Todd Wood on the Neandertal Interbreeding Question

Todd Wood has weighed in on the Neandertal hybridization results in ArXiv with some of his own thoughts and he raised the spectre of incomplete lineage sorting...something that entirely escaped my thought process when I was waxing about the ArXiv paper yesterday. But as he, himself, points out, there are problems with this explanation. He writes:

Since incomplete lineage sorting is a random process, I would not expect to see any modern human population significantly more similar to Neandertals than any other modern human population. Modern human populations diverged well after the Neandertal divergence, so whatever genes were still very similar to Neandertals should have been evenly divided. Even more interesting is the geography: the ancestors of the modern humans with Neandertal-like genes come from the same geographic region as the Neandertals. If incomplete lineage sorting is to blame for Neandertal-like genes in modern humans, that geographic pattern would just be a weird coincidence. That's not to say that one couldn't make a case for incomplete lineage sorting, but we would need some new evidence to really seal the deal (such as new samples of Eurasians that don't have Neandertal-like genes or Africans that do).

He is right. If modern humans and Neandertals diverged as much as 250 ky BP as many people think, then drift would ensure that the two genomes would be more highly divergent than they appear to be. He is also correct about the geographical coincidence. As I wrote yesterday, the interbreeding conclusion would tend to be supported by the appearance of Neandertal-reminiscent traits in the earliest moderns from the area.

The authors argue that linkage dis-equilibrium, which is when you have allele combinations that are present in two related populations in higher percentages than can be accounted for by chance, supports the recent interbreeding model, a conclusion that Wood agrees with (although how a young-earth creationist can agree with that kind of model, I will never know).

Denisova Update

John Hawks does not think that the recent data from the paper by Skoglund and Jakobsson holds up under scrutiny. He cites striking discrepancies between the analysis presented by these two with a paper that came out by Reich et al. (http://dx.doi.org/10.1016/j.ajhg.2011.09.005) called “Denisova admixture and the first modern human dispersals into southeast Asia and oceania.” Their salient finding is that there were very few Denisovan markers in mainland Asian populations.

Hawks suggests that the reason the two papers differ so greatly is that in the paper by Skoglund and Jakobsson, the analysis is picking up only slight differences in gene frequencies between populations, and that, while there is hybridization and migration into the area by Denisovans, it is small in amount. He writes:

We aren't very far from a more definitive answer of this question, as the data continue to accumulate every day. What I find interesting is the way that models can generate these 1% differences in ancestry proportions, depending on sampling and the pattern of migration assumed to have happened in the past. Two estimates that differ by less than a percent are not really different. This paper provides the suggestion of a more widespread Denisovan legacy, and I accept that as a possibility.

It is clear that there was some interbreeding between these populations. How much remains to be seen.

Hat tip to Paul Pavao.
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