Fossils, Teeth, and Sex: New Perspectives on Human Evolution

By Charles E. Oxnard | Go to book overview

CHAPTER 5
The Intervening Period: African Ancestors?

Australopithecines and Habilines
Human Ancestors?
Sexual Dimorphism in Australopithecines -- Current Views
New Studies of Individual Measures of Australopithecine Teeth
All Australopithecines Taken Together
Gracile and Robust Groups Taken Separately
Afarensis, Africanus, Robustus, Boisei -- Taken Individually
Homo habilis -- a Separate Issue
New Studies of Australopithecine Teeth -- All Measures Combined
Canonical Variates Analyses -- High-dimensional Displays
Conclusions -- Australopithecine Fossils

Abstract: We first review briefly current ideas about the evolutionary relationships of the australopithecines. These ideas imply that one or another, or even more, of the australopithecines are indeed human ancestors; and that the remainder are either also on that lineage or closely related to it.

We also review some of the challenges to those views. One of these -- now fully confirmed -- states that australopithecines are quite different morphologically from the genus Homo. A second, now largely agreed, implies that they were probably arboreal as well as being bipedal in a manner different from humans. A third suggests that as a result of these first two, we cannot say whether the australopithecines form a curious mosaic on the human lineage (accepted, reluctantly, by some) or a radiation that lies alongside the human lineage (scarcely accepted by anyone). The earlier view, that they were obviously direct human ancestors, is under fire.

We next test these ideas through new studies of sexual dimorphism. We examine what little is known about general sexual dimorphism in australopithecines and then go on to study the dimensions of their teeth, using the methods generally adopted in this book.

Univariate studies demonstrate once again, the general complexity of the phenomenon of sexual dimorphism.

They show that bimodal distributions are ubiquitous in the various australopithecine groups. The nature of that bimodality is such that it was almost certainly sexual dimorphism. It is likely that a dimorphism in variance also exists, at least in some of the species or species groups. Different patterns of dimorphism along the tooth row can be discerned. And the distributions allow us, generally, to make assessments of sex ratios in the different subgroups of these fossils.

Multivariate studies of these same dimensions are next presented. They allow us to assess the size of the multivariate sexual dimorphisms, to study multivariate sexually dimorphic patterns both between jaws and between fossil groups, and to compare multivariate positions of individual fossil groups with those of the extant apes and humans.

All of these results emphasize once again the enormous complexity of the phenomenon of sexual dimorphism. They provide fascinating information in the comparison with living apes and humans from Chapter 2. They are especially interesting in the comparison with the fossils of Chapters 3 & 4. They have important implications in the understanding of the evolution of the hominoids.


Australopithecines and habilines

Those fossils that have been found mainly in Africa, australopithecines and habilines, date from as recently as 1 million years ago or less, to possibly 4 million years ago or even more. This is a period of time that almost matches the definition of the Pliocene that extends from 1.8 to 5 million years

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