Double the migrations, double the fun

Several news stories have reported on an article by Ugo Perego and colleagues, titled “Distinctive Paleo-Indian Migration Routes from Beringia Marked by Two Rare mtDNA Haplogroups.” The Discover blog, 80beats, has a good two-paragraph summary of the results:

In the study, published in Current Biology [subscription required], a team led by geneticist Antonio Torroni analyzed entire genomic sequences of mitochondrial DNA, the genetic material in cells energy-generating units that gets passed from mothers to children. The researchers focused on the disparate geographic distributions of two rare mitochondrial DNA haplogroups which are characterized by a distinctive DNA sequence derived from a common maternal ancestor that still appear in Native Americans [Science News]. Both haplogroups appear to have arisen about 16,000 years ago.
The researchers found that all the people with the D4h3 haplogroup presently live in South America, while those with the X2a haplogroup live in Canada and the United States, which suggests that the two genetically distinct bands of early humans struck off in different directions around 16,000 years ago.

I don’t have a lot to say about this. Tracking the frequencies and geographic distribution of rare haplotypes poses different issues than doing so for common alleles. Two closely related populations might nevertheless differ in the presence or absence of rare alleles.

I really just wanted to post with reference to a broader point. If the data don’t distinguish between a single migration at one time and multiple migrations at different times, then it’s pretty much certain that they won’t distinguish between a single migration and multiple migrations at one time.

The two-simultaneous-migrations model might solve problems so far unaddressed by other models. But it’s not obvious that it solves any – there’s no test here, just a discussion of the plausibility of the scenario. Each of these scenarios for New World habitation involves the dispersal of many populations across thousands of years. That means lots of free parameters, even in the simplest of the models. Given that necessary complexity, it seems pretty likely that there’s a way for the simplest model to account for the frequencies of two rare alleles. It will take a whole lot more genetic comparisons to really test hypotheses about the founding population.

References:

Perego UA and 15 others. Distinctive Paleo-Indian Migration Routes from Beringia Marked by Two Rare mtDNA Haplogroups. Curr Biol 19:1-8. doi:10.1016/j.cub.2008.11.058