Races

[Turnintino]

Someone else here will surely be able to name the exact source (or correct me if I'm wrong), but I believe supplemental material describes the sundered races of men having developed their unique characteristics based on their ecological circumstances (I understand that's basically just a broad description of the concept of evolution in general, but it's what we've got). Coincidentally, though, they all happened to evolve into identical races even on the other shards (at least the ones we've seen, and it looks like you've completed ShB, so you've seen the First). Which does make that explanation a little harder to swallow, but... this is fiction, and making unique races for each shard we visit is more resource-intensive than the devs are interested in investing in, clearly lol.

Never say never, but I suspect we're unlikely to be given anything more in-depth than that. Plenty of room for speculation, at any rate!