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ON BEING HEADMAN* JAMES V. NEELi Over a period of 3-4 million years—150,000-200,000 generations— man evolved from a creature about 4?? feet high with a relatively massive dentition and a cranial capacity of some 500 cubic centimeters to the Homo sapiens oftoday. Along the way—ifwe may use the higher primates as a reference point—normal life expectancy may have more than doubled . While statements concerning relative rates of evolution are traditionally subject to challenge, on a per generation basis the rate ofchange, as judged by morphological criteria, seems relatively rapid when compared with the fossil records of most other mammals. Direct evidence from the ultimate criterion—demonstrated nucleotide substitutions in DNA—will of course never be available, so that the argument must be based on the indirect approach of protein and DNA homologies in species whose time of divergence can be fixed from the fossil record. Given the usual assumptions of the time of divergence of hominoids from other primates, our detailed knowledge ofthe composition ofa few proteins in primates—hemoglobin, carbonic anhydrase I, cytochrome, the fibrinopeptides—suggests that there was if anything a "primate slowdown" in the rate of evolution of these proteins [I]. However, these proteins were almost surely not those critical to the evolutionary process. Equally objective and contradictory genetic evidence suggesting that change was rapid rather than slow is provided by the exceptionally pronounced rate ofchromosomal evolution in some primates [2]. The fact is that we have not yet identified the molecular bases for the important evolutionary changes, which seem as likely to involve the control elements of the genome as those specifying protein structure [3, 4]. Let us for now agree that despite some inconsistencies in the record, man has undergone a striking evolution in the past several million years; this *This is a somewhat expanded version of a paper presented at a symposium entitled "Why Sociobiology?" sponsored by the American Society of Human Genetics, October 4-7, 1978, in Vancouver, British Columbia, Canada.|Lee R. Dice University Professor of Human Genetics, University of Michigan Medical School, Ann Arbor, Michigan 48109. I gratefully acknowledge the financial support ofthe Department of Energy and the National Science Foundation in the original investigations which led to the results encompassed in this paper. O 1980 by The University of Chicago. 0031-5982/80/2302-0136$01.00 Perspectives in Biology andMedicine ¦ Winter 1980 | 277 presentation is directed toward the population structure and the vectors under which this occurred. The Vectors ofEvolution The vectors of evolution are only three in number: chance, mutation, and selection. These vectors operate within a matrix which we term population structure. In recent years, our team has devoted a considerable effort to defining the population structure of one particular group, the Yanomama, an Amerindian tribe of southern Venezuela and northern Brazil, whose direct contacts with non-Indians (other than a few explorers) date back only to the early 1950s. They currently number some 12,000-15,000 persons and occupy an area encompassing some 100,000 square miles. Although the Yanomama now derive the majority of their caloric intake from slash-and-burn agriculture, and so represent a relatively advanced stage in human subsistence patterns, both the data on their culture [5, 6] and their reproductive patterns and deme microdifferentiation (summary in [7, 8]) suggest that they still exhibit many ofthe essential features ofpreagricultural societies, and we can use them as an approximate guide to the population structure and the nature of the above-enumerated vectors during human evolution. CHANCE AS A VECTOR OF HUMAN EVOLUTION Ifwe accept the monophyletic origin ofour species, then understanding the dynamics ofman's dissemination throughout the world is central to any effort to understand human evolution. We happened to be unexpectedly fortunate in our studies in that we came upon the Yanomama at a time of tribal expansion. In part, this may have resulted from their relatively recent access, through devious trade routes, to machetes or pieces thereof, as well as the probably relatively recent introduction of the cooking banana (Musa paradisica) as a cultivar, these acquisitions having a profound effect on their food base and their subsistence ecology . On the other hand...

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