Friday, November 30, 2007

Convergent evolution?

In an earlier post, I discussed how mtDNA evidence now shows that the Neanderthals ranged at least as far east as Lake Baikal. This finding is significant because there no longer seems to have been any geographical or ecological barrier to Neanderthal occupation throughout non-tropical Eurasia.

This point has been commented on by Michelle M. (Mica) Glantz, an associate professor in anthropology at Colorado State University. In an interview with anthropologist John Hawks, she argues that the European Neanderthals may simply have been one of several interbreeding Homo erectus populations that inhabited Eurasia. They may have been more Arctic-adapted and more specialized in various ways but they were not genetically isolated, at least not fully, from other Homo erectus populations.

If we keep moving the Neandertal boundary eastward, then wouldn't Neandertals cease being a recognizable entity that is really separate from other archaic groups in the Old World during the Middle Paleolithic? In other words, who isn't a Neandertal in this case? Certainly we do not have enough similarly aged specimens from China and other points east to make thorough comparisons, but really the specimens we do have are usually not included in any of our analyses that are concerned with European Neandertals. Exceptions to this, like Rosenberg et al. (2006) study of Jinniushan, show that Asian specimens often look like they are part of the same cline as European Neandertals.

This discovery is one of several challenges to the traditional view, which sees the Neanderthals as being not only distinct from other archaic humans but also intermediate on the line of descent from Homo erectus to modern humans. Indeed, the Neanderthals increasingly look like an evolutionary dead end. Modern humans seem to owe most, if not all, of their ancestry to a demic expansion that started in East Africa some 80,000 years ago and began to spread out of Africa some 50,000 years ago. There may have been some intermixture with archaic humans already present in Europe and Asia, but even this scenario is looking more and more problematic. We can now compare mtDNA from late European Neanderthals with mtDNA from early modern Europeans and there is no measurable gene flow from the former to the latter (Caramelli et al., 2003). Perhaps some minor intermixture did occur here and there, enough to provide the modern European gene pool with a few advantageous Neanderthal genes. It could not have been greater, however, than the surreptitious insertion of certain viral and bacterial genes into the human genome.

Why, then, did scientists place the Neanderthals above Homo erectus, even to the point of classifying them as a subspecies of Homo sapiens? Because Neanderthal brains were so big, like ours. This resemblance now appears to be just a case of convergent evolution. When humans first spread out of Africa, some 1.8 to 1.7 million years ago, their brains increased in size wherever they entered non-tropical environments, apparently because such environments were mentally more demanding (need to cope with seasonal variation in temperature, to identify and/or create shelters, to hunt over larger and riskier territory, etc.). This in situ evolution seems to have progressed the furthest in populations that we call ‘Neanderthal.’

Then, 50,000 years ago, humans again spread out of Africa, probably because a change in their neural wiring gave them an edge over archaic humans already established in Eurasia. This second wave continued to evolve as it spread into different environments with different adaptive landscapes—a subject that will be the focus of an upcoming PNAS article by Greg Cochran, Henry Harpending, John Hawks, Robert Moyzis, and Eric Wang.

The second wave out of Africa is called Homo sapiens whereas the first wave is called Homo erectus. But these are just names that simplify reality. There was and has been considerable variation and evolution within both ‘species.’


Caramelli, D., Laluez-Fox, C., Vernesi, C., Lari, M., Casoli, A., Mallegni, F., Chiarelli, B., Dupanloup, I., Bertranpetit, J., Barbujani, G., & Bertorelle, G. (2003). Evidence for a genetic discontinuity between Neandertals and 24,000-year-old anatomically modern Europeans. Proceedings of the National Academy of Sciences USA, 100, 6593-6597.

Rosenberg KR, Zuné L, Ruff CB. 2006. Body size, proportions, and encephalization in a Middle Pleistocene archaic human from northern China. Proceedings of the National Academy of Sciences USA, 103, 3552-3556. doi:10.1073/pnas.0508681103

No comments: