The Stoned Ape Theory was proposed by Terence McKenna in his 1992 book Food of the Gods.
The theory is that using entheogenic substances (e.g. psychedelic mushrooms, ayahuasca, etc.) caused a radical shift in human consciousness and, subsequently, human culture.
However, there's no good evidence to support this theory. McKenna's book, even for its day, was based on precious more than conjecture and huge leaps in logic.
Also, over the past 20 years we've been able to learn much more about the linguistic, artistic, and cultural evolution of humanity. The preponderance of evidence shows that we evolved these things very slowly over tens of thousands of years.
There are still many mysteries to how humans became who we are, and I think there's great promise in using entheogenic substances to help people with depression, end of life issues, and (ironically enough) drug abuse.
There's no need to overstate their importance to past cultures in order to justify why and how we should use them in today's culture.