ARCHAIC AFRICAN AND ASIAN LINEAGES IN THE GENETIC ANCESTRY OF MODERN HUMANS

A 3-kb region encompassing the beta-globin gene has been analyzed for allelic sequence polymorphism in nine populations from Africa, Asia, a nd Europe. A unique gene tree was constructed from 326 sequences of 34 9 in the total sample. New maximum-likelihood methods for analyzing ge ne trees on the basis of coalescence theory have been used. The most r ecent common ancestor of the beta-globin gene tree is a sequence found only in Africa and estimated to have arisen similar to 800,000 years ago. There is no evidence for an exponential expansion out of a bottle necked founding population, and an effective population size of simila r to 10,000 has been maintained. Modest differences in levels of beta- globin diversity between Africa and Asia are better explained by great er African effective population size than by greater time depth. There may have been a reduction of Asian effective population size in recen t evolutionary history. Characteristically Asian ancestry is estimated to be older than 200,000 years, suggesting that the ancestral hominid population at this time was widely dispersed across Africa and Asia. Patterns of beta-globin diversity suggest extensive worldwide late Ple istocene gene flow and are not easily reconciled with a unidirectional migration out of Africa 100,000 years ago and total replacement of ar chaic populations in Asia.