If there really was interbreeding between Neanderthals and anatomically modern humans, how and where might it have happened?
There certainly might have been some gene flow among hominid species in earlier times. After all, Homo heidelbergensis (the common ancestor of Neanderthals and anatomically modern humans) somehow managed to settle both Europe and Africa about half a million years ago, so communication must have been possible, at least occasionally. It may have been im possible most of the time because of the Sahara Desert, which is a potent barrier today, just as it was during the ice ages. The Sinai Peninsula, also often a desert in its history, may also have been an important barrier, since it was the only land connection between Africa and Eurasia. More than that, Neanderthal alleles that were advantageous outside of Africa may not have been so in Africa and thus might not have spread to anatomically modern humans there.
We have reason to think that the modern humans who expanded out of Africa some 50,000 years ago had changed in important ways—had, for example, probably acquired sophisticated language abilities. A Neanderthal allele that had not been particularly useful in the genetic context of near-modern humans 100,000 years ago might have been useful to the more advanced people who were expanding out of Africa.
Logically, if admixture occurred at all, it had to happen somewhere in Neanderthal-occupied territory, which means Europe and western Asia. As modern humans expanded their territory, they must have encountered Neanderthal bands again and again. The two kinds of humans coexisted for a few thousand years before the Neanderthals disappeared, at least in some regions. This looks to be the case for the Chatelperronian culture of France and northern Spain, and there are traces of a similar culture in Italy. If there was trade, or if there was enough contact to transmit toolmaking techniques, there was sexual contact as well—depend on it. If in the future we look at very large genetic datasets from huge numbers of individuals, we might find a few traces of neutral Neanderthal genes.14
If we found a few individuals with Neanderthalish mito-chondrial DNA (mtDNA) or Y chromosomes, we might be able to determine whether matings occurred mostly between
Neanderthal males and modern females or modern males and Neanderthal females. We do this type of analysis routinely today and have found, for example, that the maternal ancestry for most Mexicans is Amerindian, whereas most of their paternal ancestry is Spanish (which simply means that male Spanish explorers sometimes mated with Amerindian women). At this point, though we have found no Neanderthal Y chromosomes or mtDNA in modern humans, we cannot rule out significant introgression, because Neanderthal mtDNA and Y chromosomes may well have been neutral or deleterious in modern humans. In either case they would be unlikely to have persisted until today, particularly if the amount of gene flow was small. This does not mean we did not inherit beneficial gene sequences (see section on "Genetic Evidence" later in this chapter).
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