Repeated failures that led to the eventual success in human evolution.

The effective population number estimated in humans is 10,000 (Nei and Gram1984; Nei 1987; Takahata 1993 ) . This rather small number may contradict the fact that humans had been occupying vast areas in Africa and Eurasia for a long time since Homo erectus first migrated out of Africa l-2 Mya (e.g., see Lewin 1984; Fagan 1990). One possible cause is a recent bottleneck, or a sudden reduction in population size, but a severe bottleneck is incompatible with the unusual polymorphism at the major histocompatibility complex (Mhc) loci (Takahata 1990; Klein et al. 1993). In this letter, I argue that a more likely cause is frequent extinction and recolonization (turnover) of subpopulations in the lineage leading to modern humans. Wright ( 193 1) introduced the concept of effective population number (N,) in order to reduce complicated breeding structure in actual populations to the equivalent ideal case (Crow and Kimura 1970, p. 345 ) . In an ideal population, N, is equal to the number of breeding individuals, whereas in an actual population N, depends on various factors such as sex ratio, differences in offspring numbers, temporal changes in population size, and overlapping generations (for review, see Wright 1969). Wright ( 1969) also pointed out that if ( 1) a species has subdivided population structure and (2) local populations are liable to frequent extinction with restoration of a few stray immigrants, the species passes repeatedly through extremely reduced states of N,. Slatkin ( 1977), as well as Maruyama and Kimura ( 1980), formulated this problem on the basis of a traditional approach in population genetics: that is, to use the variance of gene frequencies and the probability of identity between two randomly chosen genes. Maruyama and Kimura ( 1980) derived formulas of N, under the finiteisland model of population structure (Malecot 195 1; Maruyama 1970) and the stepping-stone model (Kimura and Weiss 1964). Their approximate formula for a diploid species in the finite-island model is given by