Humans evolved from apes incredibly fast. Our brain size doubled in less than two million years. According to McKenna, this all happened because of psilocybin mushrooms. He began forming this theory when he observed wild baboons in Africa turning over cow pies looking for grubs. And ya know what else grows on cow shit? That's right, psychedelic mushrooms! It is entirely possible that apes would have unintentionally ingested these mushrooms along with their bugs.
Okay, so we've got a bunch of stoned apes but what does that have to do with evolution? First of all, very small doses of psilocybin greatly improve visual acuity. A monkey that sees better is going to be more successful with hunting and gathering. Therefore, the stoned ape is more likely to survive than his square brother. In slightly larger doses (we're still talking about very small amounts) the mushroom acts as a sexual stimulant, so the stoned ape breeds more too. The mushrooms also help to break down social boundaries so it's likely to help diversify (and thereby strengthen) the gene pool.
As the doses get even heavier (now we're cookin!) our ape begins to experience synesthesia. Sight and sound are all mixed up and linguistic thinking begins for the first time. Suddenly we've got a bunch of tripping, fucking, talking monkeys on our hands. Before you know it they've started a band:
Ah, fuck it. Let's just watch a cartoon.
Hear it (and much more cool shit) in McKenna's own words here.
