It's becoming slightly amusing at times for sure
A trippy plant caused us to evolve greater intelligence?
Riiiiiight...
I'm not so sure of the veracity of the theory but I can still see how it would work theoretically.
The reason being that physically in terms of evolution there is no food that readily drastically alters your brain / mind / consciousness or physiology like psychedelics do, apart from maybe toxins or poisons.
If for example we were talking about the possible evolution of our ability to run then a food in our diet that we found which doubled blood flow to you legs would have provided a tremendous advantage for natural selection. Or if talking about the evolution of art and language then altered states of consciousness would have provided a tremendous advantage in terms of natural selection. If talking about foods with an effect on gene expression then populations that ingested the food that randomly increased genes relating to brain size would be more likely to survive in terms of mental capacity evolutionarily.
The one thing that has profound effects on consciousness totally across the board are these primitive psychedelics. Our brains are wired for them, you could even argue that the consciousness altering states and wiring has evolved in part due to us evolving with them. Receptors definitely came about first. They have endogenous ligands, so there's no reason to believe that an exogenous substance would be needed to cause their development, especially seeing as how a good deal of neurotransmitters (serotonin, GABA, dopamine, etc) are almost ubiquitous among mammals. For example the endocannabinoid system regulates, among other things, appetite and the sensation of pain. Its endogenous ligands include anandamide, 2-AG, and NADA.
GHB is actually a naturally occurring substance in the CNS; it's produced in the brain via the reduction of succinic semialdehyde. If a drug doesn't interact with existing neurotransmitters in any way, then no effects are elicited, and as such, there's no reason for the user to continue consuming the substance. DMT is another neurotransmitter already in our brains, there are many others, and mushrooms are simply 4-HO-DMT.
Now the reason why mushrooms in particular are good candidates for this is that at low threshold dosages they drastically increase visual acuity (
Fischer, Roland; Hill, Richard (1970). "Psilocybin-Induced Contraction of Nearby Visual Space". Agents and Actions 1 (4): 190–197). For a species of tree dwelling primates and hunter gatherers this would provide a tremendous advantage in hunting for food and climbing trees. And they would have to come down out of the trees out of their comfort zone to do this, as the only place this miracle hunting food grew was on the floor of the forest, thus starting the human evolutionary process.
I can see very little way round the fact that one of the greatest things to influence the evolution of consciousness would have been one of the things that most drastically alters it. Mckenna called this the stoned ape theory of human consciousness.