Harvard Magazine has a nice piece up on David Reich’s biography and research. The section where Reich addresses the strange issues regarding Neanderthals jumped out at me, as I’ve had the same confused thoughts:
Reich is not resting on his laurels. As the data accumulate, and the tools become more sophisticated and powerful, he has begun revisiting some of his own prior interpretations of human prehistory, and coming to terms with what he describes as “weird signals in the current data.” Mitochondrial DNA shows that modern humans and Neanderthals are much more closely related to each other in the maternal line than either is to Denisovans. The Y chromosomes of modern humans and Neanderthals, passed only in the paternal line, are also much more closely related to each other than to Denisovans. “But then if you look at the whole genome, on average, Neanderthals and Denisovans are more closely related to each other than either one is to modern humans. Having an entirely male line and an entirely female line saying one thing, and then the rest of the genome saying something else, is weird.” The explanation some people give is that there may have been modern human mixture with Neanderthals further back in time than currently understood, somewhere between 250,000 and 400,000 years ago, and that contributed a few percent to the Neanderthal genome. “But it’s very surprising that only a few percent contribution would be the source of both the Y chromosome and mitochondrial DNA for Neanderthals.” Maybe contemporary non-Africans are actually Neanderthals, and later waves of modern human DNA from Africa swamped the rest of non-Africans’ genomes so “non-Africans are best described as Neanderthals, with 98 percent modern human mixture, or something profoundly philosophically unsettling like that,” he says, half seriously.
“There’s going to be a challenging of our understanding of these relationships” in the years ahead, Reich continues, “based on analyzing the data in more sensitive ways and looking at it from new perspectives. We’re taking a couple of steps back and realizing that key events and relationships are different, in a deep way, from the first-pass model we have collectively developed. The model that I’ve had a role in building is teetering. I find that exciting, as well as destabilizing. And I’d like to be part of trying to figure out the truth.”
Neanderthals clearly have a bit of “modern” human-like DNA from paleo-Africans. On the order of a few percent. But strangely, their Y and mtDNA seem to fit into a lade with modern humans, as opposed to the Neanderthal’s Denisovan cousins. This isn’t impossible; over time rare lineages will replace common ones. But what’s the chance that both Y and mtDNA from humans would replace that of Neanderthals? (probability of fixation of a new mutation is 1 over the number of gene copies)
Don’t you know the paper which demonstrated that the Y and mt of the Neanderthals are due to an introgression of Homo Heidelbergensis from Europe and I wrote that that disproved the “Out of Africa” theory being Africans only archaic A00 and B? This was close to the Huang Shi theory against the “Out of Africa”. I am quoting only by memory. I apologize if there is some mistake in what I wrote, but I remember very well all that.
Possibly Neanderthals are an African pre-AMH dispersal that admixed into a larger population of Denisovans? Maybe in some phased way that accounts for both y and mt (multiple dispersals where one and then the other switches). With relatively small populations maybe this sort of improbable sounding thing could be more likely?
Or could Reich be suggesting that Neanderthal y or mt is actually a retention and Denisovan y or mt is derived from an earlier wave in Asia, introgressed?
Maybe contemporary non-Africans are actually Neanderthals, and later waves of modern human DNA from Africa swamped the rest of non-Africans’ genomes so “non-Africans are best described as Neanderthals, with 98 percent modern human mixture, or something profoundly philosophically unsettling like that,”
I wondered if that might be in play to explain populations with more archaic morphology at Red Deer Cave in China; where you might have some Denisovan survivor that got washed over repeatedly by a larger AMH population, down to indistinguishable ancestry, but the alleles for old style crania introgressed…. Morphologically they were “Denisovan with 99.9% AMH admixture”. Seems unlikely though.
Easiest explanations seem to me that either Neanderthals and Denisovans form a real clade, but Denisovan uniparental lineages introgressed from a more diverged population (as Matt entertained); or moderns and Neanderthals form a real clade, but moderns have autosomal introgression from a more diverged population, which didn’t contribute any living uniparental lineages.
I’m just a layman here, but is that plausible?
As Razib notes, in a fitness neutral scenario, the likelihood of an introgressed Y-DNA or mtDNA becoming predominant is remote. But, if AMH admixture into Neanderthals introduced Y-DNA or mtDNA that was fitness enhancing, then it isn’t unlikely at all.
I might have suggested (for Harvard) that they give due credit to Mallory and to Gimbutas for the Kurgan Theory of Indo-European introgression into Europe. Gimbutas was admittedly a bit weird but Mallory assuredly wasn’t, and David Anthony had given credit to both.
Some time ago there were a series of tweets where Iosif Lazaridis suggested the opposite: Neanderthal uniparental introgression into AMH.
@Gioiello Tognoni, Mentioned something about Shi Huang and google search brings up something called Maximum Genetic Diversity hypothesis.
There is a recent video on YT about this https://www.youtube.com/watch?v=UALD-0mkuxk But it’s not in English or subs so hard to follow.
https://twitter.com/shi_huang5 Seems to have been followed by Razib.
I don’t know what all this is about and there is sparse accessible info online. Could someone give a brief gist.
@Var
https://www.pnas.org/doi/full/10.1073/pnas.1814338116
https://pubmed.ncbi.nlm.nih.gov/32973032/
This should be the paper which demonstrated the introgression of the uniparental markers from Homo Heidelbergensis. I wrote a lot about that, but unfortunately my 20000 letters are out because either I was banned or the blogs are out now. The last is Anthrogenica, but I was banned in 2013 after 1290 letters and a thread dedicated to me: “Rathna’s assessment of genetic materials”.
@ DaThang
I remember those tweets from Iosif and I believe he went even further than that, that “African” Homo Sapiens and Neanderthal were actually a clade, and instead of Neanderthals being a descended from an earlier African migration, AMH were rather migrants from Eurasia that split from earlier from Neanderthals.
Any comments about this piece in the New Yorker on attempts to identify the genetic relationships (and evolutionary history) of the hoatzin?