The out-of-Africa theory may be in the ascendant, but it got a little more complicated this past year. Geneticist Michael Hammer of the University of Arizona in Tucson reported evidence that after the initial exodus roughly 100,000 years ago, some human ancestors in Asia may have migrated back to Africa, leaving a genetic stamp on populations there that was later carried out of Africa again on subsequent migrations.
Hammer’s argument is based on his analysis of a small stretch of dna called yap. Located on the Y chromosome, it is the male equivalent of mitochondrial dna—it doesn’t code for a protein, and it passes from father to son altered only by a steady accumulation of random mutations. As a result, populations that have interbred recently will have more similarities in their yap dna than will populations that have long been separated. By looking at the differences in yap dna in ...
