The whole concept of different human species may be a taxonomic error of historic proportions despite the morphological differences we see in the fossil record. Information on the genome of archaic humans is a very recent development in a centuries old field so it is still catching up, especially at the level of educational material.
Based on my reading of the paper about the sequenced neanderthal bone [1] and a global genetic variation study [2], the difference between neanderthals and modern humans isn't that much bigger than the natural variation within the modern human genome. That difference is much smaller (on the order of 10-40x) than the difference between modern humans and chimpanzees and given the multiple genetic bottleneck events in our evolution, I think it's much more accurate to look at all the different species of archaic humans as breeds of modern humans that happen to show a larger difference with older samples because of the limited founding population and the diversity of our ancestors (of which we have very few biased samples).
Where to draw the line in speciation is always controversial but my theory is that once tool use really got going by the second stone industry, early humans started artificially self-selecting for intelligence just like we later did with dogs and eventually the modern human "breed" was born. By the million year ago mark, roughly the time evidence of fire started showing up in the archaeological record, I think the species that is modern humans was already long spreading and out competing other apes. I think shortly after this point is when we started developing clothing and moving into the colder climates, leaving evidence at places like Atapuerca.
We're not used to seeing diversity in humans, as the extant population is remarkably same-ish apart from very extremely minor traits like skin colour or eyelid variations. There's quite a bit of genetic diversity within the African continent, but even there it's remarkable how well we all cluster together, genetically.
So given a fossil record where we find people without chins and broader shoulders, etc. we go crazy with the species boundaries.
But given how rare fossilization conditions are, we're only seeing a tiny fraction of what was out there anyways, so it seems remarkably bold to classify with such confidence.
The reality is that nature doesn't "care" about species. There were likely bagloads of individuals who sat on a continuum of a diversity of traits that have since disappeared because of population bottlenecks at, disease, famine, homogenization, etc.
Based on my reading of the paper about the sequenced neanderthal bone [1] and a global genetic variation study [2], the difference between neanderthals and modern humans isn't that much bigger than the natural variation within the modern human genome. That difference is much smaller (on the order of 10-40x) than the difference between modern humans and chimpanzees and given the multiple genetic bottleneck events in our evolution, I think it's much more accurate to look at all the different species of archaic humans as breeds of modern humans that happen to show a larger difference with older samples because of the limited founding population and the diversity of our ancestors (of which we have very few biased samples).
Where to draw the line in speciation is always controversial but my theory is that once tool use really got going by the second stone industry, early humans started artificially self-selecting for intelligence just like we later did with dogs and eventually the modern human "breed" was born. By the million year ago mark, roughly the time evidence of fire started showing up in the archaeological record, I think the species that is modern humans was already long spreading and out competing other apes. I think shortly after this point is when we started developing clothing and moving into the colder climates, leaving evidence at places like Atapuerca.
[1] https://sci-hub.se/https://www.nature.com/articles/nature128...
[2] https://www.nature.com/articles/nature15393