Introduction

Modern humans emerged in Africa ~200 thousand years ago (kya), and expanded from an estimated census size of just over a million individuals (Hawks et al., 2000) during the Middle Stone Age (MSA) to ~7 billion inhabitants today (www.census.gov/main/www/popclock.html). The long-term human effective population size (i.e. the number of breeding adults) is inferred to be ~10 000 individuals based on studies of variation in proteins (Haigh and Smith, 1972; Takahata, 1993), autosomal DNA (Erlich et al., 1996; Takahata, 1993; Tenesa, 2007), X chromosome DNA (Cox et al., 2008), Y-chromosome DNA (Takahata, 1993), and mitochondrial DNA (Takahata, 1993). After migrating from Africa within the past 50–100 kya, modern humans have successfully colonized most of the terrestrial parts of the globe. Genetic evidence indicates that human populations underwent several major population expansion and migration events over the last 100 kya (Atkinson et al., 2009; Cox et al., 2009; Excoffier and Schneider, 1999; Harpending et al., 1993; Rogers and Harpending, 1992). Patterns of mtDNA and nuclear genetic diversity indicate that there was an initial major population expansion in Africa ~110–70 kya (Atkinson et al., 2009; Excoffier and Schneider, 1999; Harpending et al., 1993; Rogers and Harpending, 1992) and subsequent migration and expansion events around the globe at periods ~60–55 kya, ~40–25 kya, and ~12 kya (Atkinson et al., 2009; Cox et al., 2009; Harpending et al., 1993; Liu et al., 2006). Modern humans evolved during a hyper arid period in Africa and might have been restricted to parts of Africa with favorable ecological conditions (Ambrose, 1998a; Hawks et al., 2000). Population expansions and migrations during the Pleistocene, followed by exponential expansion during the Holocene, were correlated with shifts in climatic conditions and technological innovation (Pennington, 1996; Powell et al., 2009). Migration has played an important role in shaping the pattern of genetic and cultural variation in African populations (Cox et al., 2008), which currently consist of >2000 diverse ethnolinguistic groups, representing ~30% of the linguistic diversity in the world (Haspelmath et al., 2008; Lewis, 2009). Here we summarize the paleoclimatic conditions, major cultural and linguistic shifts, and major migration events in Africa which have contributed to the current patterns of genetic variation in the continent.