There are many theories in regards to why humans developed the mental capacity we enjoy today. Some say it was the consumption of red meat. Some others the fact that we became bipedal and had to "think on our feet". Some others support the invention of cooking since it expanded massively the nutritional value of our food. And lastly, some support that getting stoned with psychotropic substances might have done the trick.
One aspect of this equation that everyone seems to miss is that human evolution is complicated and for the most part it remains a mystery. There are simply way too many variables to take into account. A lucky gene mutation might have contributed to the expansion of our brains as much as a massive change in the ecosystem that somehow forced our ancestors to perceive things differently. Nobody really knows and the answer will never be as simple as most people wish it to be.
Psychotropic mushrooms and other mind altering substances have always been consumed by mammals, regardless of their sapien ancenstry. Animals do like to get stoned as much as humans do. How come none of the other species never developed the mental capacity that early homo-sapiens did? This is the biggest hole in regards to the stoned aped theory. It is a nice hypothesis but lucks in many parts. These substances are unstable. Some get panic attacks, others see visions, others just feel joy or fear. Their effect is way too chaotic.
Most anthropologists are baffled not by the fact that we gained this form of intelligence but the speed upon which it occurred. There was a moment 200.000 years ago when we had no culture, almost no tools and suddenly out of nowhere everything exploded. This sudden event though is not as mysterious as we think it is. Modern examples of how we learn today can help us unravel the mystery. For this we have plenty of evidence. It is much like reverse engineering the basic facets upon which knowledge and what we call "grasp" of a subject occurs.
Learning is not linear. If you put 1 hour of reading you won't get an hour worth of expertise on the subject. If you have tried to master something in the past, especially something you have difficulty understanding at first, you would know that you often spent hundreds of hours before you started getting "eureka" moments. At one point, out of nowhere while being lost, confused and frustrated with the salad of juggled up information in your head, you wake up one morning and you find yourself getting it. You might have spent thousands of hours in intellectual chaos but in just one day everything gets sorted out and you feel like you are on top of the world.
This is exactly in my opinion what caused the explosion of human intellect. Steadily but slowly we started scribbling on cave walls, experimented with primitive types of cooking, started hunting in groups and getting real meat, we left the savannas, became bipedal, had early births with the brain developing mostly as part of the general ecosystem, invented fire. All these were parts and pieces and we couldn't put them together to make a decisive pivot. But one day, much like a young programmer that finally "masters" a language, tribes started having eureka moments by amassing all this knowledge into one mode of thinking that aided culture, survival and development. After that the curve of intelligence eased out and while steadily increasing it hasn't changed much. Same exact thing happens to one when they master something. They continue to grow but the rate of knowledge decreases even if their knowledge expands.
So there you have it. Humans did not just pick up some mushrooms and saw the light. Instead they slowly learned from their environment, carrying out the knowledge from one generation to the next, until one day one generation finally got it. The rest is history, or in our case, still a hypothesis.