User blog comment:CreatorTheta/Human Evolution Theory in RWBY/@comment-3146930-20140623192955/@comment-24994749-20140624135117

Well, the question where faunus and humans came from also came up in this forum thread before.

Going with a more evolutionary line of thinking:

I think that originally there was a species similar to our Homo Sapiens. Some catastrophic event might have wiped out large parts of the population of all living species in the planet. The Homo Sapiens of that world was divided by the new climatological conditions into two populations isolated form each other (think our America and Europe after the Ice Age ended and the bridge joining Alaska and Russia melted).

If enough time goes by, natural selection will kick in and, the two population groups subjected to different environmental conditions evolved into two different species. your Homo Magnus and the faunus.

Hence the opening narration saying:

"we are remnants, byproducts, of a forgotten past."

Common traits like aura and semblance between the species might be the result of shared natural conditions despite the different isolated environments.

Humans with different skin colours and naming style might be the result of a similar process of migration than the one in our world resulted into different ethnics. Faunus with different animal traits might have come to be the same way on their own side of the "divided world".

The creatures of Grimm might have also appeared during this period of evolution and, by the time the natural barrier separating faunus and humans dissipated, both groups found each other, not solely faced with this new branch of their new species but also with the Creatures of Grimm who wanted to kill them both at all cost.