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took as they roamed over Africa, Europe, 

 the Middle East, and far Asia. Can the 

 traces of these migrations be detected by 

 looking at allele frequencies, or does the 

 spread of culture overwhelm the spread of 

 genes? A striking cline — or regular grada- 

 tion — of frequencies of alleles of some 

 genes, such as ABO, extends across Eu- 

 rope and correlates in space and time with 

 the spread of farming from its source in 

 the Middle East. A likely explanation is 

 that farmers, able to multiply in numbers 

 faster than their neighboring hunter-gath- 

 erers, overwhelmed and mixed with them. 

 This new, slightly mongrelized group of 

 farmers repeated the process as farming 

 spread to the north and west. On the other 

 hand, a much more recent spread of 

 Bantu-speaking peoples accompanied by 

 agriculture in southern Africa has not left 

 such obvious traces on the genes. 



And finally, is there any correlation be- 

 tween the traces of migration seen in some 

 of our genes and the spread and history of 

 human languages? Sometimes. Again, a 

 fairly striking correlation is found in Eu- 

 rope. In Austraha, however, no correlation 

 can be seen among the genes of the abo- 

 riginal populations, the distribution of 

 their languages, and the fairly simple pat- 

 terns of colonization from Australia's 

 north that we know must have taken place 

 starting some 60,000 years ago. 



The book is unlikely to settle any of 

 these controversies and, indeed, is certain 

 to stir some because of its unabashedly 

 idiosyncratic methods. The authors state at 

 the outset that they are going to concen- 

 trate on their own methods of interpreting 

 the data, because giving full justice to the 

 approaches of others would make the 

 book far longer than its current thousand 

 or so very large pages. The authors are to 

 be commended, however, for laying out 

 all the data, warts and all, showing how 

 they analyzed it, and hedging virtually all 

 their conclusions with the caveats that im- 

 perfect data demand. 



The first problem with this compilation, 

 impressive as it is, has to do with the im- 

 mense span of time for which we have no 

 genetic data. Because our genetic portrait 

 of humankind is necessarily based on re- 

 cent sampUngs, it is unavoidably static. 

 Historical records of human migrations 

 cover only a tiny fraction of the history of 

 our species, and we know surprisingly ht- 

 tle about how long most aboriginal peo- 

 ples have occupied their present homes. 

 Language, too, is so labile and so easily 

 overwhelmed by history that it can only 

 take us, at the most, ten or twenty thou- 



sand years into the past. We are pretty 

 close to the position of a viewer who tries 

 to infer the entire plot of the film Queen 

 Christina from the few final frames show- 

 ing Garbo's rapt face. 



Once we have looked as far back as we 

 can — to the invention of agriculture and a 

 little way into the Neolithic — how much 

 more of our distant history can we infer 

 from the present-day distribution of alle- 

 les? Very little, I think. As the authors ac- 

 knowledge throughout the book, the dis- 

 tribution of genes has as many 

 explanations as there are genes them- 

 selves. 



Take the Duffy blood group. One allele 

 of this gene confers absolute immunity 

 against a particular type of malaria. This 

 allele is present in sub-Saharan Africa be- 

 cause of malaria — not migration — and 

 may have made its appearance only tens of 

 thousands of years ago. The ABO poly- 

 morphism, on the other hand, is miUions 

 of years old, and therefore probably far 

 more complex than that of Duffy. Even 

 though we have known about it for the 

 better part of a century, we have still not 

 managed to discover the major reason that 

 we (and our close relatives the great apes) 

 have this polymorphism. 



Many of the maps in The History and 

 Geography of Human Genes were con- 

 structed with a technique called principal 

 component, or PC, analysis, which 

 sounds — and is — dauntingly statistical. 

 To construct one of the maps, eighty-two 

 genes were examined in many populations 

 throughout the world. Each population 

 was represented on a computer grid as a 

 point in eighty-two dimensional space, 

 with its position along each dimensional 

 axis representing the frequency of one of 

 the alleles in question. The line, rotated 

 through all the dimensions, that best fits all 

 the points is called the first PC. It can also 

 be understood as the measure that best 

 summarizes all the variables. Other PCs 

 can be obtained that summarize the left- 

 over data. 



Suppose that all our genes behaved the 

 same way — that is, they all had alleles with 

 a high frequency in Africa, intermediate 

 frequencies in Europe and Asia, and even 

 lower frequencies in Austraha. Then the 

 first PC would account for all or most of the 

 information in the data set. It is just such a 

 pattem that the eighty-two-gene map ap- 

 pears to show. Tliis is misleading, however, 

 because the first PC accounts for only about 

 a third of the data, and the other two thirds 

 are made up of conflicting trends. Which, if 

 any of them, do we beheve? 



84 Natural History 6/94 



