Paleoecological patterns at the Hadar hominin site, Afar Regional State, Ethiopia

Reconstructing paleoecological patterns associated with hominin taxa, such as Australopithecus afarensis, is important for understanding possible evolutionary mechanisms involved in extinction and speciation events. It is critical to identify local, regional, or pan-African causal factors because patterns at these different levels may affect separate populations of the same species of hominin in unique ways. Habitat reconstructions of 12 submembers of the Hadar and Busidima formations (approximately 3.8-2.35 Ma) are presented here along with faunal differences in these submembers through time. Habitats with medium density tree and bush cover dominated the landscape through much of the earlier time period in the Hadar Formation. The lowermost Sidi Hakoma Member is the most closed habitat. The Denen Dora Member shows the influence of frequent floodplain edaphic grasslands with high abundances of reducin bovids. There is an influx of ungulates in the Kada Hadar Member (approximately 3.2--approximately 2.96 Ma) that indicates a more arid habitat populated by mammals that were recovered from earlier deposits further south in Ethiopia and Kenya. In the younger deposits from the Busidima Formation at Hadar, the landscape was open wooded grassland with some floodplain environments. The fossil assemblages from the Busidima Formation show a substantial species turnover. Although high numbers of A. afarensis specimens are associated with the lower Sidi Hakoma Member, they clearly inhabited a variety of habitats throughout the entire Hadar Formation. Australopithecus afarensis from Laetoli through Hadar times appears to have been a eurytopic species.     

Associated cranial and forelimb remains attributed to Australopithecus afarensis from Hadar, Ethiopia

A partial skeleton from Hadar, Ethiopia (A.L. 438-1) attributed to Australopithecus afarensis is comprised of part of the mandible, a frontal bone fragment, a complete left ulna, two second metacarpals, one third metacarpal, plus parts of the clavicle, humerus, radius, and right ulna. It is one of only a few early hominin specimens to preserve both cranial and postcranial elements. It also includes the first complete ulna from a large A. afarensis individual, and the first associated metacarpal and forelimb remains. This specimen, dated to approximately 3Ma, is among the geologically youngest A. afarensis fossils and is also one of the largest individuals known. Its ulnar to mandibular proportions are similar to those of the geologically older and much smaller A.L. 288-1, suggesting that body size increased without disproportional enlargement of the mandible. Overall, however, analysis of this large specimen and of the diminutive A.L. 288-1 demonstrates that the functional morphology of the A. afarensis upper limb was similar at all body sizes; there is no evidence to support the hypothesis that more than one hominin species is present at Hadar. Morphologically, all apparent apomorphic traits of the elbow, forearm, wrist, and hand of A.L. 438-1 are shared uniquely with humans. Compared to humans, A.L. 438-1 does have a more curved ulna, although A.L. 288-1 does not, and it appears to have had slightly less well-developed manipulatory capabilities of its hands, although still more derived than in apes. We conclude that selection for effective arboreality in the upper limb of Australopithecus afarensis was weaker than in non-hominins, and that manipulative ability was of greater selective advantage than in extant great apes.     

Upper Molar Cusp Dimensions of South African Australopithecines

Recently, a number of articles have been published on the mechanisms of dental development, and in particular on the genetic control of dental formation. As a specific example of the potential of this kind of research for paleoanthropological studies, cusp areas of upper molars of two species of South African Australopithecines (Australopithecus africanus, A. robustus) have been examined. The results suggest notable differences between the two species, possibly related to markedly different developmental pathways that are still far from clear in their details.     

Critique of ‘Australopithecus afarensis’ as a single species based on dental metrics and morphology

Leonard andHegmon (1987) compare a series of dental metrics of ‘Australopithecus afarensis Johanson, White, andCoppens, 1978’ with criteria for modern apes, to test the hypothesis that ‘A. afarensis’ represents a single species. They also compare the morphology of the lower third premolar. The dental breadth of ‘A. afarensis’ shows a wide range of variation, particularly in the lower third premolar morphology which displays greater variation than in modern apes—yet the study concludes that the single species hypothesis cannot be rejected. The study is flawed by applying criteria for pongids inappropriate for a hominid. When ‘A. afarensis’ is compared with criteria for hominids, the range of variation in dental size, breadth, and third premolar morphology is greater than that in any hominid species. The single species hypothesis is, therefore, once again rejected. Moreover, the name ‘A. afarensis’ is preoccupied byPraeanthropus africanus (Weinert) and must be dropped.     

Evolution of the mandibular third premolar crown in early Australopithecus

The Pliocene hominins Australopithecus anamensis and Australopithecus afarensis likely represent ancestor-descendent taxa—possibly an anagenetic lineage—and capture significant change in the morphology of the canine and mandibular third premolar (P₃) crowns, dental elements that form the canine honing complex in nonhuman catarrhines. This study focuses on the P₃ crown, highlighting plesiomorphic features in A. anamensis. The A. afarensis P₃ crown, in contrast, is variable in its expression of apomorphic features that are characteristic of geologically younger hominins. Temporal variation characterizes each taxon as well. The A. anamensis P₃ from Allia Bay, Kenya expresses apomorphic character states, shared with A. afarensis, which are not seen in the older sample of A. anamensis P₃s from Kanapoi, Kenya, while spatiotemporal differences in shape exist within the A. afarensis hypodigm. The accumulation of derived features in A. afarensis results in an increased level of P₃ molarisation. P₃ molarisation did not evolve concurrent with postcanine megadontia and neither did the appearance of derived aspects of P₃ occlusal form coincide with the loss of canine honing in hominins, which is apparent prior to the origin of the genus Australopithecus. A. afarensis P₃ variation reveals the independence of shape, size, and occlusal form. The evolution of the P₃ crown in early Australopithecus bridges the wide morphological gap that exists between geologically younger hominins on the one hand and extant apes and Ardipithecus on the other.     

What's Your Angle? Size Correction and Bar–Glenoid Orientation in “Lucy” (A.L. 288-1)

There has been much debate as to the locomotor repertoire of Lucy (A.L. 288-1) and other specimens of Australopithecus afarensis, ranging from fully committed bipeds to species that spent a significant time in the trees as well as on the ground. We examined the bar–glenoid angle, a character purported to indicate arboreal propensities, and its implications for this specific debate and the more general challenge of extracting behavioral information from fossils. We examined the bar–glenoid angle in ontogenetic samples of Pan paniscus, Pan troglodytes, Gorilla gorilla gorilla, Gorilla gorilla beringei, Pongo pygmaeus, Homo sapiens, and A.L. 288-1 (Lucy). We found that there is no allometry in the bar–glenoid angle for the great apes, but a weak correlation for humans. Moreover, the data scatters for the African apes and humans converge at the smaller size ranges, and Lucy's value for bar–glenoid angle falls precisely in this area of overlap. Therefore, we conclude that the bar–glenoid angle is not tightly correlated with function and, as such, cannot be used as a morphological signal of arboreal behavior, especially in the smaller size ranges, at which arboreal and nonarboreal species overlap. Our work does not resolve issues concerning Lucy's precise locomotor repertoire but adds new information to consider. The total morphological pattern, plus an appreciation of the underlying variance in morphological and behavioral characters in extant species, is key for making functional inferences from the morphology of fossils.     

Revision of the subspecies ofAustralopithecus africanus (primates: Hominidae), including a new subspecies from the late pliocene of Ethiopia

The subspecies ofAustralopithecus africanus Dart, 1925 have been revised in a morphological and statistical analysis. Four subspecific names were previously proposed, but only one was found to be valid. The subspeciesA. africanus transvaalensis (Broom, 1936), from the Plio/Pleistocene of South Africa, cannot be sustained due to an insufficient sample, and is combined with the nominate race,A. a. africanus. The type ofA. africanus afarensis Tobias, 1980 is a mistake in identification and notA. africanus, but a pongid. The population ofA. africanus from the late Pliocene of Ethiopia does indeed represent a relatively small-toothed geographical race for which the nameA. africanus aethiopicus was conditionally proposed; and the lectotype for it, A.L. 288-1, is notA. africanus, but the type ofHomo antiquus Ferguson, 1984. The trinominalaethiopicus is thus unavailable for the Ethiopian race, which is redescribed as a new subspecies,A. africanus miodentatus n. ssp., and the mandible A.L. 266-1 is designated as the holotype.     

Morphological affinities of the Australopithecus afarensis hand on the basis of manual proportions and relative thumb length

The hands of apes and humans differ considerably with regard to proportions between several bones. Of critical significance is the long thumb relative to other fingers, which is the basis for human-like pad-to-pad precision grip capability, and has been considered by some as evidence of tool-making. The nature and timing of the evolutionary transition from ape-like to human-like manual proportions, however, have remained unclear as a result of the lack of appropriate fossil material. In this article, the manual proportions of Australopithecus afarensis from locality AL 333/333w (Hadar, Ethiopia) are investigated by means of bivariate and multivariate morphometric analyses, in order to test the hypothesis that human-like proportions, including an enhanced thumb/hand relationship, originally evolved as an adaptation to stone tool-making. Although some evidence for human-like manual proportions had been previously proposed for this taxon, conclusive evidence was lacking. Our results indicate that A. afarensis possessed overall manual proportions, including an increased thumb/hand relationship that, contrary to previous reports, is fully human and would have permitted pad-to-pad human-like precision grip capability. We show that these human-like proportions in A. afarensis mainly result from hand shortening, as in modern humans, and that these conclusions are robust enough as to be non-dependent on whether the bones belong to a single individual or not. Since A. afarensis predates the appearance of stone tools in the archeological record, the above-mentioned conclusions permit a confident refutation of the null hypothesis that human-like manual proportions are an adaptation to stone tool-making, and thus alternative explanations must be therefore sought. One hypothesis would consider manipulative behaviors (including tool-use and/or non-lithic tool-making) in early hominines exceeding those reported among extant non-human primates. Alternatively, on the basis of the many adaptations to committed bipedalism in A. afarensis, we propose the hypothesis that once arboreal behaviors became adaptively insignificant and forelimb-dominated locomotor selection pressures were relaxed with the adoption of terrestrial bipedalism, human-like manual proportions could have merely evolved as a result of the complex manipulation selection pressures already present in extant non-human primates.Both hypotheses are not mutually exclusive, and even other factors such as pleiotropy cannot be currently discarded.     

Dental development in South African australopithecines. Part I: Problems of pattern and chronology

It is well known that humans take about twice as long as apes to mature. The traditional view that such delayed maturation was already present in australopithecines has been called into question during the past several years. We have approached this problem by looking at patterns of dental development in gracile and robust australopithecines from South Africa and comparing them to patterns found in extant humans and apes. We have employed both 2 and 3 dimensional computed tomography in our research. The dental growth patterns in these two australopithecine morphs differ, particularly in M1/I1 development. The robust australopithecines are more humanlike and the gracile australopithecines more apelike in this feature (“humanlike” and “apelike” are not used in any taxonomic sense). Pattern and chronology of dental development must be considered separately. Several major problem areas for future research are identified, most of which revolve around the issue of intra- versus interspecific variation.     

Laetoli toes andAustralopithecus afarensis

The probable misfit between feet, particularly toes II–V, of 3.0-million-year-oldAustralopithecus afarensis from Hadar, Ethiopia, and the 3.5-million-year-old hominid footprints at Site G, Laetoli, Tanzania, casts doubt thatA. Afarensis made the Laetoli trails. We suggest that another species ofAustralopithecus or an anonymous genus of the Hominidae, with remarkably humanoid feet, walked at Laetoli. It would be imprudent to declare thatHomo was present at Laetoli 3.5 million years ago (my) because there is no evidence of brain expansion, advanced tool manufacture, or other non-locomotor hallmarks of the human condition at Site G.     

Australopithecus afarensis and the single species hypothesis

Ferguson (1989) has recently argued that the variability seen in the fossils assigned toA. afarensis is far more than expected for a single hominid species, and therefore proposes they represent multiple taxa. In particular, he utilizes data on variation in dental metrics and in premolar morphology in support of this hypothesis. A re-evaluation of these data finds the above conclusion to be unwarranted. Variation in dental metrics providesno basis for separating this sample into multiple taxa, regardless of the analog that is used (i.e. modern primate species or fossil hominid species). Additionally, data on P₃ morphology indicate that thepattern of variation seen in the Laetoli/Hadar sample is comparable to the sexual variation seenwithin a single hominoid species. Overall, the balance of the evidence at present indicates that the fossils from Laetoli and Hadar represent a single hominid species,A. afarensis.     

Estimating canine tooth crown height in early Australopithecus

Canine tooth size reduction and the associated reduction in canine dimorphism is a basal hominin character that also provides important evidence for models of behavioral evolution. Two specimens of Australopithecus anamensis (KNM-KP 29287 and KNM-KP 29283) that do not preserve the canine crown, but do preserve the root or alveolus, appear to suggest that canine size variation and canine dimorphism in this species may have been greater than in other hominins. We evaluate canine root and crown dimensions in a series of extant hominoids, and estimate canine crown height in Australopithecus afarensis and A. anamensis. Our results demonstrate that it is possible to generate estimates of canine crown height from basal canine crown and root dimensions with a moderate degree of accuracy. Estimates of maxillary canine crown size for A. anamensis are slightly larger than those of A. afarensis, and are approximately the same size as canines of modern female chimpanzees. Estimated mandibular canine crown height is very similar in the two species. Variation within the A. anamensis sample of estimated canine crown heights is similar to that of modern humans, suggesting a low degree of sexual dimorphism. Inclusion of estimates for KNM-KP 29287 and KNM-KP 29283 does not substantially increase either the estimate of overall canine size or variation for A. anamensis.     

Was Australopithecus anamensis ancestral to A. afarensis? A case of anagenesis in the hominin fossil record

We tested the hypothesis that early Pliocene Australopithecus anamensis was ancestral to A. afarensis by conducting a phylogenetic analysis of four temporally successive fossil samples assigned to these species (from earliest to latest: Kanapoi, Allia Bay, Laetoli, Hadar) using polarized character-state data from 20 morphological characters of the dentition and jaws. If the hypothesis that A. anamensis is ancestral to A. afarensis is true, then character-state changes between the temporally ordered site-samples should be congruent with hypothesized polarity transformations based on outgroup (African great ape) conditions. The most parsimonious reconstruction of character-state evolution suggests that each of the hominin OTUs shares apomorphies only with geologically younger OTUs, as predicted by the hypothesis of ancestry (tree length=31; Consistency Index=0.903). This concordance of stratigraphic and character-state data supports the idea that the A. anamensis and A. afarensis samples represent parts of an anagenetically evolving lineage, or evolutionary species. Each site-sample appears to capture a different point along this evolutionary trajectory. We discuss the implications of this conclusion for the taxonomy and adaptive evolution of these early-middle Pliocene hominins.     

Anterior dental evolution in the Australopithecus anamensis–afarensis lineage

Australopithecus anamensis is the earliest known species of the Australopithecus–human clade and is the likely ancestor of Australopithecus afarensis. Investigating possible selective pressures underlying these changes is key to understanding the patterns of selection shaping the origins and early evolution of the Australopithecus–human clade. During the course of the Au. anamensis–afarensis lineage, significant changes appear to occur particularly in the anterior dentition, but also in jaw structure and molar form, suggesting selection for altered diet and/or food processing. Specifically, canine tooth crown height does not change, but maxillary canines and P₃s become shorter mesiodistally, canine tooth crowns become more symmetrical in profile and P₃s less unicuspid. Canine roots diminish in size and dimorphism, especially relative to the size of the postcanine teeth. Molar crowns become higher. Tooth rows become more divergent and symphyseal form changes. Dietary change involving anterior dental use is also suggested by less intense anterior tooth wear in Au. afarensis. These dental changes signal selection for altered dietary behaviour and explain some differences in craniofacial form between these taxa. These data identify Au. anamensis not just as a more primitive version of Au. afarensis, but as a dynamic member of an evolving lineage leading to Au. afarensis, and raise intriguing questions about what other evolutionary changes occurred during the early evolution of the Australopithecus–human clade, and what characterized the origins of the group.     

The South African australopithecines: A palaeodemographic analysis

Anthropologists, demographers and historians alike are continually seeking information about demographic profiles of prehistoric and ancient human populations. There are many different approaches relevant to the problem, yet direct evidence of the demographic structure of any archaeological population is primarily provided by analysis of human skeletal and dental remains. This offers a possibility of extending demographic inferences back to Pliocene — Early Pleistocene times, which, in turn, would enhance our understanding of the principles of human survival, adaptation, social interaction and demographic evolution of man. Data on the age distribution of South African australopithecines has been analysed using life-table analysis, based on a stationary population model. The estimated demographic profile is the evaluated and interpreted within a framework of biological, cultural and ecological circumstances. It is concluded that palaeodemography, if carefully undertaken, can play a real and pragmatic role in understanding the demographic history of man.     

French Contribution to the study of Human Origins: The case ofAustralopithecus afarensis

For a long time, French scientists have been involved in the study of human evolution and especially of human origins. Their key works in Eastern Africa have led to the discovery of major fossil hominid sites, especially in the Afar region in Ethiopia, where numerous remains ofAustralopithecus afarensis have been unearthed. The major contribution of the French scholars to the interpretation of the Hadar sample was to demonstrate the impact of the postcranial features on taxonomy and phylogeny. Two groups were identified in the sample and by comparison with modern populations of wild primates, these groups are assigned to different taxa. The other major impact was to show that early hominid bipedalism was an exact replica of modern human bipedality.