Australopithecus Species Research Articles

Evolutionary relationships among early hominids

Although cladistic analysis provides one of the most useful approaches to discovering the phyletic relatiosships among the species of Australopithecus and early Homo, methodological problems continue to beset any attempt to apply it in this context. Two of the most pressing problems are redundancy of traits due to similarity of underlying function and overrepresentation of some anatomical or functional systems in trait lists. In an attempt to mitigate these problems, we collapse our list of 77 traits into sets of traits using two methods. The first method segregates traits into seven groups by anatomical region. The second method segregates traits by function into five complexes which correspond fairly well with recognized trends in the evolution of early hominids (adaptation for heavy chewing, reduction of the anterior dentition, basicranial flexion, increased orthognathism and encephalization). The most parsimonious cladogram describing the relationships among the early hominid species obtained using the original 77 traits or the summary scores from the functional complexes, places Australopithecus afarensis as a sister group to all other hominids. Australopithecus aethiopicus occupies the next branch, leaving A. africanus, A. robustus, A. boisei and Homo as a monophyletic group. The cladogram next separates A. africanus from the remaining hominids and finally divides Homo from A. robustus and A. boisei. Summary scores for anatomical regions produced three equally parsimonious cladograms, one of which was identical to that described above. This result implies that there was a large amount of parallelism in hominid evolution, and that adaptations for heavy chewing evolved separately in the lineage leading to A. aethiopicus and in the lineage leading to A. robustus and A. boisei. Another implication is that Homo descended from an A. africanus-like ancestor and diverged from A. robustus and A. boisei relatively late in hominid evolution by reducing the extent of its adaptation to heavy chewing. Most current phylogenies are not compatible with the cladogram obtained in this study, but are instead compatible with a cladogram obtained when traits related to heavy chewing are used exclusively. The “heavy chewing” cladogram reverses the positions of A. aethiopicus and Homo. Perhaps the reason why most current cladograms resemble the “heavy chewing” cladogram is the over-representation of traits related to heavy chewing in most trait lists.

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.

Variability and sexual dimorphism in canine size of Australopithecus and extant hominoids

Among extant hominoids degrees of sexual dimorphism and combined-sex coefficients of variation of canine teeth dimensions are highly correlated. Based on this relationship and coefficients of variation of four species of the genus Australopithecus, we predict degrees of canine dimorphism for these extinct hominids. The estimates show that A. afarensis is as dimorphic as the pygmy chimpanzee, A. boisei slightly less dimorphic than the pygmy chimpanzee, A. robustus slightly more dimorphic than the lar gibbon, while A. africanus overiaps with the lar gibbon as well as a modern human sample. These estimates represent degrees of canine dimorphism substantially lower than results based upon prior sexing of individual specimens. The relationship between canine dimorphism and body weight dimorphism is also analyzed. All four species of Australopithecus are considerably less dimorphic in canine size for their body weight dimorphism than expected. This dissociation of canine size dimorphism and body weight dimorphism is shared with modern humans, and thus represents a unique hominid trait. We interpret the moderate to strong body weight dimorphism in australopithecines as the result of intra- and intersexual selection typical of a polygynous mating structure, while the rather mild canine dimorphism is interpreted in terms of the “developmental crowding” model for reduction in canine size.

Dental Evidence for the Diet of Australopithecus

Explaining human structure in functional and adaptive terms and constructing a picture of behavioral changes in the course of human evolution are among the goals of evolutionary studies. Over the years many attempts have been made to interpret the behavior and ecology of Australopithecus, the earliest undoubted member of the human family. Such attempts have used information about environments offossil deposition, the nature of the death assemblages, cultural associations, and the animals and plants with which australopithecines are found. One of the most important sources of information about australopithe­ cine diets has been the anatomy of the animals themselves, especially the teeth . It is this last source that is examined critically here. The basis for most reconstruction of the behavior of extinct animals is the analogical argument. We observe a pattern of behavior in a living species and note that such behavior is associated with an anatomical specialization. When a similar specialization is observed in an extinct species, that animal is inferred to have behaved in a similar way. By building up a catalog of such behavioral inferences, the broad patterns of behavior and ecology of a paleospecies emerge. Of course, this commonsense approach has difficulties, many of which are reviewed and debated in recent literature (3, 31, 58) . Many theories have been advanced about the dietary adaptations of Australo­ pithecus and early Homo. Some of these theories have utilized the evidence of australopithecine dental structure. J . T. Robinson (46) proposed that diets differed in the two species of Australopithecus then known. The larger species, A. robustus. was primarily a vegetarian whereas the smaller species, A . africanus. ate meat as well as vegetable foods. Largely in agreement with Robinson, Jolly (25) envisioned primitive Australopithecus. as exemplified by

Relative cheek-tooth size in Australopithecus.

Until the discovery of Australopithecus afarensis, cheek-tooth megadontia was unequivocally one of the defining characteristics of the australopithecine grade in human evolution along with bipedalism and small brains. This species, however, has an average postcanine area of 757 mm2, which is more like Homo habilis (759 mm2) than A. africanus (856 mm2). But what is its relative cheek-tooth size in comparison to body size? One approach to this question is to compare postcanine tooth area to estimated body weight. By this method all Australopithecus species are megadont: they have cheek teeth 1.7 to 2.3 times larger than modern hominoids of similar body size. The series from A. afarensis to A. africanus to A. robustus to A. boisei shows strong positive allometry indicating increasing megadontia through time. The series from H. habilis to H. erectus to H. sapiens shows strong negative allometry which implies a sharp reduction in the relative size of the posterior teeth. Postcanine megadontia in Australopithecus species can also be demonstrated by comparing tooth size and body size in associated skeletons: A. afarensis (represented by A.L. 288-1) has a cheek-tooth size 2.8 times larger than expected from modern hominoids; A. africanus (Sts 7) and A. robustus (TM 1517) are over twice the expected size. The evolutionary transition from the megadont condition of Australopithecus to the trend of decreasing megadontia seen in the Homo lineage may have occurred between 3.0 and 2.5 m.y. from A. afarensis to H.habilis but other evidence indicates that it is more likely to have occurred between 2.5 to 2.0 m.y. from an A. africanus-like form to H. habilis.

Australopithecus, Homo erectus and the single species hypothesis.

AN enormous wealth of early hominid remains has been discovered over the past few years by expeditions within eastern Africa. Evidence has been presented for the existence over a considerable period of time of at least two contemporaneous hominid species1. Some of this evidence is compelling, but some less so for a variety of reasons such as the lack of association, fragmentary specimens, geological uncertainties, equivocal anatomical differences and suchlike. Many of these new specimens are of great antiquity and have led to suggestions that an early form of the genus Homo was contemporary with at least one species of Australopithecus. The evidence presented here deals not with the earlier stages of human evolution, but with the unequivocal occurrence of H. erectus from the Koobi Fora Formation, east of Lake Turkana (formerly Lake Rudolf).

Gestation period for Australopithecus.

LEUTENEGGER1 has provided estimates of the foetal size at birth of two species of Australopithecus. Huggett and Widdas2 drew attention to the relationship between foetal age and weight in mammals, and we have been assessing both specific foetal growth rates and length of gestation (J. F. D. F. and A. St G. Huggett, unpublished data). It is quite clear that in the higher primates the foetal growth rate is 0.06. Using this and the estimates of foetal sizes at birth in the Huggett/Widdas formula2 we obtain gestation periods for A. africanus of 257 days and A. robustus 300 days.