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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Austin.
ReplyDeleteIve been following this On Dienekes' blog,
The paper raises some interesting questions, in fact it asks questions than it answers.
One thing they did not mention, was that the people we call Denisovans, and like to associate with Oceana and south east asia, is in fact a populatiom that spread into eurasia ahead neanderthal and AMH, from western europe, as is evidenced by the sequencing of the Atapuerca "homo heidleburgensis.
So, just like all other homonins to preceed and follow them, they would have left some residual populations along the way to se asia and oceana.
Modern humans and the neanderthal ahead of them would have to have moved through these populations as they moved east. I thgink its safe to say that there was more than one admixture event and likley in both directions.
There are two key pieces of evidence that are glossed over, in the paper, the first is the fact that tibetans and han chinese have the altitude adaptaion mutation that comes from denisovans, yet native americans did not inherit the same mutaion. That is very odd if we look at EE as the progenitors of Native Americans. To me this indicates that the ancestors of the amerindians split off of the eurasian line before they mixed with the high altitude denisovans.
But what of that small percentage of denisovan dna found in NA populations, and how does it square with the austrailian, PNG, Oceana evidence?
So as i see it the evidence is pointing to several episodes of admixture with denisovans.
First, when the ancient eurasian ancestors first moved through the remnant denisovan populations in siberia they picke a smidge of D ancestry, but not from the high altitude adapted denisovans.
When the Out of Africa southern coast wave of AMH came along, some of moved into S asia moving north up the rivers and into the highlands, they mixed with the altitude adapted denisovans and became a component of modern EE, which shares shares the eurasian compnent of NA ancestors.
Some of the southern wave moved straight on to australia through coastal SE asia, with their S asian denisovan component, while a group went on to PNG and oceana.
We must also remember that the first AMH into australia has no genetic relations to modern aboriginal austrailians and our MRCA was 125k years ago. And that the extremely archaic and robust Kow swamp people do have a relation to modern australians but have only been there for a short period.
After seeing a cast of a Kow swamp skull in person, next to other early AMH specimens, i would say they were denisovan descended, it almost looks like HE.
There is another possiblity to explain the equal parts PNG/Austrailian ancestry in N ative Americans, is that a very early wave into SA was infact Australaisin people, which could be attested to by the archaic skulls from SA, and the trail of pebble tool complexes found along the west coast of north and central america.
And since it is looking like these people moved back north at the end of the ice age to form the basis for the Haskett/sluiceway/mesa tool complex in North america. This is how the denisovan ancestry got spread into the neanderthal leaning ancient north americans.
Austin,
ReplyDeleteAre you aware of this presentation,
http://www.ele.net/kunz/mesamonte.htm,
It from the BLM archeologist making the case for a south to north early movement in the new world, and ties El Jobo points from tiama-tiama in Venzualea to mesa/sluiceway and haskett, of which the largest of its kind was just published.
http://westerndigs.org/over-1000-ancient-stone-tools-left-by-great-basin-hunters-found-in-utah-desert/
Thanks for the link. The more I think about it the more I have doubts. The peopling of America from Barrow to Cape Horn must have taken quite some time. And the Mesa Verde Link shows well developed toolage in Alaska similar to those found at other sites in America during that same period.
DeleteThis could mean either (a) these people moved really fast and spread their know-how across the Americas in 1,000 years or...
(b) They had been in the New World for some time, and spread out taking plenty of time. And this leads to the slight differences noted in the different stone tools.
Slide #24 asks an interesting question: how come the oldest tools are found in South America instead of North America.
Your suggestion further up of an early wave of Australasian people into SA is quite interesting.
Definitively we need more findings, especially older ones.
Thanks for your comments!
I live in Australia and have noticed striking resemblance between peoples of Aboriginal/European background with people of Patagonia. Recently DNA links have been discovered.
Delete