Stoned Ape Theory

So it turns out the long search for the answer as to how, exactly, human consciousness emerged was right under our noses all along...

Link.

To summarize: McKenna theorizes that as the North African jungles receded toward the end of the most recent ice age, giving way to grasslands, a branch of our tree-dwelling primate ancestors left the branches and took up a life out in the open -- following around herds of ungulates, nibbling what they could along the way.

Among the new items in their diet were psilocybin-containing mushrooms growing in the dung of these ungulate herds. The changes caused by the introduction of this drug to the primate diet were many -- McKenna theorizes, for instance, that synesthesia (the blurring of boundaries between the senses) caused by psilocybin led to the development of spoken language: the ability to form pictures in another person's mind through the use of vocal sounds.

About 12,000 years ago, further climate changes removed the mushroom from the human diet, resulting in a new set of profound changes in our species as we reverted to pre-mushroomed and frankly brutal primate social structures that had been modified and/or repressed by frequent consumption of psilocybin.