Friday, October 25, 2013

Denisovan DNA East of Wallace's Line

As naturally and inevitably as gators eat the dogs of careless owners in Florida, the one time I carelessly leave town for personal reasons--to attend the wedding of Honduran friends in Louisiana--Science ejaculates a gob of interesting articles. There are too many of them to review in one post, so I will tackle them one at a time and hope to finish one day.

First up is this discussion in the "Perspectives" section:

Cooper, A. and C. B. Stringer (2013). Did the Denisovans Cross Wallace's Line? Science 342: 321-323.

Wallace's Line is the imaginary boundary that winds north to south through the islands of modern Indonesia marking the frontier between the mainland Asian fauna to the west and the Austronesian ecosystems to the east. Cooper and Stringer attempt to explain why small "remnant" Denisovan DNA is only found in modern human populations east of the Wallace line when the only known Denisovan lived far away in southwest Siberia. The characteristics of the Denisovan DNA sequence imply a large and diverse population in Asia. The authors, however, reject the obvious idea that most east and southeast Asian peoples once had Denisovan DNA that was later "overwritten" by subsequent gene flows. Their evidence for rejecting this interpretation is pretty strong: the DNA of isolated, remote, and evidently "indigenous" Asian populations thought to predate later migrations, such as that of the Andaman Islanders, lacks Denisovan DNA, precisely where one might most confidently expect it to be found. Moreover, recent analysis of the ancient DNA from a Chinese specimen dating from ca. 40,000 ya shows no trace of Denisovan introgression. They therefore suggest that the Denisovans who crossed the Wallace Line encountered the ancestors of Homo floresiensis, the hobbits, who were there at least 800,000 ya if not 1 mya. Perhaps, they conjecture, the Denisovan DNA signal was better preserved east of the Wallace Line because of the small size of the populations involved, which might have encouraged interbreeding, or because of selective advantages conferred by some of the genes upon rapidly advancing human populations. I didn't find their final answer to be very satisfying, but their control of the data was impressive and their discussion thought-provoking.

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