This is known as the &quot;Stoned Ape Theory&quot; and was (to my knowledge) first popularized by Terrence McKenna.<p><a href="http:&#x2F;&#x2F;www.lycaeum.org&#x2F;~sputnik&#x2F;McKenna&#x2F;Evolution&#x2F;" rel="nofollow">http:&#x2F;&#x2F;www.lycaeum.org&#x2F;~sputnik&#x2F;McKenna&#x2F;Evolution&#x2F;</a><p>From the link above:<p>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.<p>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&#x27;s mind through the use of vocal sounds.