10k years ago, Dixing and Yashou were already separate groups, and all evidence points to Shen Wei et al being born on Earth. Given that these were aliens advanced enough to make an interstellar spaceship, I'd guess that they'd be able to genetically engineer themselves. (Terraforming is expensive – and given that Earth already had sentients, I'd guess they would've preferred to do the more ethical thing of changing themselves to suit the environment, vs potentially driving the Earthlings into extinction by changing the environment to suit them.) As for whether Dixing and Yashou are cross-fertile or not, idk, perhaps we should ask Shen Wei. :P