August 25, 2005

More on Ancient Alaskan mtDNA

I had previously posted about ancient DNA extracted from a prehistoric Alaskan sample (On Yourk Knees Cava Man, or OYKCM). In this news article, Brian Kemp, the scientist responsible for this research elaborates about the work which has been submitted to Nature for publication. Interestingly, by comparing the ancient DNA with that of modern humans with the same haplotype, and taking into account the 10,300 years separating the two, he was able to measure the mutation rate, finding it to be faster than what was previously thought:
In the late 1990s, scientists used DNA studies to propose that people first advanced upon the continent from Asia as much as 40,000 years ago. But data from numerous archaeological sites across the Americas have placed the migration at closer to 10,000 to 12,000 years ago.

Kemp has used OYKCM as a measuring stick to come up with dates much closer to the archaeological record. "Because we know that this guy represents the oldest known example of this lineage, that places a minimum date on the emergence of the lineage," he explains.

In other words, OYKCM represents one end of the measuring stick. At the other end are the 47 people who belong to his haplotype. According to the rules of the molecular clock, this makes it possible to measure the genetic changes between OYKCM and the modern samples and calculate the time it would have required for those changes to occur.

"My calibration shows that the changes were occurring two to four times faster than previously thought," Kemp says. "It means some people have overestimated the time. It wasn't so long ago."

Of course ancient DNA is susceptible to so-called phantom mutations occurring after the subject's death. So, even if OYKCM's mtDNA was exactly similar to that of living humans, damage in the intervening 10,300 years may have caused it to appear different. Thus, the mutation rate may be overestimated due to this problem. It will be interesting to see whether this problem is addressed in the published paper, since there are ways to distinguish between genuine and post-mortem mutations in ancient DNA.

No comments: