Is it possible that East Asians are a hybrid between modern and achaic humans?

Question

As I understand it, all populations outside Africa have at least 2% Neanderthal ancestry. In eastern Asia and I think Papua New Guinea in particular that percentage could be more than 4%.

Now you also have the Denisovans from which a full genome sequence from a small bone fragment have been found in Denisova cave in central Asia. The latest research suggests that some Asian groups have about the same Neanderthal ancestry as other non-African populations but more Denisovan ancestry.

Question: If we theorize that East Asians (Chinese, Koreans, Japanese) with their characteristic features are not the result of recent evolution as per the "recent out of Africa Theory" but are instead the result of hybridization between modern humans and some form of archaic human (just like the Neanderthals and Denisovans) that have yet to be found.

To what degree could this theory be proven or disproven from genetic studies?

Answer 1

Firstly, as one of the comments said, there is nothing particularly unique about the set of 'features' that East Asians have, other than having fairly high levels of Denisovan ancestry. Any region in the world has some kind of unique phenotypic features if you look hard enough.

Genetics has fairly unambiguously shown that all non Africans originated from the out-of-Africa (OOA) event; Europeans, East Asians, Native Americans, Aboriginal Australians etc, are all from the same smallish source population that left Africa some time between 40-50kya. Of course there was plenty of migrations out of Africa that predated the out of Africa event, but there’s no strong evidence that they left any trace in existing populations. There’s been some hints by e.g. Pagani et al 2016, but this is very hard to prove convincingly, in part due to things like archaic admixture. If it did happen, it was likely a very small contribution.

Simply, you can fit models of human population history to allele frequency differences using something like Treemix and get graphs like this:

Which show that East Asians are nested exactly where you would exepct to see them following an OOA event (i.e. equally divergent from Africans as Europeans, Native Americans etc).

Humans and Archaics are very highly diverged in relative terms and if East Asians were some kind of mixture between modern humans and some unknown Archaic, they would stick out like a sore thumb on a model like this.

You could also look at something like cross-coalescent split times, which look at the divergence in genealogical trees across the genome between pairs of populations. Again, if East Asians were some uknown mixture, these trees would be hugely divergent from those you see in e.g. Europeans. They aren't:

Answer 2

There is evidence that Europeans and Africans have admixture from other hominin groups, so it is not just Asian populations that have such admixture. We are all such hybrids, if anyone is.

The reference you may be looking at shows that in east Asia there may have been more than one period of interbreeding with Denisovans.

There is new evidence that there may have been several groups in Africa that merged to become what is now the human race even.

It’s pretty settled that we all have strong connections to Africa and Neanderthals and other brethren hominids.

The examination of DNA has assembled complicated migration histories - leaving and coming back into Africa - migration across and back to Europe.

I think take-home in general here is that we were not really separate species and our ancestors didn't really see each other as being so different that we would not mate. All these subspecies coexisted for thousands of years and we probably interbred as often as we were in the same place. There were very few of us at the time... we just didn't meet each other so often.

I wouldn't jump to the conclusion that the Denisovan inheritance is unique to Asians somehow. The racial genetics of old have also been long debunked in terms of differentiating races into distinct subspecies. In short: the oft-referenced racial groups have all interbred with each other and with archaic hominins to a degree that makes their distinctions fuzzy.