Comment: I posted this (despite the fact that Terence McKenna's voice drives me insane) because I think the Stoned Ape Theory is an entertaining piece of speculation, not because I think it's true. Something like this could have happened, I guess, but you can say almost anything happened a hundred million years ago if you're not too worried about the evidence, or, in this case, the lack of evidence.
Aliens could have mated with our monkey ancestors or intelligent mushroom spores could have invaded their primitive brains, leading to the evolution of language and tool-making abilities. Who knows? I don't think that's what happened, but I can't say for sure because there's no evidence one way or another to account for the emergence of human consciousness (whatever that is).
The assumption behind all this speculation about the emergence of the truly "human" mind is that a huge gulf exists between the minds of humans and the minds of animals that has to be accounted for somehow. Personally, I don't think this gulf exists, but I don't have any evidence to support that other than my own observation of human behavior. Maybe we're as special and radically different as we like to think we are. Maybe we aren't. Until we can enter into the minds of animals (PDF), we'll never know for sure.