Yes, it’s reasonable.
It also leaves room for lots of speculation about just what difference there was in a small percentage of humans, around 90 to 70 thousand years ago that would favor them so heavily that in a few dozen millenia their genetic heritage would entirely dominate the species. Unfortunately, whatever your favorite hobby horse is, it remains conjecture what exactly caused it. I think it was the development of language.
Tris
Fascinating. Man, I wish I had a time machine and could go back to find out!
To my mind, genocide infers two things:
I saw a on the UK’s Horizon programme on I believe Channel 4 (sorry no cite that I could find) that posited that Neanderthal was actually well evolved for ice-age existence. They could catch things in deep forests and could survive well on the ice and snow and cold, their right arm was actually highly evolved as their spear arm and very very strong and quite efficient at stabbing and thrusting a spear into prey, and Neanderthal were amazingly strong with a build like a weightlifter but even larger. They died out, according to this programme, when the climate changed and the retreating ice left behind plains and more open forests and the animals (prey) changed as well. Modern homo sapiens were better evolved for plains / tundra / open conditions because they were built more like runners and so could use their superior speed, endurance and lighter build to better pursue prey in more open terrain. Neanderthal didn’t have the same talents, and gradually died out.
They also estimated that Neanderthals gradually died out over about 5k years if I recall correctly and continued to move north following the ice; not really a genocide in classic terms, more like the extinction of a species due to climate change much like polar bears are facing now.
Finally the programme also posed the idea that Neanderthals and homo sapiens would likely not have been in direct contention and may never have seen each other; they lived in very different areas, with Neanderthal usually found mostly at the edge of the ice (in that time, middle Europe) whilst homo sapiens started in central Africa and gradually moved north.
So IMHO, as well as not by definition genocide, I kinda doubt that modern humans came about as a result of genocide of Neanderthals or proto-humans, although if they had met and the climate hadn’t changed (I.E. ice retreating) then I wonder who would have won - modern humans were faster and lighter, but Neanderthal were stronger and much tougher. Strikes me as a pretty even fight - both were pretty intelligent animals.
ETA - I think the think that defines the start of civilisation is moving to farming from a hunter-gatherer lifestyle. That didn’t happen until the ice retreated. This tells me that the ‘spark’ that led to humans dominating the world started about 50k years ago when we started to farm our staples rather than have to go out and get them, as it’s less time consuming and means we can spend more time developing things like writing and communications.
One final thought - what makes a civilisation is capturing knowledge. Our brains have not substantially evolved in 50k years - not enough time has gone past. But what has evolved is how we capture information and transfer it to our young.
So to me that’s what makes Humans vastly superior to any other species and ensures our dominance of the world as we know it. And also what differentiates societies - Rome beat Gaul because the Roman records were preserved and they could learn from their mistakes. The technology gap meant little as other posters have already shown, the ability to capture the knowledge required and move it forward with innovation and improve on it was superior with the Romans and why they ruled the known world for so long.
Nitpick: Not subspecies, but species (H.erectus, or some variant). Modern humans seem to have left Africa once before, getting into the Levant around 100k years ago. But that population appears to have died out before the next migration out of Africa which led to the rapid expansion over most of the globe.
You can get one of those, you know.
First, build yourself a wish-granting machine; THEN, wish for a time machine.
Well, the sub-species was because if modern humans numbering more than the numbers from the mtDNA evidence existed before or during the expansion, that population (the one without living descendants) would be a subspecies. Human, but without some characteristic that made the ancestor group so much more successful.
Speculation, I admit.
Tris
No. Mitochondrial DNA works differently from chromosomal DNA. Chromosomal DNA undergoes sexual reproduction, which means it has the amazing ability to drift around a population and spread from a few mutant individuals to millions. Mitochondrial DNA is simply cloned from mother to daughter, and doesn’t spread or compete.
The first possessors of the ‘miracle’ gene would continue to have sex with whatever women they found. These women’s inferior chromosomal DNA would be weeded out versus the men’s superior miracle gene, but the women’s mtDNA would just continue to get passed along.
If mtDNA can be traced back to 1000 women 70 thousand years ago, it means those were probably the only women that existed on the planet to have sex with. It’s not likely that other women existed, but no one with the ‘miracle’ gene got around to having sex with them. Such segregation is possible, but it’s more likely that all the other women were simply dead.
Related is your idea that there was a sub-population of humans that took over Africa. This is a restatement of “segregation,” which sometimes happens. However, more likely, evolution went on as normal and it is the genes, not populations, that spread.
SOP for millennia (until really very recently) has been to kill all the men and rape and enslave the women.
That’s not what defines a subspecies.
All the Tasmanians have died out, but they were not a subspecies. What they lacked for survival was cultural, not genetic.
Unless, of course, the first possessor of the “miracle gene” was a woman…
What does? What defines a species?
As you know, there isn’t universal agreement on that. But for mammals, the BSC* is most often used. A species is a population that can be distinguished from other populations and that freely interbreeds in the wild, producing fertile offspring. Subspecies are populations that are distinguishable from each other, that can interbreed and produce fertile offspring, but don’t do so (or do so only rarely) due to some physical barrier (usually a natural barrier). Populations that are distinguishable, but which vary gradually over geographical areas, are not generally considered subspecies.
Now, those definitions don’t stifle debate, especially when we’re talking about extinct populations-- a situation that brings a whole new level of complexity to the problem. Also, there is no recognized body to approve/disapprove of the classification of populations into subspecies.
Still, identifying populations with largely different mtDNA lines would not be enough evidence to establish a population as a supspecies. That can be done in extant human populations today.
*Biological Species Concept
I agree, but I wanted to let others know that there is no “One True Way” of defining a species. There are experts in beetles who have made species where experts in mammals wouldn’t even consider it a sub-species.
By some definitions of sub-species, the Ainu would have possibly qualified- if they weren’t human. I disagee with those people, but still.
There is still debate over whether the Neanderthal are a species or sub-species.
What about the Ainu that make them especially qualified? I’d like to know more about the Ainu, but if we are proposing human sub-species, surely the usual suspects would be better qualified.
Who then gives birth to a boy with the miracle gene, and he… doesn’t have sex?
Yeah, any reason you picked the Ainu? There are probably better candidates out there considering how long the Ainu have been in contact with ethnic Japanese. I guess one might argue that before the age of exploration there were natural barriers preventing interbreeding between Europe/Asia and the Americas and Australia and Oceana. But that was also before we had the Linnaean system of classification, too. 
There probably always will be. But until we find evidence that the two populations actually did interbreed, then it makes sense to call them separate species, since there was a 5-10k year time period when they overlapped geographically.