Genetic Evidence

The first investigations of modern humans attempting to detect remnants of ancient lineages looked at mitochondrial DNA and Y chromosomes. Both are of interest because they are inherited from just one parent (the Y chromosome from the father, the mtDNA from the mother) and because they do not recombine. Extensive sampling has shown no evidence of variants that might have existed in archaic human populations such as Nean-derthals.22 The data, in other words, are consistent with low (or zero) gene flow from archaic to modern populations. This pattern might also have arisen if Neanderthal mtDNA and Y chromosomes didn't mesh well with the genetic background of anatomically modern humans and reduced fitness in some way. In that case, they would have dwindled with time and could be rare or nonexistent today even if they had once been moderately common in modern humans.

A number of recent reports, however, provide evidence that people do retain some autosomal alleles from archaic humans.23 Some of these reports have detected odd patterns in our genome as a whole, whereas others have looked closely at particular unusual genes.

V. Plagnol and J. D. Wall found that the pattern of linkage disequilibrium—that is, of the history of chromosomes having broken and reformed—among SNPs (single nucleotide polymorphisms, or single base differences between chromosomes) in the human genome was inconsistent with an unstructured ancient population, estimating that 5 percent of genetic variation among both Europeans and sub-Saharan Africans originated in archaic humans such as the Neanderthals.24 This is interesting, in that evidence for introgression is nearly as strong among Africans as it is among Europeans. This is what one would expect to happen if many of the alleles picked up from Neanderthals or eastern archaic humans were generally advantageous and spread very widely. There may also have been significant archaic populations somewhere in Africa: There are some apparently archaic variants found only in Pygmies, which suggests an African origin. Conditions for fossilization are poor in much of Africa west of the Rift (for example, chimpanzees have almost no fossil record)—and there may well have been hominid populations other than anatomically modern humans in the blank spots in those Africa maps. Since some of these alleles are found at high frequencies in people today, and since the overall level of admixture was probably low, they probably gave a fitness advantage— in other words, were adaptive.

P. D. Evans and his colleagues at the University of Chicago looked at microcephalin (MCPH1), a very unusual gene that regulates brain size.25 They found that most people today carry a version that is quite uniform, suggesting that it originated recently. At the same time, it is very different from other, more varied versions found at the same locus in humans today, all of which have many single-nucleotide differences among them. More than that, when there are several different versions of a gene at some locus, we normally find some intermediate versions created by recombination, that is, by chromosomes occasionally breaking and recombining. In the case of the unusual gene (called D for "derived") at the microcephalin locus, such recombinants are very rare: It is as if the common, highly uniform version of microcephalin simply hasn't been in the human race all that long in spite of the high frequency of the new ver-

sion in many human populations. The researchers estimated that it appeared about 37,000 years ago (plus or minus a few tens of thousands of years). And if it did show up then, Neanderthals are a reasonable, indeed likely, source.

Another interesting possibility involves FOXP2, a gene that plays a role in speech that was replaced by a new variant some 42,000 years ago.26 This is very recent in evolutionary terms, and there is evidence that the same version of that gene existed in Neanderthals.27 If the new FOXP2 allele is really that recent in modern humans, it is likely that the migrating humans picked it up from Neanderthals, since that's about the time they encountered them in their expansion out of Africa. The idea that we might have acquired some of our speech capabilities from Neanderthals may be surprising, but it is not impossible. The timing of the acquisition is certainly consistent with the creative explosion. If it is true that we gained the gene by means of intro-gression, then the version of FOXP2 in the Neanderthals is likely to be older and have more variation than it does in modern humans. Further sequencing efforts on the skeletal remains of Neanderthals should eventually confirm or refute this possibility.

If FOXP2 is indeed a "language gene" and responsible, perhaps, for some of the creative explosion of modern humans in Europe and northern Asia, it would explain a major puzzle about modern human origins. There were at least two streams out of Africa 50,000 years ago, one northward into Europe and central Asia, and another eastward around the Indian Ocean to Australia, New Guinea, and parts of Oceania. There is no trace of any creative explosion in populations derived from the southern Indian Ocean movement, who brought and retained Neanderthal-grade technology and culture.28

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