Message #264831

[Cadavid]

2. Ancient DNA is finally placing a framework around the vexed question, 'how can we pick a new species from among the fossils'?

For decades, anthropologists have been locking horns over how many species there might be in the human evolutionary tree; with estimates presently ranging from 5 to more than 25 species.

So far, we've lacked an independent, objective, way to test our ideas. But, surprisingly, this is now emerging from comparisons of the human genome with those of our archaic cousins.

For example, for over 100 years anthropologists have argued about whether the Neanderthals are a separate species to modern humans, or merely a sub-species of our kind.

DNA has now given us an answer, and it should satisfy even the more hard nosed of anthropologists; although, experience tells me some of my colleagues will go the grave believing otherwise.

Neanderthal, Denisovan and other archaic DNA is found unevenly throughout the human genome, occurring in hotspots, with vast deserts separating large stretches of archaic genes.

One example is the human X-chromosome which is largely free of archaic DNA. This indicates that natural selection weeded out archaic genes, and also that male hybrid offspring of archaic and modern human matings were probably infertile.

Anyone with a passing interest in the species questions will recognise immediately the importance of such a finding: humans and Neanderthals were different species, even if one applies the very strict criterion of 'interbreeding', so widely assumed to be indicative of species differences.