GENETIC ADMIXTURE IN THE LATE PLEISTOCENE

The replacement hypothesis of modern human origins holds that the orig inal population of modern humans expanded throughout the world, replac ing existing archaic populations as it went. If this expanding populat ion interbred with the peoples it replaced, then some archaic mitochon dria might have been introduced into the early modern gene pool. Such mitochondria would be recognizable today because they should differ fr om other modern mitochondria at several times the number of sites that we are used to seeing in pairwise comparisons. In this paper we ask w hat can be inferred from the absence of these ''divergent'' mitochondr ia from modern samples. We show that if the effective number of female s in our species has been large for the past 40,000 years, then the le ver of admixture must have been low. For example, if this effective nu mber exceeded 1.6 million, then we can reject the hypothesis that more more than 2/1,000 of the mitochondria in the early modern population derived from admixture with archaic peoples. We argue elsewhere that r egional continuity would be detectable in the fossil record only if th e rate of admixture exceeded 76%. Here, we show that this level of adm ixture would require the effective female size of the human population to have been less than 1,777 for the past 40,000 years. (C) 1996 Wile y-Liss, Inc.