Given it really seems to be a genetic difference, we'd have to assume their genes were changed at some point on the Source. So, who do we know who were masters of genetic manipulation, were in conflict with the Miqo'te and had a vested interest in surpressing their population, did not exist on the First, and in fact have canonically shown the ability to alter their genes on an intrinsic level, which is the core reason why travel to the First was made possible to begin with?