Sexual Dimorphism as a Problem in Human Evolution
The General Problem of Ancestors in Morphological Studies of Human Evolution
The Sex of Ancestors as a Special Problem
Sexual Dimorphism in Living Primates -- the Current Picture
New Views -- the Existence of Multiple Sexual Dimorphisms
Implications for Evolution of Great Apes and Humans
Abstract: There are, of course, many differences between the sexes that are directly related to the reproductive act and process. But there are also many other anatomical differences between the sexes that are only indirectly related to reproduction and are manifest all over the body. The sum total of these differences is called sexual dimorphism. Though relatively small in humans, sexual dimorphism is quite enormous in some of our closest living relatives, gorillas and orang-utans.
The current picture of this sexual dimorphism is that it is a single, unidimensional phenomenon that is displayed to greater (e.g. gorillas, orang-utans) or lesser (e.g. humans) degrees in the different primate species. It is, moreover, generally believed to be primarily related to differences in overall bodily size between the sexes. The evolutionary implication is, that in the past human sexual dimorphism must have been very much greater than it is today, perhaps more like that in the living apes.
With this in mind we examine the results of new anatomical studies of the overall form of the body in primates, using various morphometric tools (especially those of canonical variates analyses and high-dimensional displays). These studies show that sexual dimorphism is more complex than a single unidimensional spectrum. They suggest that several different sexual dimorphisms exist in the living primates. They imply that there must have been a number of different evolutionary directions in which sexual dimorphisms evolved. And this means that current views of sexual dimorphism in human ancestors are to be questioned.
Understanding the human lineage is one of the main problems in studying human evolution. But many fossil hunters seem to believe that this means that their task is to find the fragments of the precise human ancestor in the field. Likewise, many laboratory workers seem to believe that this means that their task is showing that a particular fossil remnant is that ancestor. Even in the public mind, studying human evolution seems to be this matter of going from 'missing' to 'found' links.
The problem seems to be the
discovery of ancestors.
But what is the reality of such an endeavour? Even from a population as large and concentrated as that of any major metropolitan area, and over as many as hundreds of generations, the statistical chances of any particular individual ever becoming fossilised and found by a palaeontologist millions of years later must be almost infinitesimal. How much less must be the chances of finding representatives of populations of perhaps only a few thousand, scattered over an area of the world as large as Africa or Asia, during periods of time measured in hundreds of thousands, even millions of years. Once we are a few thousand years beyond a death, perhaps a few tens of thousands of years at the outside, the chances of ever finding anything fossilised that can be said to be a direct ancestor of anything in our time, are small indeed.
If finding ancestors is not our main aim,
what then is?
Our aims are multiple. One aim is to compare the structures of living forms one with another and with such fossils as by chance have come to light. Such comparisons allow us to evaluate the amounts and kinds of morphological differences that have existed among related biological organisms. From