That&#x27;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 &quot;These two definitely couldn&#x27;t mate&quot; (e.g. Australopithecus afarensis and Homo neanderthalensis), but there&#x27;s always going to be a large gray area. In fact, here&#x27;s an interesting quote from the OP article:<p>&gt;  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 <i></i>(with a small amount of interbreeding)<i></i>.<p>So actually, it seems we are confident that Homo sapiens <i>was</i> capable of reproduction with some of the other &quot;species&quot; we&#x27;ve identified.