Terence McKenna's attempt at a mainstream scholarly monograph, his hypothesis is that our pre-Homo sapien ancestors formed a symbiotic relationship with psilocybin mushrooms in the grasslands of Africa many thousands of years ago. Those who ingested the sacred shroom gained a slight survival advantage due to increased visual and sensory acuity, spatial and abstract reasoning, and other benefits of a threshold dose of psilocybin. Greater doses introduced the first numinous experience, resulting in ecstasy (in the strict sense of the word). Thus we were propelled into what the Indian Vedas (circa 6000-2000 b.c.) refer to as "ancient times", when monthly rituals of mushroom-taking occured (soma) involving ego dissolution, ecstatic dance, sexual orgy, and group mind behavior.
The 20th century mind is nostalgic for the paradise that once existed on the mushroom-dotted plains of Africa, where the plant-human symbiosis occurred that pulled us out of the animal body and into the tool-using, culture-making, imagination-exploring creature that we are.

- from Alien Dreamtime

McKenna's book then leads us to the time when climatic changes made entheogenic substances rare. This resulted in a paradigm shift from the feminine, Goddess-worshipping, polytheistic, polygamous, pastoral life maintained by regular mushroom-taking to the patriarchal, ego-centric, monotheistic, monogamous monotonous life maintained by ego-producing drugs like alcohol. Our post-modern exploration of such things as rave dance culture and psychedelic drug culture are in fact a rediscovery of our lost cultural mode. McKenna calls this the Archaic Revival.