I have a similar theory to McKenna. We essentially became symbiotic with psychedelic mushrooms to some extent. Without them, we are not functioning properly anymore.
The effects of the mushrooms are not a defense mechanism - it takes too long to work and is perceived as enjoyable - but it could be a symbiotic relationship with mammals, though. Some mammals, such as primates and rodents, consume psychedelic mushrooms as a food source. This can lead to the dispersal of spores and the spread of fungal species.
When humans began herding cattle, we entered into a fundamentally symbiotic relationship with the mushrooms that grew from their dung. We invented religions based on the mushrooms and drew tons of cave art based on them and their entheogenic experience. We learned to preserve them in urns filled with honey. Their beneficial effects shaped us as we evolved.