> Is there evidence for that mating could only occurr "to an extent," like with horses and donkeys? I am not sure what "to an extent" really means in this cont

> Is there evidence for that mating could only occurr "to an extent," like with horses and donkeys? I am not sure what "to an extent" really means in this context, but modern-day Europeans and Asians have some distinctly Neanderthal and/or Denisovan genes. So, unlike mules, the children of (anthropologically modern) humans and Neanderthals were able to reproduce. In particular this has led some biologists to a rethinking of the Homo sapiens species as being many different (mostly extinct) subspecies: - Homo sapiens neanderthalis - Homo sapiens denisovan - Homo sapiens idaltu [1] - Homo sapiens sapiens (all currently living humans) An important disclaimer, as without context this stuff comes uncomfortably close to racist pseudoscience: compared to every other living species of primate, there is vastly less genetic variation among humans. This is despite the fact that there are far more humans than there are chimpanzees, gorillas, etc. Among the common chimpanzee there are many different subspecies, with distinct genomes. Chimpanzee subspecies are physiologically capable of cross-reproducing, but have intense social barriers that prevents it from happening in the wild (except in cases of rape). By contrast, modern humans only have one subspecies: Homo sapiens sapiens. Genetic differences between human groups are extremely subtle compared to chimpanzees, despite greater physical variation, and even the most bitter ethnic conflicts are nothing like the ancient socio-biological barriers found in chimps (they are transient on a biological timescale, and individual social/sexual relationships can still survive and flourish). So the ancient world of modern humans, Neanderthals, and Denisovans should really be seen as an ancient world with a variety of Homo sapiens subspecies, in an era where are genome was as variable and chaotic as the modern chimpanzee. What specifically happened to the Neanderthals and Denisovans is an open question, though their minimal contribution to European and Asian genomes suggests they weren't around very long as a distinct subspecies once modern humans came over from Africa. So, going back to "to an extent" - the children of modern humans and Neanderthal were biologically successful from the perspective of Charles Darwin, but a culturally disastrous from the perspective of a Neanderthals. [1] https://en.wikipedia.org/wiki/Herto_Man https://en.wikipedia.org/wiki/Herto_Man