Board Thread:Theories and Speculation/@comment-1512913-20150116120105/@comment-1512913-20150116123310

I speculate that the Ootsutsuki seeded many planets, hence humans being their creation (akin to Titans in Warcraft). That would quite well explain the similarity in appearance. Also Kaguya's statement about Earth being her nursery may very well support my hypothesis.

So essentially, the Ootsutsuki seeded many planets with their creations for the purpose of creating the Shinju thus chakra, meaning its existence had been their doing and thus wasn't random, hence how they were aware of it, they just didn't know which planet successfully produced it. If true, this makes humans essentially lab rats. Makes sense, doesn't it?