Everyone is an Alien

The large majority of the sentient beings in known space are human, at the genetic level anyway. The fact of it is that so many humans have had traits added or removed or had themselves cosmetically or functionally altered that nobody's really just human anymore.

As you near the vaults, and within the vaults themselves, human purestrains become more commonplace. Even these are from different atmospheres and gravity wells, however, and although they may be cosmetically similar, most are different at one biological level or another.

Some of the better known alien races are the Aquarians, a race that thrives on loose energy, the Ximm, a race of three-eyed xenophobes that claim to be able to see into strandspace unaided, the Librans, a race that naturally produces anti-gravity, and the Vendekost (singular Vendek) a ferocious race of feline bipeds. Also known to exist are a race of automorphs known as the Titanians, who are very easy to meet, but very hard to find.

To that mixture add a few dozen more familiar alien races and a few dozen unfamiliar ones, along with genengineered alpha-races, and AIs, and known space is just brim full of people that can't productively mate with one another. This of course, does not stop them from giving it the old Harvard try.