Would still like to know how mankind deviated into so many different variants

[Kakurady]

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.

Lore implies Hyurs (like real-world humans) are just really good at long-distance migrating.

In the character creation screen, it says Elezens regard themselves as native to Eorzea, while Hyur are said to have migrated to it from surrounding areas.

Of course, that having happened several eras ago, no-one in universe or out would know how reliable that is.