If they bred and had fertile offspring, then, by definition, they were not separate species.
Same Genus, different Species. Example. Lions and Tigers breed together. Produce fertile litter. Litter breeds with another Lion or Tiger. Produces a Litigon.
“If they bred and had fertile offspring, then, by definition, they were not separate species.”
Having fertile offspring is a major characteristic of a distinct species, but not the only factor.
Cattle and buffalo breed well, and are different species
Dogs and coyotes breed well with fertile offsprint
False killer whale and bottle-nose dolphins
Domestic cats and lots of different kinds of wild cats breed
Grizzly bears and polar bears have viable offspring
Etc.
They really want them to be anything but homo sapians.
Then they can talk about "extinct lines" when what happened was groups of humans were isolated for a bit and then as humanity spread out and grew in number those populations were found and began to interbreed.
Have to wonder how much of this comes from a desire to clone these people and treat them as property because they "aren't really people".
Yes, I am a cynic. Or a realist. Hard to tell the difference any more.