A paper by Pengfei Qin and Mark Stoneking (Denisovan Ancestry in East Eurasian and Native American Populations bioRxiv preprint first posted online April 3, 2015; doi: http://dx.doi.org/10.1101/017475), takes a look at the Denisovan content in modern Homo sapiens genes.
Below is one of the maps posted in the supporting information (from page 8) which shows (blue shades) the higher introgression of Denisovan DNA into humans in Australia, New Guinea and Melanesia, while the Neandertal content is given by the red shades (higher in Asia, America and Europe. Green tint is the neutral African area.
They put forward two possible ways for the admixture of Denisovan and Homo sapiens genes. Allow me to quote them:
"However, there are (at least) two potential alternate scenarios that could explain these results. First, Denisovan admixture could have occurred in a population that was ancestral to both EE/NA (East Asia and Native American) and Oceanian populations; second, admixture could have occurred in a population that was ancestral specifically to Mamanwa, Australians, and New Guineans (as suggested previously (2)), followed by a back-migration from New Guinea to mainland East Asia. This putative back-migration would then have spread both New Guinea and Denisovan ancestry throughout East Asia and Siberia, and ultimately to the Americas."
The first option (admixture in Siberia near Denisova), would be followed by additional dilution in East Asia of Denisovan genes when more modern humans migrated into East Asia and admixed with the population that had originally received the Denisovan input. While this did not take place in Oceanian peoples.
In my opinion the back-migration out of Australia is unlikely. But what if... there was a sizeable populations of Denisovans in South East Asia and Sahul. Humans arrived and mixed with them. The higher the density of Denisovans or ratio of Denisovans to H. sapiens, the higher their "input" of genes into the mixed offspring of human - Denisovan matings. So PNG, Australia had a higher ratio of Denisovans than East Asia... or America (why must we always assume that the introgression took place once in one place?). Because admixture could have taken place in several locations: America, Asia and Austronesia.
Who are these Denisovans? The most likely candidate is an ancestor that lived in Asia and had left Africa before Humans and before Neandertals... that spells Homo erectus. They did live in Sahul and China (maybe America but that is not considered by mainstream science).
The interesting thing is that Neanderthal (see the red hues in the map above) genes are very strong in America among Native Americans too, suggesting (a) Modern Humans mixed with them in Asia and brought the Neandertal genes with them into America or (b) Admixture took place in America: modern humans met and mated with Neandertals in the New World.
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