Assimilation of Archaic Humans into Modern Human Populations
Posted: Thursday, December 26, 2002
by Vinayak Eswaran
From: Current Anthropology
A Diffusion Wave out of Africa: The Mechanism of the Modern Human Revolution?
CURRENT ANTHROPOLOGY Volume 43, Number 5, December 2002
EVIDENCE OF HYBRIDIZATION
One interesting aspect of the interaction between modern and archaic humans is that hybridization may have occurred between them. Even proponents of the recent-African-origin model do not deny such hybridization, even if they discount the possibility that it finally led to genetic assimilation. The genetic evidence seems to show few obvious signs of assimilation. That few fossils of clearly hybrid morphology (e.g., Duarte et al. 1999) have been found also suggests that little if any hybridization occurred.
However, this model indicates that the latter inference is not necessarily correct. It shows that it is possible that there was a progressive and complete hybridization of the archaics at the wave front even while there was a low rate of assimilation of archaic neutral genes into the emergent modern population. The model further suggests that, far from being an occasional occurrence of little significance, hybridization, along with natural selection for anatomical modernity, could have been the principal reason for the disappearance of the archaic morphology, thus explaining the apparent "extinction" of archaic humans.
The simulations show that African parentage levels full sharply to zero at the wave front, implying that gene flow is associated only with the wave and does not penetrate beyond. So, if the advantage of anatomical modernity was fundamentally linked to morphology, all signs of hybridization would have appeared only at the wave front; hybrids that attained full modernity would have shown few signs of hybridization, at least in their primary metrics. The simulations show the wave front to be barely 800 km in width, and the region within which clear signs of hybridization would have appeared could have been as narrow as 300 km. Hybrids ahead of this narrow region were close to archaic, while those behind were essentially modern. Thus if the diffusion wave traveled through 3,000 km of Europe between 45,000 and 25,000 years ago, only 10% of the fossils of that period could be expected to have clearly mixed morphologywhich may explain the rarity of obvious hybrids in the fossil record.
Another empirical observation that could be explained by these simulations is the relatively rapid transition that has been recorded to occur at the local level (Mellars 1989). The transition from "progressive" archaic to "essentially" modern could have taken as little as 2,000 to 3,000 years, the time required for the 300-km core of the wave front to pass over a site.
DISAPPEARANCE BY HYBRIDIZATION
The archaics at the wave front would have been progressively hybridized even while the fraction of moderns there increased because of natural selection. An obvious way to demonstrate this hybridization is in terms of the archaic/modern parentage ratiothe ratio of the average African parentage of the local archaics (including not-fully-modern hybrids) to that of moderns in the same area. Because the African parentage of a "pure" archaic is always zero, any nonzero value is due to hybridization. As the maximum African parentage in an area would presumably be found in the moderns, the archaic/modern parentage ratio would normally be less than or equal to unity. The ratio could be used to measure the degree of hybridization in the local archaics, larger values of the ratio indicating a more hybridized population. A ratio of zero would indicate that no hybridization had occurred. A ratio of close to unity would indicate that the local archaics had nearly as many African neutral genes as the local moderns, which could only be due to a complete hybridization of the archaics.
FURTHER EVIDENCE OF ASSIMILATION
Other evidence suggests that assimilation did occur. For example, the unique and ancient ( > 200,000-year-old) polymorphisms that exist in some Asian populations and the high coalescence time obtained in a Melanesian population (Harding et al. 1997, Fullerton et al. 1994) can most easily be explained by archaic assimilation into modern populations. Similar genetic evidence from another locus determining hair and skin pigmentation has suggested assimilation from Eurasian archaic populations (Harding et al. 2000).
There is also fossil evidence in Asia of morphological continuity across the archaic-modern transition (Pope 1992), reinforced by fossil evidence from across the world (Wolpoff et al. 2001). Assimilation could also explain the long-standing continuity of anatomical features such as shovel-shaped incisors in East Asians. Many such traits were presumably neutral. Simulations of the Monte Carlo model (not shown here) indicate that single advantageous alleles would have been even more readily assimilated from archaic populations. Such assimilation from resident archaics could have aided the genetic adaptation of the moderns to local ecological conditions as the wave moved far from Africa.
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