How do we constitute what is a species and what is a variant or subspecies of another species? I mean, it's harder for fossil specimens, since we mainly only have morphology to go off of, not much genetic information. And molecular analysis-based taxonomy has consistently shown us that morphology can be way off the mark (see modern Scleractinian taxonomy). A species is a clade of animals that can have fertile offspring with each other right? Well, why are Neanderthals and Denisovans considered separate species from Homo sapiens even though they WERE able to have fertile offspring with us, shown by admixture in modern human populations? Or how do we know we couldn't reproduce with H. erectus? Morphological differences could just be chalked up to intraspecific/phenotypic variation.