Interpreting DNA Evidence

Last week, I posted a link to a NYT article that discussed recent DNA studies of people and cattle in Italy. These studies found genetic similarities between current populations of both species in Italy and those in various regions of the Near East. This link was then interpreted as evidence that the ancient people known as the Etruscans arrived in Italy from the Near East (either directly or indirectly) sometime in the last three or four thousand years.

John Hawkes' has a post on his anthropology blog that examines another such attempt to use DNA evidence to reconstruct ancient migrations, this one published in Science. The results have been controversial, which has led to much scholarly back-and-forth. I thought this would be a salutary example of the dangers of overinterpreting DNA evidence. So what's the controversy?

For those wanting more detail, Hawkes ably sums up the issues involved, but in brief the evidence is this: DNA testing of people from North Africa found that they possess specific variations in their mitochondrial DNA similar to people living in the Near East. Based on the differences between these lineages and other mitochondrial DNA, we can estimate the time at which these groups branched off from their neighbors. In this case, the answer is about 45,000 years ago. The authors argue that this indicates that there was a migration into North Africa from the Levant around that time, displacing the earlier people that lived there. The date corresponds to the first peopling of Europe, and the authors suggest that both movements were part of the same pattern of migration.

Not so fast, say two other scholars. There are other ways for genes to move around. A later movement of peoples might have brought over these lineages. Or there might have been gradual diffusion of genes (via a series of short-distance interactions) instead of large-scale population movements. The archaeological evidence suggests that there was no large scale migration from Europe or the Near East into North Africa until much more recently, around 5000 BC.

For now, the evidence is insufficient to decide between the alternatives: 40,000 year-old mass migration? Slow diffusion? Recent mass migration? These are precisely the same problems that come into play in the Italian studies. Just because you share a common ancestry with someone 5000 years ago in the Near East doesn't mean that a bunch of people came over from the Near East at precisely that moment. There are lots of ways that genetic material can circulate.