Sunday, 3 November 2013

Denisovans crossed the Wallace line

Wallacea is bordered to the west by the Wallace Line
and to the east by the Lydekker Line (Wikimedia Commons)

Denisovans were archaic hominins distinct from Neanderthals and modern humans (previous post). There is evidence for gene flow between all three populations and putatively a fourth. The greatest amount of introgressed Denisovan DNA is found in the aboriginal peoples of New Guinea and Australia (here).

This is remarkable for several reasons. Firstly, it is a long way from Denisova in Siberia to New Guinea. Secondly, there is no trace of Denisovan DNA in modern humans from mainland Asia or from Southeast Asia west of the Wallace Line. As pointed out in a recent essay (here), the most likely explanation is that Denisovans crossed the Wallace Line into Wallacea (see map) and that exchange of genes with modern humans occurred there. Traces of Denisovan DNA do occur in modern populations from Wallacea as well as further east to Polynesia and Fiji.

Does that mean there was no contact between Denisovans and the ancestors of present day Asian populations? Not necessarily. Some Denisovan genes may have been retained east of the Wallace Line because they conferred resistance to disease (purifying selection). Further west Denisovan DNA may have been "overwritten" in the course of time.   

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