I do think people overstate how similar we were though -- we don't have any super clear examples of neanderthal artwork (people make arguments, but this stuff isn't exactly Chauvet https://en.wikipedia.org/wiki/Neanderthal#Art). To me, this seems to indicate that their brains might have worked pretty differently.

Does it?

Men like to spread their seed, and I'm not sure what the rules were regarding giving women a choice in the matter. I'm guessing rape wasn't in their vocabulary though.

Sometimes a river is that boundary, or a preferred prey species, a mating strategy, or odor preference.

Otherwise, we cannot distinguish bear, dog, or great-cat species.

It seems to me that if e.g. American black bears and Asian black bears can interbreed, then we could call them all one species—black bears—and put all their subspecies together in into that taxonomic category. Maybe with some optional taxonomic level between "species" and "subspecies" for describing their phenotypic groupings.

But I see, looking at various sources, that those two types of bears are indeed considered separate species. Why do we do that? What's better/more useful about drawing the species boundary there?

"Subspecies" is a concession to what lumpers call splitters.

Why not just define “species” by that clear formal boundary, and then call everything that doesn’t manage to reach that line “subspecies”?

Sometimes the product of mating between species becomes, instantly, another species, if they prefer mating with one another over either progenitor. That just happened, with some birds, in the Galapagos.

Legally, there are no endangered subspecies, only endangered species. So, claiming some variety is "just a subspecies" may mean they get no legal protection against extermination. To me that's more than enough reason for a species.

https://www.theguardian.com/commentisfree/2014/may/02/neande... (and there is link in the article to the study itslef)

"After careful study of archaeological records, scientists in the Netherlands found evidence to suggest that Neanderthals were just as advanced in culture, weaponry and hunting as our human forebears. According to those scientists, the misunderstanding came about because people had been comparing Neanderthals to their successors, who had more advanced tools, rather than their contemporaries. Which is rather like assuming I am more advanced than my parents because I know how to work an iPhone. But this doesn't make my parents any less intelligent … just obsolete and unable to function in this modern, fast-paced world."

So have modern humans.

Certainly not unheard of for modern humans though...