistocene ancestors.

The Supraorbital Torus: "A Most Remarkable Peculiarity" [and Comments and Replies]

The supraorbital torus is found only in some genera of the primate order. Because no muscles of consequence attach directly to it, it has been considered nonfunctional. However, invitro strain-gauge experiments demonstrate that when the anterior teeth are loaded, the supraorbital region acts as a bent beam, pulled downward on each end by masticatory muscle forces and pushed upward centrally by bite force. Clinical and experimental data indicate that in response to repeated dynamic bending stress, adaptive cellular activity reconstructs skeletal material until bending stresses are neutralized. With these facts in mind, the hypothesis that supraorbital development is, in part, a predictable ontogenetic response to in-vivo bending stresses which concentrate over the eyes during anterior tooth loading was tested by means of a biomechanical model. The bent-beam model states that supraorbital bending is a function of the area moment of inertia of the forehead (relative to the direction of the bite force) and of the bending moment. When this model was tested on a series of Australian Aboriginal crania, significant relationships were found between browridge development and measures of forehead area moment and bending moment. It was concluded that the torus functions to resist bending stress concentrated over the eyes during anterior biting and that its development is proportional to the amount of such stress which cannot be resisted by the unadorned frontal bone.

Implications of Border Cave Skeletal Remains for Later Pleistocene Human Evolution [and Comments and Reply]

Recent excavations have helped to clarify the cultural and stratigraphic sequences at Border Cave, and human skeletal remains recovered at this South African site may well be associated with a Middle Stone Age industry. A partial adult cranium is of particular interest, and this was originally described as quite different both from African Negro and from Bushman individuals. Eleven measurements were taken on the fossil, and these provide a basis for comparison of Border Cave with other crania drawn from extant African populations. Discriminant analysis shows clearly that the cranium lies close to the Hottentot male centroid and is within the range of modern African variation for the measurements employed. It is heavily constructed but not archaic in the fashion of Florisbad or Broken Hill. If the case for antiquity is regarded as firm, then the Border Cave skeleton suggests the presence of Homo sapiens sapiens in southern Africa before 50,000 B.P. and perhaps as early as 115,000 B.P. Hottentot or Bushman-like people may thus have inhabited southern Africa for a long time. This would be consistent with a phyletic view of human evolution, postulating an in situ transition from archaic to more modern man. Whether migration and replacement of populations may have occurred elsewhere is another question, and there is no reason to exclude this as a possibility on a local level, even if waves of replacement did not sweep the Old World late in the Upper Pleistocene. The course of evolution outside of southern Africa cannot be determined from the evidence considered here.

Homo erectus and Middle Pleistocene hominins: brain size, skull form, and species recognition.

Hominins that differ from Homo erectus, the Neanderthals, and recent humans are known from Middle Pleistocene localities across the Old World. The taxonomic status of these populations has been clouded by controversy. Perhaps the most critical problem has been an incomplete understanding of variation in skull form. Here, both H. erectus and later mid-Pleistocene hominins are the focus of an investigation aimed at clarifying the relationships among brain volume, basicranial dimensions, neurocranial shape, and certain facial characters. Brain size in H. erectus averages about 950 cm(3), while in a series of Middle Pleistocene crania from Africa and Europe, volume is about 1230 cm(3). If encephalization is the primary mechanism operating in the mid-Pleistocene, then diverse aspects of cranial form cannot all be treated as independent variables. Correlation is utilized to examine the associations among measurements for more than 30 H. erectus crania that are reasonably well preserved. A similar approach is used with the Middle Pleistocene sample. Patterns of covariation are compared in order to assess integration. Next, factor analysis is applied to the H. erectus specimens in an attempt to identify modules, tightly integrated traits that can evolve independently. Studies of the variation within H. erectus are followed by direct comparisons with the Middle Pleistocene population. Discriminant functions facilitate the description of intergroup differences. Traits that vary independently from brain volume include anterior frontal broadening, lateral expansion of the parietal vault, elevation of the lambda-inion chord, and rounding of the sagittal contour of the occipital. This finding helps to resolve the problem of species recognition. Neurocranial proportions as well as characters from the cranial base and face can be incorporated into a differential diagnosis for the mid-Pleistocene sample. Evidence presented here supports arguments for speciation in the Middle Pleistocene.

Ramapithecus and Hominid Origins (and Comments and Reply)

The importance of Ramapithecus has traditionally been in the claim that it represents the earliest hominid. It is argued here that the development of Ramapithecus interpretations has been dependent upon the currently accepted theory of hominid origins. In turn, theories of hominid origins have been influenced by the emerging understanding of the ramapithecine adaptive radiation. Recent evidence strongly suggests an ancestry for Pongo among the ramapithecines, and it is suggested that the earliest hominids as well as the African apes also evolved from a ramapithecine species. The origin of the hominids is viewed as a speciation event in a late African ramapithecine species, and it is hypothesized that competition between the early hominids and the emerging African apes resulted in divergence and character displacement in both lineages. The realization of hominization in this latest ramapithecine, combined with the numerous primitive characteristics of the earliest hominids, helps bridge the gap between humans and their predivergence ancestors. The resulting interpretation of the ramapithecines preserves many of the earlier claims about their phylogenetic significance.

Fossil Skulls from Dmanisi: A Paleodeme Representing Earliest Homo in Eurasia

The Plio-Pleistocene site of Dmanisi has yielded much evidence bearing on the morphology and behavior of the earliest hominins from western Eurasia. Human remains, animal bones and stone artifacts were deposited at Dmanisi during a brief interval following the close of the Olduvai Subchron (1.77 million years ago). The hominin fossils now include an adult braincase (D2280), the partial skull of a young adult (D2282/D211), a large mandible (D2600), a small subadult skull (D2700/D2735), an edentulous cranium with lower jaw (D3444/D3900), and postcranial bones attributed to several individuals. The crania have capacities ranging from 600 to 775 cm3. Supraorbital tori and other vault superstructures are only moderately developed. Although there is variation related to ontogenetic age and sex dimorphism, it is appropriate to group the Dmanisi individuals together. Most probably, all of the hominins are sampled from one paleodeme. This population resembles Homo habilis in brain volume and some aspects of craniofacial morphology, but many of these features can be interpreted as symplesiomorphies. Other discrete characters and measurements suggest that the Dmanisi skulls are best placed with H. erectus. There are numerous similarities to individuals from the Turkana Basin in Kenya, but a few features link Dmanisi to Sangiran in Java. This evidence can be read to support an early dispersal of H. erectus from Africa to Eurasia. However, an alternative hypothesis must also be considered. Morphological comparisons of the fossils and findings from geochronology are consistent with the view that H. erectus evolved in Asia. Only later did representatives of this species disperse from western Asia to Africa and eastward toward Java and China.