Board Thread:Speculation House/@comment-24994749-20140808215337/@comment-24994749-20141209223624

There was a theory, that was proposed when the WoR Grimm came out, about Grimm having a non-biological origin, and rather being weapons left behind by a civilization from a war before humans arrived to Remnant. They suggested that humans happened to match the attack criteria of the Grimm, so when humanity appeared in Remnant (by evolution or arriving from outer space) the Grimm that were dormant activated again. There would a "Mother Grimm" that is capable of producing more units to continue attacking targets as Grimms are killed by hunters.

Going by that idea, maybe civilization X was at war with the ancestors of the faunus. Modern faunus are the result of having evolved or mutated to gain animal traits and avoid Civilization X weapons. Many generations later, humans arrived, migrating form another planet, Interstellar style and awoke the Grimm.

Alternatively, going by the theory that humans and faunus are subspecies of one common recent ancestor (hence why they can interbreed), the Grimm were created by Civilization X as part of a war between two factions of it (think the animated movie "9" from 2009). The Grimm got out of control, the faunus come from a faction that evolved to no longer match the Grimm attack criteria (or were created like in "9") humans are the other faction of the Civilization that survived in scarce numbers, facing near extinction (when human numbers rose significantly, the Grimm activated again).