PART XI — HUMAN ADAPTATION TO ENVIRONMENTAL STRES5 



equality is somehow incompatible 

 with "all men are created equal," or 

 that the division of labor is based 

 on "natural inequalities." Thus, the 

 political doctrines of our society are 



considered to be based on unsound 

 biological assumptions. On the other 

 side, there seems to be resistance to 

 the idea that any racial characteristic 

 is adaptive; this is the result of ex- 



tending the concept of genetic fitness 

 to an ideal of individual worth. In 

 any case, the entire controversy is, 

 or ought to be, irrelevant to the 

 ideology or aims of our society. 



ASPECTS OF MAN'S ADAPTATION IN THE TROPICS 



From our vantage point in the 

 temperate zones, we look upon man 

 in the tropics as having undergone 

 extensive adaptations. In fact, how- 

 ever, man probably arose in tropical 

 zones, living in small bands in the 

 rain forest; from the standpoint of 

 evolutionary biology, it is we of the 

 highly civilized countries who are 

 now making adaptations to a rapidly 

 changing world. 



In recent years, some groups of 

 investigators have undertaken exten- 

 sive, multidisciplinary studies of the 

 surviving groups of primitive man, 

 these almost without exception in 

 tropical or subtropical zones. In the 

 broadest sense, it is the purpose of 

 these studies to define the popula- 

 tion structure of primitive man, and 

 to appreciate the various pressures 

 (disease, nutritional, etc.) which, in- 

 teracting with that structure, pro- 

 vided the milieu within which human 

 evolution has occurred. It seems ap- 

 propriate to point out that, in many 

 respects, ours is the first generation 

 of scientists to have the facilities for 

 studies of much deeper significance 

 than those of the past, and the last 

 generation to have the opportunity, 

 since relatively undisturbed primitive 

 man is rapidly disappearing from the 

 face of the earth. 



With respect to this matter of un- 

 derstanding better the population 

 structure of primitive man in the 

 tropics, the geneticist is concerned 

 with such matters as: the amount of 

 inbreeding; the extent of polygyny; 

 birth rates; neonatal, infant, and 

 child death rates; marriage patterns, 

 etc. These factors define the stric- 



tures that are placed on the evolu- 

 tionary process. 



Genetic Differentiation 



The studies of most groups of 

 investigators agree in suggesting that 

 early man in the tropics was charac- 

 terized by high levels of inbreeding. 

 They suggest that infertility was un- 

 common and the reproductive per- 

 formance of woman relatively uni- 

 form. On the other hand, because 

 of the institution of polygyny, male 

 reproductive performance was some- 

 what more variable than in the United 

 States today. The health of the chil- 

 dren appears to have been better 

 than those of most peasant popu- 

 lations. 



Studies of the frequency of a vari- 

 ety of genetic markers in the isolated 

 villages that comprise most tribal 

 populations reveal a marked degree 

 of genetic microdifferentiation — i.e., 

 there are rather large genetic dif- 

 ferences between the villages that 

 compose a tribe. These villages are 

 engaged in a constantly shifting pat- 

 tern of hostilities, one with the other; 

 that is, competition between demes is 

 a varied risk. This population struc- 

 ture is one that population geneticists 

 feel is particularly conducive to rapid 

 evolution. 



Health Patterns 



With respect to the matter of inter- 

 action with agents of disease, mem- 

 bers of these tropical populations 

 have unusually high levels of gamma- 

 globulin. Because of transplacental 



transfer, a child thus comes into the 

 world with considerable resistance to 

 local pathogens, conferred on it by 

 its mother. As the child comes into 

 active contact with the many disease 

 agents in its surroundings, it will to 

 some extent be protected by pla- 

 centally transmitted maternal anti- 

 bodies, so that it may have an oppor- 

 tunity to build up its own resistance 

 more slowly. 



The commitment of many of these 

 primitive groups to regulation of pop- 

 ulation numbers is noteworthy. By 

 a variety of means — intercourse ta- 

 boos following the birth of a child, 

 prolonged lactation, abortion, and in- 

 fanticide — the entry of new life into 

 the population is rather rigidly con- 

 trolled. Under these circumstances, 

 an infant may be nursed for as long 

 as three years; in general, the nutri- 

 tion of children is quite excellent. 

 In this respect, many civilized com- 

 munities may have lost an adaptation 

 well recognized by primitive man. 



Goiter — With respect to specific 

 diseases, a number of examples may 

 be cited as to how markedly many 

 of these people differ in their physio- 

 logical adaptations from ourselves, 

 and how rapidly these adaptations 

 are altered by acculturation. An out- 

 standing example is with respect to 

 iodine. Some of the least-touched 

 groups of South America live in 

 areas where iodine is in very short 

 supply, and yet they do not show 

 goiter. On the other hand, endemic 

 goiter is a prominent feature of civ- 

 ilized populations living under con- 

 ditions of short iodine supply. Studies 

 indicate that at least one primitive 

 group in South America (the Yano- 



378 



