Diversity among human populations the result of inbreeding with pre-Homo sapiens species?
An interesting article about the origin of our species, Homo sapiens sapiens, which I found that presents the theory that the high diversity found among human populations today is the result of inbreeding between member of homo sapiens sapiens with predating species like Homo erectus. The article concludes that Homo sapiens sapiens developed outside of Africa somewhere in Aurasia from a proto-Homo sapiens sapiens species which left the African continent at an earlier point.
African Eve, Eurasian Adam
The Age and Origin of the Human Species
Ronald A. Fonda
The speciation event that produced Homo sapiens sapiens could not have occurred contemporaneously in more than a very few individuals. It follows that those few s. sapiens would have possessed a very restricted sample of the progenitor species’ genetic diversity. However, the diversity observed in current populations implies that there were never less than several thousand breeding pairs in the human ancestry (Harpending et al., 1998). Accordingly, the founding s. sapiens and their descendants must have interbred with the progenitor species (and perhaps other pre-human populations) in order to preserve the diversity which exists today. While some changes in the genome must have occurred after the speciation event, the “lifetimes” of the genetic elements considered (in the works cited here) are far longer than new estimates of s. sapiens’ age (Mountain et al., 1994). As a consequence, most of the current diversity must be the result of interbreeding with pre-human populations.
On this view we would expect to see the most hybridized elements of the modern indigenes in those areas where pre-human population density was highest, such as Africa and S. E. Asia. Also, we would expect those populations to have the greatest diversity today, because they would preserve more of the pre-human genome, which would have had much more genetic variety than was represented in the tiny, original population of s. sapiens.
In fact, we do find that Africans and some S. E. Asian populations have not only more diversity (Jorde et al., 1997), but central Africans have ancestral genetic elements as well (Tishkoff et al., 1996). It is also clear that the population which gave rise to s. sapiens had been separated from the sub-Saharan Africans’ ancestors for longer than our species’ lifetime.1
The evidence indicates that humans came from a sparse population in Eurasia; that their diversity was further reduced by the speciation event; that they subsequently expanded in every habitable direction; and that they interbred with the populations they came in contact with, producing extant hybrid populations. Hence Mountain et al. (1994) reports that in the cladistic tree “the European branch is significantly short relative to all other branches,” that “the neighbor-joining tree… places the European sample close to the center of the tree with an extremely short branch,” and further that “Europeans and northeast Asians are closely related.”
The Ngandong specimens, in particular, have occasioned much debate on account of their mixture of sapiens and erectus traits and their affinities with extant Australian populations.10 We would expect that the skulls of such hybrids would show affinities to both species, and that is why these fossils are so hard to classify. Some authorities say they are clearly erectus, while others point to modern traits, and especially that very similar skulls (from overlapping dates) are found in Australia. Moreover, the traits in question occur in the modern population. This is not merely consistent with, but constitutes strong evidence for, the view that radiating, low-diversity s. sapiens interbred with relic erectus populations to the extent that they acquired near-African diversity.
Yet another challenge exists to the claim that our species radiated out of Africa. There is a consensus among anthropologists that s. sapiens’ cultural artifacts display a higher level of cognitive function than all previous species. The technical level and diversity of their tool industry alone would have set them apart. Add to that whole new categories of behavior: the creation of representative art, the domestication of the dog etc. Thence we would expect that the populations which were hybridized with predecessor species would be intellectually and cognitively disadvantaged in relation to low-diversity, Eurasian populations. In fact we do observe that Eurasians are cognitively advantaged in comparison to high-diversity populations (Herrnstein and Murray, 1994), which clearly reveals the direction of species radiation. Expressing this view however is likely to attract such vehement abuse that few dare speak it openly. Only those few whose livelihood is not subject to the fiats of “wimmin and minorities” can openly state the truth on this subject, and even then their views are ruthlessly censored.
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