Thursday, October 24, 2013

The Dmanisi skull(s)

A couple of good discussions of the Dmanisi skull(s) and what they mean to our picture of human evolution:
The value and significance of Dmanisi extends far beyond Georgia. The window on variation at Dmanisi can be applied to the rich fossil samples from East Africa and elsewhere. When the authors of this study make that step, they determine that we have likely be over-taxonomizing, or relying too much on species identification and diversity to explain the variation we observe in fossils of this time period. All the fossil variation that in East Africa gets divided by some into Homo habilis, Homo rudolfensis, Homo ergaster, and Homo erectus, because the specimens are too variable to represent a single species, viewed from Dmanisi, suddenly look like a single evolving lineage. Or at least, make it harder to reject that idea as a starting hypothesis.

Let's look harder at this concept of "parsimony". The basic principle is that plurality should not be posited unnecessarily. But what kind of plurality?

A simple answer is that we should adopt the null hypothesis that a sample of fossils represent a single species. Multiple species would be an unnecessary plurality, unless we discover that the sample has statistical properties that are rarely or never found in known samples of a single species. This is the conclusion of Van Arsdale and Wolpoff, and of Lordkipanidze and colleagues, and indeed it is my own attitude. A single species is a good null hypothesis, and the data considered here don't reject it.

Still, saying that the data don't reject a null hypothesis is not the same as saying that the null hypothesis is true. Maybe the data have little power to reject any hypothesis. Parsimony may guide us toward the choice of a null hypothesis, but that's no reason for us to promote it, if the data are indecisive.
          .....Humans today are a poor model for understanding early Homo, because the last major layer of shared ancestry has left us too genetically uniform.
          ......The model I outline is complex. It is not a simple bifurcation of species, branching without reticulation over time. But the model I outline is necessary for Neandertals, Denisovans and MSA Africans, it is necessary within MSA Africa, it is necessary for living chimpanzees, and for living gorillas, and it is necessary during the last 50,000 years of our evolution. These are the mix of demographic and selective forces that characterize every close model we have for the evolution of early Homo.