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 modern populations—1,500 men from 60 populations in Europe, Asia, Africa, and Australia—Hammer was able to reconstruct a paternal family tree for the Y chromosome.
He found that the yap dna of some modern African men seems to have descended from much older yap in Asian populations. This leads us to believe that there was a significant period of evolutionary time when people in Asia evolved their own types of Y chromosomes, and then later spread that to Africa, says Hammer. After the African exodus, he thinks, early modern humans spent thousands of years living somewhere north of the Himalayan Mountains but south of Siberia before some returned to Africa between 50,000 and 10,000 years ago.
A sojourn in Asia is also supported by the work of population geneticist Rosalind Harding at Oxford. Harding conducted a study similar to Hammer’s but looked at a different stretch of dna from populations in Africa, Asia, Australia, Papua New Guinea, and Europe. She found evidence for a migration from Asia back to Africa more than 50,000 years ago. Instead of the simple picture of modern humans originating in Africa and then colonizing the rest of the world, she says, the new evidence presents a view of human origins that is more messy and confusing—and therefore probably more realistic. There was a lot of diversity generated in Asia, she says, and some of it went back to Africa.