But would it ever, necessarily? If Neanderthals and Homo Sapiens are really just human beings with somewhat different physiognomies, in the main, is there really any good reason to assume that human DNA would change suffiently to create a species diversification?
Supposedly, a
Homo erectus population diverged sufficiently from other
Homo erecti, producing the first
Homo sapiens. Or maybe some other species of
Homo was most directly ancestral to
Homo sapiens. In any event, there’s no reason to suppose that
Homo neanderthalensis and
Homo sapiens could not have diverged even further from each other, if only because other species of
Homo have experienced genetic divergence from
their ancestors.
Of course, it’s possible that the rate of divergence between
H. neanderthalensis and
H. sapiens would have slowed down to zero. The fact that Neanderthals and
Homo sapiens interbred, would have largely halted such divergence. But if the two groups had stopped interbreeding, then the rate of divergence would have likely gradually increased.
Certain species of shark and whatnot may have continued over millions of years with little genetic change, true, but that doesn’t stop other shark species and whatnot from experiencing further genetic change.
(My characterizing of Neanderthals as a separate species is not universally accepted, but I find the arguments for it relatively convincing.)