My reading, based on the dev answer of essentially intelligently-directed evolution, is basically that after the Sundering, the species we call 'Ancients' existed but were no longer biologically feasible; in a land of thinner aether, a body of that size and ability couldn't sustain. So using those last remaining magics perhaps semi-consciously, they developed themselves into the races we're familiar with. Perhaps including some extras that didn't survive, or gradually bred together.

The sahagin are definitely the descendants of the guys from Elpis who were really into sharks.