Population Bottleneck

Social and technological innovations in Africa during the later Middle Stone Age and early Later Stone Age (50,000 to 70,000 years ago) may Figure 3. Unrooted family tree diagram of mitochondrial DNA control region sequences of a worldwide sample of 811 humans, and 332 chimpanzees from four regions of Africa. Branch length is proportional to the amount of genetic difference between individuals, and to the time since they shared a common ancestor. Symbols indicate individual chimpanzees belonging to the same social group. West African chimpanzees have three common ancestral lineages. Genetic differences between individuals in the same group substantially exceed differences between all living humans, and between humans and Neanderthals. have facilitated population expansions and colonizations by enhancing survival in arid, unpredictable ice age environments. New stone tool technologies may have increased foraging efficiency and food supply. A system of mutual reliance and cooperation between distant foraging groups, mediated by reciprocal gift exchange, may have also increased humans' ability to survive in unpredictable environments. Further social and technological innovations may have facilitated population expansion within Africa, dispersals out of Africa, and the replacement of archaic populations, including Neanderthals, by anatomically modern humans outside of Africa.

Low levels of modern human diversity thus reflect our recent African ancestry and the effects of several population bottlenecks. In a similar fashion, colonization bottlenecks promoted rapid differentiation of northwestern Eurasians and southeastern Australasians.