Quick review. In the 19th century once the idea that humans were derived from non-human ancestral species was injected into the bloodstream of the intellectual classes there was an immediate debate as to the location of the proto-human homeland; the Urheimat of us all. Charles Darwin favored Africa, but in many ways this ran against the cultural grain. The theory of evolution was birthed before the highest tide of the age of white supremacy and European hegemony, and Darwin's model had to swim against the conviction that Africans were the most primitive of the colored races. After the waning of the ideological edifice of white supremacy, and the shock it received during and after World War II, the debates as to the origin of humanity still remained contentious and followed the same outlines (though without the charged normative inferences). But as the decades wore on many more researchers began to believe that Darwin was correct, and that the origin of humanity lay in the African continent. First, the deep origin of the human lineage in Africa was accepted, but eventually a more recent expansion out of Africa was argued for by one school. The turning point in these academic disputes was the popularization of the "mitochondrial Eve" theory of the 1980s. What some paleontologists had long argued, that anatomically modern humans have their locus of origin in Africa, was supported now by research from genetics which indicated that Africans were the most basal clade of humans on a continental scale, so that non-Africans could be conceived of as a subset of Africans. From this originates the chestnut of wisdom that Africans have more genetic diversity than all other human populations combined. By the year 2000 one could say that the "Out of Africa" triumphalism had proceeded to the point where an almost exterminationist model had taken hold when it came to the relationships of anatomically modern H. sapiens, and other groups which had evolved outside of Africa over the past million or so years, such as the Neandertals.
But the theoretical dichotomies were too coarse and absolute as it turns out. A division between multiregionalist phyletic gradualism, where H. sapiens evolved out of its hominin ancestors concurrently on a world wide scale, and a model of rapid expansion of one tribe in Africa to replace all others in totality, may have been warranted in the age of classical genetics and a morphometric analysis, but now we can look at the raw genomic material in a more fine-grained fashion. In fact, we can now look at the genomic patterns of variation among extinct hominins! Though there have long been hints that the expansion-and-replacement paradigm was too extreme from the genetic and morphological data, with the publication last spring in Science of a paper which made the claim for admixture between Neandertals and non-Africans in the range of 1-4% in all non-African groups based on a comparison of Neandertal and modern human genetic variation, one can dismiss absolutist expansion-and-replacement as self-evidently true orthodoxy. But one orthodoxy has no given way to another, and the shock to the old models presented by the data has not resulted in the coalescence of new robust paradigms. We live in a time of scientific troubles, so to speak. One of the more notable results in the Science paper from last spring was that all non-Africans had about the same admixture in relation to the Neandertal reference genome, ~1-4%. This means from the Orkneys to New Guinea. Because Neandertals were distributed only in the western half of Eurasia this implies that the admixture was an early event. By the time of modern human expansion across Eurasia, Australasia, and the New World, it had become equally distributed across the individuals within the population. Recall the contrast between African Americans and Uyghurs. Among the Uyghurs the ancestral quanta are equitably distributed from individual to individual, but among African Americans there remains substantial intra-population variance. The reason is that African Americans are quite new, an order of magnitude younger than the Uyghurs in a genetic sense, and admixture is still occurring into the African American population from the ancestral groups. The Uyghurs as we known them today genetically are probably ~1,000-2,000 years old (though their cultural origins are both more and less ancient, as a matter of linguistics in the former, and ethnic self-conception as a Muslim East Turkic group in the latter). The implication here is clear: there was a pause in the Out of Africa movement, where the proto-non-Africans mixed with a Neandertal group, possibly in the Middle East, and only began a massive demographic expansion after an unspecified sojourn. A paper from last spring makes this all explicit:
A more likely explanation for the OoA bottleneck is that Eurasia was populated by a larger population that had been relatively isolated from other modern human populations for tens of thousands of years prior to the expansion. The first fossil evidence for modern humans outside of Africa is in the Middle East at Skhul and Qafzeh between 80,000-100,000 years ago, which is at least 20,000 years prior to the Eurasian diaspora. If a population of modern humans remained in the Middle East until the expansion into Eurasia, there would have been sufficient time for genetic drift to reduce heterozygosity dramatically before the Eurasia expansion. This “Middle East isolation” hypothesis provides a robust explanation for the relative homogeneity of European and Asian populations relative to African populations (see Figures 3A-B) and is supported by a recent maximum likelihood estimate of 140,000 years ago for the time of Eurasian-West African population separation . Interestingly, a recent study of the Neandertal genome suggests that the non-African individuals, but not the Africans, contain similar amount of admixture (1-4%) with the Neandertals . The authors suggest that the admixture must have happened between the Neandertals with an ancestral non-African population before the Eurasian expansion. Given the fossil, archaeological, and genetic evidence, the Middle East isolation hypothesis warrants rigorous evaluation as whole-genome sequence data become available.
