eli5 If it’s suspected that early humans interbred with other species of humans, why would they be considered different species since the offspring were obviously fertile?
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Because as much as people have attempted to draw the line of speciation at ‘no longer interbreed to produce viable offspring’ it just isn’t that simple or how we’ve categorised species in most of history. Species is a line we draw on a continuum of physiological and genetic differences. At some distance between points on this continuum those differences become too great for them to still interbreed, but for example, we could have two extremely morphologically and genetically similar animals that can themselves interbreed, but only one of them is capable of interbreeding with a different animal – so then the two should be different species? That seems obviously silly.
Species itself is primarily useful as a way of describing different populations of similar animals, and throughout most of history has basically been built on the principal of looking different. Taxonomy itself can be an incredibly contentious field with scientists variously arguing some animals should be the same species and others entirely different.
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