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

By Charles E. Oxnard | Go to book overview

CHAPTER 4
Eight to Eighteen Million Years Ago: Sexual Dimorphisms in Early Relatives

Ramapithecines -- the Current Consensus -- Ape Ancestors?
New Ramapithecines from China -- Ramapithecus and Sivapithecus
Dental Measures Taken One by One -- Canine Sexual Dimorphisms?
Sexual Dimorphisms -- in Incisors -- in Premolars -- in Molars?
Additional Univariate Results for Ramapithecines
Measures Taken Together -- Canonical Variates -- High-dimensional Displays
Implications for Evolution

Abstract: We first review briefly the current consensus about the ramapithecines. This is the idea that they are merely ape ancestors, probably more closely related to orang-utans than to anything else alive today. What had earlier been assessed as two separate species, are currently believed to be merely the males and females of an extremely dimorphic 'ape'.

We next examine information about new ramapithecines from China, especially the fact that there have been found over one thousand teeth for this group. Such data clearly allow us to prosecute studies on these fossils in a way usually denied to studies of primate fossils: that is, in a population mode.

Dental dimensions, lengths and breadths, are investigated, separately, for each tooth position: canines, incisors, premolars and molars. When the data are grouped as ramapithecines, a single species or species group, then the results make it very obvious that if ramapithecines truly form only a single cluster of 'apes', their sexual dimorphism must have been enormously greater than anything else known from the entire span of the living or fossil primates.

In contrast, however, examination of each 'sex' of each of these 'apes' demonstrates a new phenomenon. Each 'sex' has, itself, evidence of two subgroups within it!

It is evident that the Chinese ramapithecines actually comprise two species or species groups, each showing evidence of two subgroups. Special features of these two subgroups (for example, that they are more obvious in the examination of breadth dimensions than of lengths) imply that it is not inappropriate for us to designate them the two sexes of each group.

The evidences of this sexual dimorphism in each ramapithecine, which at this point we designate: Ramapithecus and Sivapithecus, are then studied separately. Their sexual dimorphism is clearly complex and is not only different in each, but also shows differences from, and similarities to, the extant species already studied in Chapter 2 and the human fossils of Chapter 3.

This is further evident when the dimensions of the teeth are examined altogether using multivariate morphometrics. These studies allow additional comparisons with the extant apes and extant and fossil humans. The findings have major implications for human and ape evolution, although for a full discussion, we must also await the results of examining other fossils (such as the australopithecines) in later chapters.


Ramapithecines, the current consensus: ape ancestors?

A fragment of upper jaw described by Pilgrim in 1910 and 1915 was the first ramapithecine to be recognized as such. It was described by G. Edward Lewis ( 1934) as human-like. Following this, remains of similar fossils have been recovered in Africa, Europe and Asia (e.g. Leakey, 1962; Simons, 1964; Von Koenigswald, 1972, Simons, 1974; Kretzoi, 1975; Andrews and Tobien. 1977; Simons, 1978). The processes of discovery of these fragments goes on even today, Walker and Leakey ( 1984) having recently found further fragments in Africa. These fragments are mostly limited to small pieces of jaw bones and isolated teeth, and all are believed to be between approximately ten and eighteen million years old. They have been assumed to represent many different groups at different times. At first they were given many different

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