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Suggested Citation:"Human Evolution." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
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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

Suggested Citation:"Human Evolution." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
×

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.

Suggested Citation:"Human Evolution." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
×
Page 167
Suggested Citation:"Human Evolution." National Academy of Sciences. 1995. Tempo and Mode in Evolution: Genetics and Paleontology 50 Years After Simpson. Washington, DC: The National Academies Press. doi: 10.17226/4910.
×
Page 168
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Since George Gaylord Simpson published Tempo and Mode in Evolution in 1944, discoveries in paleontology and genetics have abounded. This volume brings together the findings and insights of today's leading experts in the study of evolution, including Ayala, W. Ford Doolittle, and Stephen Jay Gould.

The volume examines early cellular evolution, explores changes in the tempo of evolution between the Precambrian and Phanerozoic periods, and reconstructs the Cambrian evolutionary burst. Long-neglected despite Darwin's interest in it, species extinction is discussed in detail.

Although the absence of data kept Simpson from exploring human evolution in his book, the current volume covers morphological and genetic changes in human populations, contradicting the popular claim that all modern humans descend from a single woman.

This book discusses the role of molecular clocks, the results of evolution in 12 populations of Escherichia coli propagated for 10,000 generations, a physical map of Drosophila chromosomes, and evidence for "hitchhiking" by mutations.

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