Putting Human Genes on the Map - the History and Geography O
Back in the late 1970s, I had the pleasure of visiting the laboratory--or perhaps more properly the lair--of Arthur E. Mourant. It was hidden away in the far recesses of the British Museum of Natural History in London. Mourant, a genial man 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 compendiums. He had provided scientists working on human evolution and variation 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 collected and speculating about their meaning. Among other things, he showed me the proofs of a new book he had just finished on human genetic variation and disease.
The gray columns of figures in this book were a treasure trove. The first connection 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 between 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 rejection. Strenuous efforts by many researchers 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 varying proportions. Why are both so pervasive, 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 Cavalli-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 together 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 alleles. Much of the data had been gathered in raw form by Mourant, with later additions by Mourant's co-workers and by Cavalli-Sforza's group. More than 75,000 allele frequencies, measuring the prevalence of various alleles in nearly 7,000 human populations, are summarized--not in the gray columns of Mourant's compilation but in the form of maps and statistical 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 genetic and cultural histories of our species on a worldwide scale. Succeeding chapters deal with each continent in turn. The book is nothing less than an attempt to relate the physical appearance, language, and culture of the far-flung members of our extremely variable species to the evidence of the genes. In the course of this titanic 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 light on? One is the origin of our species itself. Did we arise within the last one or two hundred thousand years in Africa and spread throughout 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 perhaps by puzzling and highly specific flows of genes conferring human rather than prehuman characteristics on our diverse ancestors? …