There is plenty in the lore book about when the different races migrated to Eorzea. But there is nothing about which race is the race the rest of the races are variations of.

What did get revealed at the Tokyo Fan-Fest was that the 1st Shard has all the same player races Hydaelyn does, just under different names. So I'd say that's very indicative of all the races being distinct races at the time of the Shard Split.

From the looks of things, it seems Norvrant has the same geography that Hydaelyn does, just with 12,000 years of timeline difference, which is almost nothing on a geological time-scale. On a cultural time scale, that is huge though. So having all the same races have very different cultures compared to the Source races makes a lot of sense.