Moving Out of Africa
The first detected event is an expansion out-of-Africa into Eurasia dated to 1.9 million years ago using a molecular clock—an inference confirmed by the fossil record. Unlike any other model of human evolution at the time, a second expansion of nonmodern humans out-of-Africa into Eurasia occurs about 650,000 years ago, which corresponds well to the expansion of the Acheulean tool culture out of Africa.
These Acheulean populations did not replace the Eurasian populations they encountered, but rather admixed with them. Moreover, after Acheulean expansion, there was significant, albeit limited, recurrent gene flow between Eurasia and sub-Saharan Africa.
Next, there was a third major expansion of humans out-of-Africa into Eurasia at 130,000 years ago, corresponding to the fossil record of the spread of anatomically modern humans out-of-Africa beginning 130,000 to 125,000 years ago and reaching far Eastern Asia by 110,000 years before present.
Like the earlier Acheulean expansion, this third expansion out of Africa resulted in low levels of admixture with Eurasian populations, not complete replacement. Indeed, the hypothesis of replacement (zero admixture) was rejected with a 10-17 probability level, making this the most definitive conclusion of the analysis.
After this third out-of-Africa expansion, human populations subsequently expanded into Northern Eurasia (including northern Europe), the Americas, and the Pacific. Wherever humans went, patterns of genetic interchange were soon established (Templeton, 2005).
These results were extremely controversial when first published because of their strong rejection of the out-of-Africa replacement model, which posited that anatomically modern humans, when they expanded out of Africa into Eurasia, drove all the native Eurasian populations to complete extinction with no admixture. Even a mostly out-of-Africa expansion was anathema to the replacement advocates because a low level of admixture can still have strong evolutionary consequences.
Interestingly, the out-of-Africa replacement hypothesis rose in popularity due to a genetic analysis based on mitochondrial DNA (Cann et al., 1987). The evolutionary history of human mitochondrial DNA consisted of African-only branches at the most ancient part of the history followed by a mix of African and non-African branches, with no deep divergences (that is, branches separated by many mutational changes with no intermediates present in current populations).
In the 1980s, there were three principle models for human evolution: 1) the out-of-Africa replacement model; 2) the multiregional model that proposed that humans evolved toward modernity across the globe because of genetic interchange between African and Eurasian populations, and 3) the candelabra model that posited that Africans, Europeans, and East Asians evolved to modernity as separate evolutionary lineages that diverged after the initial expansion out of Africa with little to no gene flow since then.