
Originally Posted by
Lurina
The Doylist answer is that the writers, like a lot of stuff associated with the Sundering, probably just didn't want to create a definitive answer. Any clarification about the specifics of what "mankind" is in the modern day and how it diversified from basically just humans would lead to some uncomfortable questions about the ethics of what took place, and put some weird ideas in player's heads about the racial politics of the setting without doing anything to compliment the themes they had in mind.
For a Watsonian answer, you could reasonably guess that the Hyur and maybe the Elezen are directly descended from Sundered Ancients, while the other races are descended from creations or are genetic spinoffs. Aside from being visually the most similar, in the Elpis MSQ, an Ancient comments on the attributes of your race compared to their own, and the only remark Hyur and Elezen get is that their aether is really thin, while others are singled out for their bestial and unusual features. And the Au Ra backstory makes it almost explicit that they, at least, were created by the rival sides of the Zodiark vs Hydaelyn conflict.
The Hyur are also omnipresent across the entire planet in a way the other races are not, including in the New World, which seems to indicate them previously having been the dominant species of the world. Though, that's of course a post-hoc justification. The real reason is that they're fantasy humans, and making fantasy humans the generic race that's everywhere is just a thing writers do.
The issue with this explanation is: Why would the races diverge in exactly the same way in both the Source and the First, when everything else about their timelines went differently? Any explanation has to be something which can be attributed to the moment of the Sundering, not afterwards.