Why are we the only ones?

[Chronos]

Quoth fierra:

If they reproduce solely by asexual reproduction (many simple sea creatures & 99% plus of single celled creatures (not getting into issues about bacteria etc swapping dna)), then they obviously can’t produce a fertile offspring with species x, y or z as they can produce no offspring with x,y, or z.

But by that same criterion, two members of species x, which can’t interbreed because they can’t breed with anything, would be considered a separate species. I don’t think that anyone’s willing to claim that each individual bacterium is a separate species.