That's a problematic criterion when trying to classify long extinct species. We can hypothesize, and perhaps there is a point where we can confidently say "Thes

That's a problematic criterion when trying to classify long extinct species. We can hypothesize, and perhaps there is a point where we can confidently say "These two definitely couldn't mate" (e.g. Australopithecus afarensis and Homo neanderthalensis), but there's always going to be a large gray area. In fact, here's an interesting quote from the OP article: > A small group of Homo sapiens left Africa 60,000 years ago and settled the rest of the world, replacing the other human species they encountered (with a small amount of interbreeding). So actually, it seems we are confident that Homo sapiens was capable of reproduction with some of the other "species" we've identified.