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

[Lurina]

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.

Despite the Elezen's outlook, Encylopedia Eorzea says that Hyur have been present on Eorzea since before the First Astral Era.