The demography of two tribes of primitive, relatively unacculturated American Indians.

Repeatedly in the past 50 years, concern has been expressed over the possible adverse genetic implications for our species of certain demographic trends and differentials. Our relative ignorance of the degree of association between genetic attributes and reproductive attributes has permitted a wide diversity of opinion. In this confused situation, knowledge of primitive man's demographic structure should provide a helpful perspective. Specifically, since the demographic structure of a human population places definite boundaries on its response to selection, how different is our present demographic structure from that obtaining during the period when man was evolving to his present physical and intellectual state? Should it prove possible to couple the answer to this question with data on the genetic component in attributes determining survival and reproduction in primitive man, and should we obtain similar data on various groups of civilized man, then for the first time we would begin to possess the perspective thus far so difficult to obtain. The approaches to understanding the demographic structure of our remote ancestors are chiefly two. Firstly, something can be learned of the age and sex structure of early human populations (and, by inference, of reproductive patterns) through the study of skeletal remains. Secondly, one can turn to intensive studies of the scattered, surviving groups of so-called primitive man, on the assumption that the reproductive performance of these groups is a reasonable approximation to that obtaining during long periods in human prehistory. Primitive groups undergo rapid demographic changes subsequent to their contact with technologically more advanced cultures. Some of the reasons for this are obvious-introduction of new diseases; restriction -of tribal territory, with changes in nutritional patterns; changes in ancient mores with contact with Christian missionaries-but there remain additional, poorly understood reasons. Be this as it may, there is at best a brief period during which the observer can obtain a reasonably accurate view of the precontact demography, and, even then, collecting data of the requisite accuracy is rendered difficult by the lack of written records, short life spans, and, frequently, a reluctance to mention dead relatives. Furthermore, of course, there is no assurance that the pattern of the surviving primitives is typical of our ancestors. But if, on the other hand, there are demographic similarities between all or most such groups that clearly set them apart from civilized man, one can have confidence in the emergence of certain common denominators. During the past five years, we and our associates have undertaken a broadbased investigation, which includes demographic studies, of two tribes of South American Indians, the Xavantes of the Brazilian Mato Grosso and the Yanomama of southern Venezuela and northern Brazil. The former enjoyed a brief period of peaceful contact with Brazilian soldiers and colonizers in the late