This paper proposes that the worldwide transition to an anatomically modern human form was caused by the diffusive spread from Africa of a genotype-a coadapted combination of novel genes-carrying a complex genetic advantage. It is suggested that the movement out of Africa was not a migration but a "diffusion wave"-a continuous expansion of modern populations by small random movements, hybridization, and natural selection favoring the modern genotype. It is proposed that the modern genotype arose in Africa by a shifting-balance process and spread because it was globally advantageous. It is shown that the genotype could have spread by directionally random demic diffusion, but only under conditions involving a low rate of interdeme admixture ("interbreeding") and strong selection. This mechanism is investigated using a quantitative model that suggests explanations for many puzzling aspects of the genetic, fossil, and archaeological data on modern human origins. The data indicate significant genetic assimilation from archaic human populations into modern ones. A morphological advantage of the modern phenotype-possibly reducing childbirth mortality-is proposed as the cause of the transition. The evidence of this and previous human "revolutions" suggests that the shifting-balance process, proposed by Sewall Wright, was particularly important in human evolution -possibly because human populations had a small-deme social structure with low interbreeding rates that allowed it to operate. This may explain the relative uniqueness of human evolution. © 2002 by The Wenner-Gren Foundation for Anthropological Research. All rights reserved.