The premature tweet has some legs to it, though -- it's accompanying a live stream link from Ubisoft which shows what appear to be cave paintings. The livestream has been going for 4 hours now with still nothing to show except for a repeatedly changing teaser splash, but if you look at the paintings you can make out rhinoceroses, saber-tooth tigers, wooly mammoths and more.
So, what is Far Cry Primal? If I had to guess (and I don't have to, but I will) I'd say it's some sort of Ice Age style open world game. Bows, Arrows, that little raccoon chasing an acorn. The whole bit. It's almost as good as 80s schlock action madness, but without the T-Rexs which spit laser beams it's hard to say whether it will be as good as Blood Dragon. Would you play a caveman Far Cry? What would be this game's Cassowary or Honey Badger?