PART III
HUMAN EVOLUTION
Human evolution figures remarkably in Tempo and Mode by its complete absence. The paleontological record of human evolution illuminates general issues of rate and pattern of evolution, and human evolution was a subject about which Simpson had much to say in later years. But the paleontological record of mankind's history was much too scanty at the time of Tempo and Mode. Not so at present. Henry M. McHenry, in Chapter 9, elucidates that human morphological evolution was mosaic. Bipedalism appeared early; the enlargement of the brain, much later. Some locomotor features changed only well after our ancestors had evolved bipedal gait. Dentition and face remained quite primitive for some time after the evolution of a distinctively hominid cranium. McHenry projects the haphazard pattern of brain-size increases over a reconstruction of the phylogenetic relationships among the, at least, eight known hominid species, from Australopithecus africanus to Homo sapiens.
The genetic diversity of the human histocompatibility complex is wondrous. At least 41 alleles are known at the B locus, 60 at C, 38 at DPB1, 58 at DRB1, and more than a dozen at each of three other loci. This gene complex serves to differentiate self from nonself and in the defense against parasites and other foreign invaders. The alleles at any one locus are quite divergent, the living descendants of lineages that recede separately for millions of years into the past. Francisco J. Ayala and colleagues rely on the theory of gene coalescence to conclude, in Chapter 10, that our ancestral lineage has been at least 100,000 individuals strong, on the average, for the last 30 million years. If a population
retrenchment occurred at any time, the bottleneck could not have been smaller than a few thousand individuals, a conclusion that is also buttressed by computer simulations. These results contradict the claim propagated by the media that all modern humans descend from a single woman or very few women that lived 200,000 years ago.
