Neander Vale Man and Magnon Cave Man have been known for over a century, with the Magnon Man recognisably us. They and we overlapped. Our genes show evidence for that overlap - but not much.
(Here I shall lay out my usual fine-tunings of Cro-Magnon itself. Cro-Magnon 1 is a Black Death era victim - oopsie. The others are now classed Aurignacian, living after that Campanian Ignimbrite eruption; no-one living after 37kBC ever met 50%+ Neanders. In turn Aurignacian DNA - which survived to the Magdalenian era - didn't weather the Neolithic turnovers any better than the Neanders had weathered that eruption. Nobody anymore talks "Cro-Magnon" in an ancestry context, because they're close to a dead end. They talk of "Early Modern Humans".)
Up to the late 2000s of our era, most of us assumed that the Neanders were simply wiped out. We just didn't know how: that volcano was one of many suspects, or perhaps conspirators. Some authors counter-surmised that the Neanders interbred with us; but this debate went more in the popular press. Larry Gonick with his Cartoon History was most influential to my generation, and then Gregory Cochran's and Henry Harpending's 10000 Year Leap. The scholars doing peer-review felt limited by the evidence, and well into our millennium even the DNA was not showing mixture, even at low levels.
A classic clash between Bayesian common-sense on the one side, and inductive Popperian reasoning on the other.
As of 2010ish our DNA extraction got good enough to find traces at the 3% level or less. The plebs had won this one against the nerds: Neanders live amongst us. Soon after that we found in the Siberian Denisova cave (a Frenchman might call it, "Cro-Denis") another population. This caveman was descendate from the same Pleistocene out-of-Africa movement which led to Neander; but - by the time of the remains - already itself very different from Neander. And we've conjectured further offshoots from Cro-Denis' post-Neander ancestor; these contributed disproportionately to Melanesian DNA. Human populations remain haunted by other ghosts besides. Some ghosts entered some African genomes.
For our purposes here: before modern humans mixed with Asians, those modern humans had already mixed with Neanders, presumably in the Near East. Everyone from the Outer Hebrides to Tasmania to Greenland shares in the Neander legacy...
... slightly. One reason it took so long to find any admixture at all in modern peoples, is that said people's ancestry had become more African and less Neander as the generations progressed. Some of the balancing is accounted for by a great return of now-hybrid Europeans back to north and east Africa, on the occasion of the Ice Age ~15kBC. We are absolutely talking whole tribes, with the women: U5 mitochondria show up in the Maghreb, and N1 in Somalia. More important was a dilution in Eurasia: a "basal Eurasian" population, xenophobic enough to avoid contact with Neanders, finally relented so far as to surrender to now-hybrid tribes. A third factor, which we're getting into, was modern-human selection against Neander DNA although, of course, these stretches would cluster where it mattered and not over the "junk" stretches.
Cochran and Harpending further surmised that Neander DNA revolutionised the non-African brain. The African brains and - by then - the Eurasian brains had each reached its equilibrium by, oh, 100kBC. Even that early, there was some outflow from this population to that. Cro-Denis itself was part Neander, and early modern humans had bumped into Neanders - early modern women, specifically. Just not on the scale of the great out-of-Africa 70kBCish. And the sporadic fluid-swapping didn't change anyone's Palaeolithic way of life. Those 70kBC revolutionary changes - so claimed C&H - demanded a Neander-human hybrid, of 70kBC Eurasian proportions. The hybrid could develop in directions Neanders, and Africans, could not on their own.
It turns out Neander DNA stretches - and the various Denisovans' - do have gene-expression to affect the brain. We were told last year that scientists have grown Neander neurons in the lab. Neander grey looks different from African grey at the cellular level. And from our grey. Which pointed away from Neander influence on us, and from C&H.
We may, at last, have our DNA evidence - in that direction (h/t hbdchick). Natalie Telis, Robin Aguilar, and Kelley Harris have posted "Selection against archaic DNA in human regulatory regions". Those regions affect muscle... and the brain. In our ancestors Eurasian muscle was allowed to deteriorate, which means that tools and (necessarily) skills had got better; so the real action was happening on the brain. Bluntly, Neander DNA wasn't good for the head; and the less of it there was, the better our toolset got. Which would "severely constrain", as is politely put, Cochran and Harpending.
Neanders and most Denisovan subgroups couldn't ever maintain much of a population size. The large heads, larger than ours, made breeding difficult. Also, the climate was harsh. (Razib points to southeast Asia being different, but there you run into other problems - like the volcanoes, and in our 100k year time-scales interglacial global warming flooding the shores and leaving behind islands.) Lower populations, and isolated populations, induced inbreeding and genetic drift. Some of those genes caused the Neander traits to drift. In the case of the brain, badly.
Thomas Wynn thought in 2012 that the Neander brain would become ultraconservative, fixed into patterns to survive in their isolated refugia. We today call such tendencies OCD or even autism.
But Cochran and Harpending would say, that's what our Eurasian ancestors needed, was some instinct to stay in the valley we know, even in winter. It led to time-preference. If Neander neurons didn't last amongst our ancestors, perhaps they lasted long enough to protect our developing brains over that last Ice Age.
UPDATE 7/27 11:26 AM - to 7/25 you go.
UPDATE 9/9 - Consider not the brain but the skull. Perhaps the larger Neander cranium allowed modern human brains to grow for longer...?
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