Very interesting discussion. The question of the species as a fuzzy concept goes back to Darwin himself: “From these remarks it will be seen that I look at the

Very interesting discussion. The question of the species as a fuzzy concept goes back to Darwin himself: “From these remarks it will be seen that I look at the term species, as one arbitrarily given for the sake of convenience to a set of individuals closely resembling each other, and that it does not essentially differ from the term variety, which is given to less distinct and more fluctuating forms”. Darwin’s statement can be understood today because if you reason in terms of a conceptual discontinuity between species you allow creationists of any kind to imply the intervention of a divinity (something that still happens today); and also: how would you cope with species in statu nascendi? Notwithstanding that, a sharp criterion that consent us to put a sound demarcation is the issue of “genome incompatibility”: once a genetic barrier has been established between the individuals of any two populations, as the (possible) hybrids are not viable or are sterile, we know that it cannot be reversed by natural means, and the genetic pools of those populations will diverge more and more with time. Therefore, they should be considered as separate species. What about Neanderthals and sapiens? It seems that hybridization occurred when the sapiens who invaded Europe some 50,000 years ago encountered the Neanderthals, who had remained separated from the common ancestor for at least 500,000 years (I do not consider this as an established fact, as it is possible, however improbable, that the segments of Neanderthal’s genome we observe in today Europeans are remnants of common ancestry, which have been preserved in Europeans and lost from today Africans by selection or drift). This would be a strong argument in favor of them being a single species. I personally prefer this view also because it helps to support a less “supremacist” view of our biological roots, showing that not only is our species subdivided in phenotypically recognizable populations, but that it was once subdivided in morphologically distinct subspecies.