Box Recent African Origins hypothesis

This hypothesis proposed the origins of anatomically modern humans as a small isolated ancestral population in Africa from which a number of individuals emigrated to colonize the rest of the world and replace, without significant genetic mixing, archaic (pre-modern) human populations such as the Neanderthals in Europe (Fig. 8.7) (Stringer and Andrews 1988). In terms of genetic diversity, the migrating individuals represented a limited subset of African genetic variation. Subsequently much of this diversity is thought to have been lost by genetic drift during repeated or severe transient reductions (bottlenecks) in population size. By contrast, the remaining African populations retained and continued to accumulate genetic diversity.

Figure 8.7 Dispersal of modern humans from sub-Saharan Africa. Earliest fossil and archaeological evidence (dates shown) supports genetic evidence for dispersal from Africa within the last 100 000 years. Reprinted by permission from Macmillan Publishers Ltd: Nature (Hedges 2000), copyright 2000.

populations (Watkins et al. 2003). Clustering of non-African populations is seen in such analyses, with separation from African populations by a long branch (Fig. 8.6) (Xing et al. 2007). The analysis of polymorphism in mobile DNA elements has provided important evidence supporting the Recent African Origins hypothesis (Box 8.5) (Xing et al. 2007).

The analysis of neutral human genetic diversity has provided important insights into our evolutionary past, complementing data from paleontological and archaeological research (Garrigan and Hammer 2006). Analysis of diversity in mitochondrial DNA and the Y chromosome provided significant early data to support a single origins model of anatomically modern humans in Africa (Box 8.5) (Cann et al. 1987; Stringer and Andrews 1988; Vigilant et al. 1991). More recent data analysing genetic diversity from the X chromosome, and among the different autosomes, suggest significant heterogeneity within the human genome in terms of evolutionary history and greater time to our most common ancestor (Harding et al. 1997; Harris and Hey 1999; Garrigan and Hammer 2006). Rather than completely replacing the archaic Homo populations without admixture, there may have been significant genetic contributions from these groups to anatomically modern humans.

Garrigan and Hammer (2006) describe how genetic diversity data may relate to different models for human origins. In contrast to a single origin model from a single geographically localized interbreeding population (deme), the multiple origins models propose gene flow among a number of ancestral demes before the transition to anatomically modern humans, or after the emergence of that population (Fig. 8.8). The unexpectedly distant origin of different haplotypes (Box 2.8) within the current human genome support the descent of anatomically modern humans from multiple archaic Homo subpopulations but this remains controversial. Many important questions remain as to the origins of anatomically modern humans and their subsequent global spread. The effective ancestral population size is thought to have been small, approximately 10 000 individuals (in contrast to great apes such as gorilla and orangutan). Fossil evidence supports an origin of anatomically modern humans in the last 200 000 years: hominin skulls from the Omo Valley in Ethiopia, for example, have been dated to 195 000 years ago (McDougall et al. 2005).

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