We continue to comment on things related to paleontology. This is not our field, and we've commented mainly on what we believe are premature if not insufficiently baked speculations about what evolved for what in our early history of a species, around 2 million years ago. But there is a more recent (paleontologically) story that is of interest and is closer, at least, to what we at least know something about.
For decades there have been debates about whether or in what sense Neandertals, a kind of fossil relative of ours that existed from about 400,000 to about 40,000 years ago in Europe, and similar creatures elsewhere, co-existed with our immediate ancestors. Neandertal-looking fossils disappeared 40,000 years ago, but more modern looking fossils seemed to have become common about that time. The question has been whether these were different species, where and for how long they might have been cohabiting contemporaries in Europe, and whether there was any mating between them.
Not long ago a relatively complete set of Neandertal DNA sequence data was reported, that confirmed that overall Neandertals seemed to have a deeper but somewhat separate common ancestry relative to us modern humans. But bits of the Neandertal sequence seemed too similar ours to be part of that picture. That suggested that there was, in fact, some admixture. In turn, that means that Neandertals and our 'modern' looking ancestors weren't separate species in the clear-cut classical sense. But the evidence is not overwhelmingly strong and it's statistical in the sense that common ancestry times and degrees of relationship among DNA sequences have to be judged in relation to probabilistic models of deep population history.
How that apparent admixture happened is a matter of demography--population size, location, contact and mating patterns. If there were doubts about the fact of contemporaneity of the two groups, then clearly they could not have mated, and the genetic evidence would need some re-thinking.
Now there are reports of a human-ancestral fossil from England that has been recently dated to about 40,000 years, the first or oldest known modern-looking fossil in that part of Europe. This is touted in the media, as usual, because all stories are touted as greatly as the investigators and co-conspiring journals can manage. Still, if it is a 'modern' ancestor--the fragmentary nature of the bits of tooth and bone make that still open to discussion, it is evidence of the potentially long (thousands of years) co-habitation of modern ancestors and doomed Neandertals in Europe.
If the evidence holds up, the evolutionary dynamic question is what extent and kinds of contact and interaction would be expected to have taken place, and whether this is consistent with the sequence data as we described above.
The story, like all such stories, gets a lot of press ink, but one thing is important to think about before we get too excited. There are already substantial differences among modern humans from South American to Africa, peoples who have been separated for around 100,000 years. But they are clearly not separate species. When people from one location travel to another, as Europeans and Africans and Asians have come to North America for example, they naturally mate. There is admixture--but not species admixture. So, first of all, Neandertal-premodern mating would not be a shocking surprise.
Secondly, the definition of 'species' is well-known to be problematic. We know rather clearly from the Neandertal sequence data already available, that the Neandertals were about as close to us, relative to our mutual relationship to common ancestry with our closest relatives the chimps, as we would expect from the fossil dates. So, if Neandertal sequences were from 40,000 years ago, and Neandertals split 400,000 years from our own lineage, but we had about 8 million years of overall common ancestry, you'd expect that 400,000 years ago, when we and the Neandertals were one species, we were already 15/16ths identical to each other compared to chimps (because 400,000 is 1/16 of the way from 8,000,000 years ago to today. We were all essentially 'human' by then.
Thus even by the most species-separating, melodramatizing advocate of 'admixture', the Neandertals and humans were essentially already the same species in terms of their overall DNA. Whether or not we 'could', or just didn't, inter-mix.
Of course if Neandertal and our various ancestors never lived nearby, then of course no sort of direct 'admixture' between the two could have taken place. So if the DNA suggests something like that did occur, and if the existing fossil dates, reinforced by the dating of the new British find (no tea was found at the site, by the way), at least the opportunity for mating was there. Opportunity may be nine points in the law, and the DNA suggest at least some admixture.
Still, thinking about it, while it's very newsy, there is nothing particularly scientifically important about the question from a genetic point of view. Yes, there could be traits found in Neandertals but not in our lineage. Bigger brow-ridges may be an example. But we vary among ourselves in important traits (like disease susceptibility), and clearly in many physical traits as our 'racial' variation shows. This may have a lot to do with genetics, case by case, but it has very little to do with species or admixture.
For these reasons, despite the genocentric focus of all the news, the really interesting questions are not genetic ones. They are cultural ones. What were the Neandertal and pre-modern populations like in size, behavior, environmental preferences, language, technologies, and so on? What were their mating patterns? When they saw each other, what kinds of relationships did the two groups have? If there was intermating, were there recognized kinship affiliations among the adjacent villages of the two groups? Did they share religious, body-decorating, food preparation and preference, and so on patterns?
It is these things, not genes, that could have been far more important in determining what happened. It is true that genetic differences, even in something like body smell, could have inhibited mating. But why do so many go so far out of their way to suggest things, than to try to find cultural explanations?
In part this is because genes mean grants, the news media eat it up, and they're the causes-du-jour of our time. It is possible, certainly, that there were genetic barriers (even our favorite self-promoting criterion of intelligence). But then to suggest that and yet see some intermating evidence, leads to forced explanations (such as, essentially, rape of Neandertals by aggressive, superior, violent 'moderns'). That, too, is catchy and may attract media as well as professional attention. But so far, it's not supported by any actual evidence beyond the imagination.
Again, right under our noses is the most fascinating question of all: how one human-like species completely displaced or replaced another. The Neandertals disappeared, if they were indeed more than a 'racial' group at the time. But before that, even more surprising if current views are close to correct, about 100,000 or fewer years ago, premoderns spread out of a spot somewhere near east Africa, and the long-established Homo erectus creatures disappeared.
It's difficult to imagine how these things happened, other than by accepting superficial, nearly evidence-free speculations. But in any case, those questions seem more interesting, and are certainly more challenging, than always focusing on genes.