In particular, he was interested in the altered states that could be caused ingesting high doses of psychedelic mushrooms, and DMT, which he believed was the apotheosis of the psychedelic experience. Although he avoided giving his allegiance to any one interpretation (part of his rejection of monotheism), he was open to the idea of psychedelics as being "trans-dimensional travel"; literally, enabling an individual to encounter what could be ancestors, or spirits of earth.
In his book Food of the Gods, McKenna proposed that the transformation from humans' early ancestors Homo erectus to the species Homo sapiens mainly had to do with the addition of the mushroom Psilocybe cubensis in its diet - an event which according to his theory took place in about 100,000 BC. He based his theory on the main effects, or alleged effects, produced by the mushroom.
The mushroom, according to McKenna, had also given humans their first truly religious experiences (which, as he believed, were the basis for the foundation of all subsequent religions to date).
The above information was taken from McKenna's Wikipedia page. More about McKenna can be found here.