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Putting 



Human 



Genes on 



the Map 



With decades of data and big 



computers, scientists are 



beginning to visualize the 



complexities of human diversity 



by Christopher Wills 



Back in the late 1970s, I had the plea- 

 sure of visiting the laboratory — or perhaps 

 more properly the lair — of Arthur E. 

 Mourant. It was hidden away in the far re- 

 cesses of the British Museum of Natural 

 History in London. Mourant, a genial man 



The History and Geography of Human 

 Genes, by Luigi Luca Cavalli-Sforza, 

 Paolo Menozzi, and Alberto Piazza. 

 Princeton University Press, $175; 1,032 

 pp., illus. 



who looks rather like Mr. Punch, presided 

 over a large room lined with cabinets filled 

 to overflowing with papers. For decades, 

 he and a few devoted co-workers had kept 

 track of our growing knowledge of the 

 human gene pool, summarizing the work 

 of thousands of scientists in huge com- 

 pendiums. He had provided scientists 

 working on human evolution and varia- 

 tion with a distillation of studies that had 

 been written in a dozen languages, in a 

 hundred parts of the world. We spent a 

 couple of fascinating days going over 

 some of the reams of data that he had col- 

 lected and speculating about their mean- 

 ing. Among other things, he showed me 

 the proofs of a new book he had just fin- 

 ished on human genetic variation and dis- 

 ease. 



The gray columns of figures in this 

 book were a treasure trove. The first con- 

 nection between stomach cancer and the 

 ABO blood groups had been published in 



1953. By the time Mourant summarized 

 the literature in 1978, an astonishing 5,000 

 studies had looked for connections be- 

 tween ABO blood groups and virtually 

 every major disease. About 15 percent of 

 them showed an association. 



Other gray columns of his figures told 

 about another, less-known human blood 

 group called MN, which is confined to the 

 surface of our red cells. So minor is it that 

 it is usually ignored by our own immune 

 system and, unlike ABO blood groups, it 

 is not important in transfusion or tissue re- 

 jection. Strenuous efforts by many re- 

 searchers have not been able to detect any 

 association between the MN blood groups 

 and disease. 



Yet virtually every human population 

 has the M and N forms of this trait in vary- 

 ing proportions. Why are both so perva- 

 sive, and why is not our entire species type 

 M or type N? Is it simply accidental or are 

 selective forces at work? And what does 

 the distribution of these and other variant 

 forms of genes tell us about the history and 

 current state of our species? What indeed 

 can it tell us if all the genes that have been 

 discovered turn out to be as different as 

 ABO and MN? 



A new book by Cavalh-Sforza and his 

 collaborators, as massive as anything put 

 together by Mourant, attempts to answer 

 some of these questions. It is an immense 

 and laudable undertaking that pulls to- 

 gether the information on many genes 

 that, like the ABO blood group gene, are 

 polymorphic — that is, they exist in the 

 population in a variety of types called al- 

 leles. Much of the data had been gathered 

 in raw form by Mourant, with later addi- 

 tions by Mourant's co-workers and by 

 Cavalli-Sforza's group. More than 75,000 

 allele frequencies, measuring the preva- 

 lence of various alleles in nearly 7,000 

 human populations, are summarized — not 

 in the gray columns of Mourant's compi- 

 lation but in the form of maps and statisti- 

 cal analyses that make trends in the data 

 far more obvious and accessible. 



The book begins with a survey of the 

 methods used in analyzing the data and 

 then moves on to an overview of the ge- 

 netic and cultural histories of our species 

 on a worldwide scale. Succeeding chap- 

 ters deal with each continent in turn. The 

 book is nothing less than an attempt to re- 

 late the physical appearance, language, 



and culture of the far-flung members of 

 our extremely variable species to the evi- 

 dence of the genes. In the course of this ti- 

 tanic enterprise, the book summarizes 

 how much we have learned and shows 

 how far we still have to go. 



What are the many controversies that 

 the book hopes to cast Ught on? One is the 

 origin of our species itself. Did we arise 

 within the last one or two hundred thou- 

 sand years in Africa and spread through- 

 out the rest of the Old World, sweeping all 

 the poor hominids already resident there 

 into the ash heap of history? Or did we 

 arise from our immediate ancestor. Homo 

 erectus, in a series of parallel events in 

 various parts of the Old World, aided per- 

 haps by puzzhng and highly specific flows 

 of genes conferring human rather than 

 prehuman characteristics on our diverse 

 ancestors? While admitting that all the ev- 

 idence is not in, the authors come down on 

 the side of a single origin. 



A second question that the authors 

 spend a good deal of time on is the matter 

 of races. While our species is highly di- 

 verse both physically and genetically, the 

 patterns are so complex that it is impos- 

 sible to divide us into races in any consis- 

 tent way. For example, an earher genera- 

 tion of anthropologists classified the Ainu 

 of northern Japan as Caucasian because of 

 the abundant hair on their bodies, the lack 

 of an epicanthic fold on the upper eyelid, 

 their wavy brown hair, and pale skin. But 

 then- genes place them squarely among the 

 peoples of eastern Asia. The San (Bush- 

 men), at the other end of the Old World, in 

 southern Africa, have flattened faces of 

 rather Asian appearance — though again 

 without an epicanthic fold — and yellow 

 rather than dark skin. Yet the frequencies 

 of their various alleles, although unusual 

 in some respects, resemble those of their 

 African neighbors. 



The authors do not attempt an explana- 

 tion. But I suspect that since our species is 

 blessed with an abundant variety of alleles 

 of genes that contribute to outward ap- 

 pearance, a little mixing, matching, and 

 sorting out would have been quite enough 

 to have produced — anywhere on the 

 planet — the relatively trivial differences in 

 appearance on which we put so much 

 weight when we classify people into races. 



A third question concerns the various 

 patterns of migration our recent ancestors 



82 Natural History 6/94 



