User blog:Orion Invictus/Theory on how an Ultimate Human "works"

I think that, like in so many other shows, humans can breed with any sentient species in the galaxy, hence Ben 10,000 doesn't need to change into any alien, his human DNA is simply overwritten with the alien's abilities without changing much.

That would also explain the Highbreed's (former) hatred for humans, as humans are a species capable of creating hybrids with all known species.