If humans are 99.9 percent genetically identical, as President Bill Clinton is fond of asserting when he extols the Human Genome Project, that 10th-of-a-percent difference has a lot of explaining to do. How does genetic variation determine a person's unique physical traits? Can it predict someone's susceptibility to a disease?

Such questions, pertaining to the present or future, are what occupy most human geneticists. A small group, however, studies genetic variation as a clue to the past. Sometimes called molecular anthropologists, these researchers use DNA polymorphisms, or markers, to hypothesize about human evolution and population migrations. An estimated 20 labs worldwide are intensively engaged in this work, and a growing number are peripherally involved.

Two developments lately have boosted their endeavors. Scientists have discovered enough useful DNA markers over the past 20 years to form a critical mass. And techniques and equipment, some of which are spillover from genome sequencing...

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