We wouldn't be able to mate with Nakalipithecus nakayamai, so they are indeed a distinct species despite being our ancestor.
By this criterion, there are very, very few bacterial species.
Or rather, each bacterium is its own species?
Why? Bacteria exchange DNA with each other.
I seem to recall this being the reason the sexual reproduction definition of species is considered problematic. It's a convenient definition for animals but breaks down for most other things, and even the animal kingdom has some funky edge cases, like the https://en.wikipedia.org/wiki/Zebroid.