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).

Our Cheatin' Ancestors?

A new study suggests that modern humans are considerably less promiscuous than their forbears. In a story that appeared in the University of Oxford site, the culprit seems to be androgen levels:

Previous studies by the research team have shown that promiscuous species have low index to ring finger ratios while monogamous species have high finger ratios. High levels of androgens, such as testosterone, increase the length of the fourth finger in comparison to the second finger. It is thought that prenatal androgens affect finger length during development in the womb, which in turn is linked to adult behaviour. High levels of prenatal androgens are linked with competitiveness and promiscuity.

I would love to know what the r² on that analysis is. I am always extremely suspicious of a study that links a single process to a complex social behavior, especially something like this. Later, the article has this:

Dr Susanne Shultz, from the University of Oxford, said: ‘Social behaviours are notoriously difficult to identify in the fossil record. Developing novel approaches, such as finger ratios, can help inform the current debate surrounding the social systems of the earliest human ancestors.

They are notoriously difficult because they often leave few to no clues. This analysis also sidesteps the problems involved in trying to ascertain what other selective forces might have been at work in early hominid development. For example, in Ardipithecus, given that they exhibited incipient bipedality and were, at least in part, arboreal as well, there may have been selective pressures on metacarpal and phalanx length. Also, is finger length one trait dictated by pleiotropy? Or is it polygenic? Is there linkage disequilibrium going on? These sorts of questions could be extrapolated to other hominid species as well and need to be addressed before arriving at the conclusion that is stated in this study.