How did archaic humans evolve into the different populations of Homo sapiens we see today? The answer has long divided anthropologists. Some opt for the ‘out-of-Africa’ model; others for the multiregional model.
According to the out-of-Africa model, we all descend from a small group that existed some 100,000 to 80,000 years ago somewhere in eastern Africa. This group had an advantage over other humans, perhaps a superior ability to construct mental models in all four dimensions (Dubreuil, 2008; Klein & Edgar, 2002). They thus grew in numbers and progressively replaced their more archaic rivals, first in Africa and then elsewhere after 60,000-40,000 BP. Extinction was thus the common fate of all archaic humans throughout Europe and Asia, be they Neanderthals, Neanderthaloids, or mysterious Hobbits.
This claim is disputed by the multiregional model, which asserts that archaic Europeans and Asians did not die out. In fact, they went on to provide most of the gene pool of present-day Europeans and Asians. Modern humans thus have regional differences that can be traced to longstanding continental differences among earlier hominids. These continental groups evolved in parallel from the archaic evolutionary grade to the modern one, with gene flow gradually spreading advantageous alleles from one group to another.
The multiregional model no longer has many supporters, at least not in its pure form. With the recovery of DNA from Neanderthal remains, it has become clear that any genetic continuity with modern humans must be minor at best. There is nonetheless some support for a hybrid model, i.e., our current gene pool largely comes from a population that expanded out of Africa, but with significant admixtures from archaic Europeans and Asians. This view finds backing in a recent paper by Wall et al. (2009).
Wall and colleagues compared genetic polymorphisms in subjects of European, West African (Yoruba), and East Asian descent. They first looked for gene loci whose alleles show multiple mutational differences—a sign that the polymorphism either is very old or has undergone some kind of diversifying selection (i.e., a balanced polymorphism). Among these loci, they next looked for those that lay close to other polymorphic loci whose alleles likewise show multiple mutational differences. The authors reasoned that the more the apparently ancient alleles clustered by continental origin (West African, European, or East Asian), the greater the likelihood that these alleles had entered the gene pool from local archaic groups that modern humans had encountered while spreading out of Africa.
And the results? Wall and colleagues found significant archaic admixture: 14% in Europeans and 1.5% in East Asians. Curiously, no estimate was made for the West African subjects. The authors simply state: “Interestingly, we also find evidence for ancient admixture in the Yoruba.”
Well, how much? I’d like to know because there is another estimate based on a different methodology. When Watson et al. (1997) studied mtDNA lineages in sub-Saharan Africans, 87% of the lineages seemed to originate in a series of population expansions that began some 80,000 years ago. One cluster of lineages, dated to c. 60,000 BP, defines almost all non-African humans. The remaining 13% look much older and seem to be “the relics of a less dramatic and more ancient expansion event across Africa.”
As for archaic admixture outside Africa, Wall and colleagues (like John Hawks) point to the Neanderthals and other local hominids. The admixture must have been specific to Europe and then East Asia because the European and East Asian subjects had different sets of apparently archaic alleles.
In contrast, Dienekes argues that the admixture occurred before modern humans began to spread out of Africa. The archaic alleles then persisted to varying degrees in the small founder groups that ‘budded off’ from the parent African population:
Thus, the expanding African population that eventually spilled over into Eurasia, would indeed be quite inbred and homogeneous, but its gene pool would also contain traces of the smaller, less successful African populations it had absorbed. Because of their low frequency, these traces would be more susceptible to extinction in the series of bottlenecks that led to Europeans on one side and East Asians on the other, with different sets of archaic genes preserved in either region.
I lean more to Dienekes’ explanation. If Europeans and East Asians have different archaic alleles because of separate admixture, their ancestors must have parted company before encountering the Neanderthals not long after leaving Africa (c. 40,000 BP). Yet this seems counterintuitive because the genetic distance between Europeans and East Asians (and hence their time of separation) is much less than that between Africans and non-Africans.
There is in fact a convergence of genetic, linguistic, and archeological evidence for a late split between ancestral Europeans and ancestral East Asians. A Y-chromosome study indicates that all North Eurasian peoples descend from a common ancestral population dated to about 15,000 BP (Stepanov & Puzyrev, 2000; see also Armour et al., 1996; Santos et al., 1999; Zerjal et al., 1997). The language families of northern Eurasia, particularly Uralic and Yukaghir and more generally Uralic-Yukaghir, Eskimo-Aleut, Chukotko-Kamchatkan and Altaic, share deep structural affinities that point to a common origin and not simply to word borrowing (Cavalli-Sforza, 1994, pp. 97-99; Fortescue, 1998; Rogers, 1986). Archeological evidence (characteristic lithic technology, grave goods with red ocher and sites with small shallow basins) also suggests a common cultural tradition throughout Europe and Siberia 20,000 to 15,000 years ago (Goebel, 1999; Haynes, 1980; Haynes, 1982). Finally, dental and cranial remains from southern Siberia (23,000-20,000 BP) indicate strong affinities with Upper Paleolithic Europeans (Alexeyev & Gokhman, 1994; Goebel, 1999).
Rogers (1986) places the European/East Asian split at the glacial maximum (20,000-15,000 BP). At that time, advancing glaciers and swollen glacial lakes created a barrier along the present-day Ob, thereby dividing a nomadic Eurasian population that hunted reindeer and other herbivores within a steppe-tundra belt stretching from Western Europe to Beringia.
A word of caution
Finally, a word of caution. Like any statistical analysis, the Wall et al. approach will yield a certain number of false positives. Many loci are highly polymorphic because selection has favored allele diversity, such as with balanced polymorphisms, and not because they are very old. And some will lie close on the genome to other balanced polymorphisms that likewise look older than they really are. And the alleles of these adjoining loci will often cluster by continental origin because they face different selection pressures in sub-Saharan Africans, Europeans, and East Asians—or just because of pure chance.
These false positives may also be more frequent among Europeans than among East Asians. If ancestral East Asians had budded off from an older European population, they would have taken less genetic diversity with them. They should have proportionately fewer polymorphisms and less diverse ones.
In conclusion, the jury is still out on this one. We won’t have a definite answer until we can match these continentally specific alleles with identical alleles on the Neanderthal genome now being reconstructed.
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