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

[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.

https://en.wikipedia.org/wiki/Speciation

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.

[Lauront]

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.

Indeed. We can assume similarly of the Void from what we know of it. It's one of my slight disappointments with how they premised the differences of the First and Source, as these are worlds with differing aetheric densities (something the lore is not shy about emphasising as making a difference in outcomes, e.g. here, but also stated in EE vol1) and passage of time... but I suppose they really did just want to do a 1:1 to Eorzea on some level, but more high fantasy.

Even without the usual environmental pressures at play, mating choices alone should've resulted in some manner of speciation that is distinctive across each world, as this influences which traits propagate... but instead we have exactly the same division of races, right down to the clans. Perhaps it just doesn't operate on the same genetic principles as do RL life forms (i.e. for hybrids, traits defining the race cluster back together within a very small number of generations, perhaps even a single, and those influencing individual traits are not so much heritable as randomised within a pool available to a race and clan, or at least not as strongly heritable as with RL humans; combined, something like this would prevent prominent speciation based on mating choices persisting), which is not exactly inconsistent with many life forms being created through magic. Barring the possibility of creations/similar entities, even other stars appear to be mono-racial as well, which makes you wonder about how life originated on each star and what influenced its development. Both the ancients and dragons, for example, possess some very great powers innately.

With that said, for the time being we just have to roll with the fact that there's precious little difference between the assortment of races/species between each Reflection, barring anything that appeared post-Sundering, and assume they're just physical forms potentially created by the ancients, which the star saw fit to dump ancient souls into.

[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.

[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.

[MikkoAkure]

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

Yes and no. Hyur obviously lived on Eorzea in previous eras. But after the 6th Calamity the Elezen were first to repopulate Eorzea, followed by the Hyur. The Eorzea Encyclopedia says that the Hyur immigrated to Eorzea in 3 waves, the first one was over 300 years into the 6th Astral Era and was what led to a large group of Elezen leaving the Shroud and creating Ishgard.

In the original 1.0 lore before Yoshi-P, there were no eras and Elezen were always the original natives of Eorzea so they had to reconcile the old lore with "everyone leaves but then they come back because Eorzea has a lot of aether".