Why did some primates emerge with brains twice the size of their ancestors, over many thousands of generations, during the course of several million years?
This hypothesis can only fit inside the huge knowledge gap. Many times, accelerated evolutionary changes signify arms races and bottle necks. An arms race like the Cambrian explosion has lots of things happening at the same time, a form of evolutionary tit-for-tat game. A bottle neck is something like the dinosaurs disappearing. The invasive species concept is, many times, kind of in between.
Since we can't access or develop a perfect map of primate migration patterns from an ideal fossil record of all historic skull sizes over time, demonstrating how the expanding brain case trend took shape, the details for the specific drivers of these changes are unclear.
We have a grand canyon sized void of information, and all we can really do is consider bridge designs that utilize natural forces to connect the empty space separating both sides. Figure, maybe, the stoned ape concept is like some epic beaver dam between us and our predecessors.
Since the human brain is an unprecedented organ, we have few avenues for corollaries. Like the big bang theory, we'll get stuck with whatever idea connects the most dots, fits inside the knowledge gap, whether it ousts other ideas or not.
Even if incorrect, the big bang fits what we see, but can never tell us why time started at all, if material existence truly began as a singularity, or whether something existed prior to moment zero. It may be that the big bang is an event that can only be obscuring prior information, due to some unknowable cosmological process that destroyed any such information predating this mysterious expansion event. Perhaps some crunch or freeze, reduced the sampleable scope of the universe we inhabit and currently observe into a pin-prick singularity we are forced to guess only broad, basic details about. How would we inform ourselves of what exists beyond the observable universe, or learn of events prior to a moment that destroyed all preceding facts or information?
As a hypothetical idea, the stoned ape hypothesis will always deal in uncertainty, at least until we discover the next ground-breaking evidence that changes the current picture. Same with the big bang: we can't really know if it's correct, so much as we can only deal in observable facts evident to our perspective, and that can be unsatisfying.
If multiple stories fit inside an evidence-starved narrative (brains doubling in size), these competing stories will sit deadlocked until a tie-breaker appears, usually with the most easily reasoned hypothesis carrying preference until rendered untenable. Science and institutional academia often shuns juicy excitement, which is where this theory's incredible story is found wanting of incredible evidence.
I'm saying we may never know the real story, in that no single theory might ever accumulate enough backing evidence to be proven correct, by virtue of the processes being examined. SAH operates in a space where we may only find utility in thought experiments.
I can see one or two points of evidence emerging, that could simply blow SAH to bits and put the nail in the coffin of debate. But just as easily, I could see a fossil jump out of the ground, and upend our thinking overnight. The evolutionary tale of human prehistory is that brittle, based on what we have in front of us right now.
The only ways I see SAH being proven, are forward looking replication of results, or longshot statistical truths, found in subtle clues which are very hard to measure, all in one place. Meanwhile, literally any other theory could burst onto the scene with a stronger, more convincing and reasonable narrative, and bury SAH so deep in its own wild and extraordinary whimsy that we never think about it again. But neither has happened yet, so we can still navel gaze on this one.
But, take anything you want. Any externality, leveraging selective pressure, but leaving scant traces of ever even having happened, and you've got a good competing hypothesis with legs under it. Instead of psychoactive substances, why not body lice, for example, as a selective externality? Although less charismatic than SAH, just as reasonable of a story, but just as tough to tease out a smoking gun.
How would body lice influence circumstances favoring for genetic brain size? We might easily hypothesize an illness spread by one kind of louse, whereby the head lice versus the body lice is the deciding factor to produce the selective brain outcome in the same time span. But why the bigger brain on the other side of it? Same as with SAH, the brain size could be an incidental trait. That's the way bottlenecks, invasive species explosions and fitness arms races work in evolution. Some outcomes are non-deterministic, so we never get satisfying proof.
