In 2017, at Psychedelic Science, the largest psychedelic science conference in the world, the famous mycologist Paul Stamets stood on stage and asked a room full of scientists to reconsider how we became human. “I want to bring back the concept of the Stoned Ape Hypothesis,” he said, calling it “a very, very plausible hypothesis for the sudden evolution of Homo sapiens”. The audience loved it. In my little corner of the world, people still do. I hear it all the time in conversation, podcasts, in my comments: magic mushrooms made us human.
Most people only know that one-line version. The full hypothesis is stranger, more detailed, and more interesting. Before I tell you what I think of it, let me give you all the details.
The idea comes from famous psychonaut Terence McKenna’s 1992 book Food of the Gods. He takes us back to the landscape our prehuman ancestors lived in. Over the last few million years, Africa was drying out. As the climate shifted, forests thinned and our primate ancestors were pushed onto open grasslands, where the diets they were used to were sparse and everything unfamiliar had to be tested as food. They began following herds of wild cattle, for meat and for the insects in their wake. And there, growing in the droppings the herds left behind, was Psilocybe cubensis: a psilocybin mushroom.
Naturally, McKenna assumed, our ancestors tried them. While I think this is very plausible, there is no direct evidence they did: no hominid site, coprolite, or bit of dental calculus has ever shown a trace of psilocybin. It is the founding assumption of the hypothesis. From there, the particular survival benefits, McKenna argued, depended on how much psilocybin they consumed. Neuroscience and psychology research shows us that the effects of psychedelics are dose dependent. For us, a low dose means subtle shifts in perception, mood, and energy, and a high dose means a full trip with visual hallucinations, ego dissolution, and the mystical experience.
For our ancestors, McKenna claimed, the benefits also climbed with the dose. At low doses, psilocybin sharpened vision, especially edge detection, the visual system’s knack for picking an object’s outline out from its background, and sharper vision made the mushroom-eaters better hunters. At medium doses it aroused them, which meant more sex and more offspring. And at high doses it dissolved the boundaries of the self entirely, producing ecstatic states, strange vocalizations, and a group cohesion he believed sparked language, ritual, and religion.
The main claim of his theory is that mushrooms sped up evolution, expanded consciousness, and made us capable of things our prehuman ancestors were not, like language, symbolic art, and religion. So how would a mushroom meal get into your genes to do this? Here McKenna offered two different answers. Sometimes he claimed the compounds were themselves “mutation-causing”, directly rewiring the brain’s evolution. That version is simply wrong. A mutagen is something that changes the DNA sequence itself, the way radiation or certain chemicals can, and psilocybin has never been shown to do that. Whether a mushroom could change something subtler than the sequence itself is a different question, which I will explore later. Elsewhere he argued that psilocybin “changed the parameters of the process of natural selection by changing the behavioral patterns upon which that selection was operating.” Evolutionary biologists call this the Baldwin effect: behavior changes first, and natural selection then favors the genes that support the new behavior. In my view, this version is plausible. The question is whether the evidence cooperates.
Let’s start with his vision claim. It traces back to a handful of small studies from the 1960s by a researcher named Roland Fischer, and McKenna’s summary of them. The problem for McKenna is that Fischer’s work did not show visual acuity improving after small doses of psilocybin. His 1970 paper found that psilocybin distorted nearby visual space; no paper of his reports the low-dose improvement.
There is a fairer, modern version of the low-dose claim, and it runs on an idea I write about often: the brain perceives by predicting, filtering the flood of the senses through what it already expects I wrote a whole essay on this.
Psychedelics relax that filtering so that more sensory information makes its way up to our conscious perception. So would a less filtered, more “present” hunter notice more, catch the outline the forest was hiding? Maybe. But hunting rewards the opposite: holding one moving shape and ignoring ten thousand shivering leaves. Relax the filtering, and everything glows with novelty, prey and leaf alike. The doses do not cooperate either. Low doses most likely do not influence this filtering at all. High doses would loosen the filtering too much; no one hunts better when the world is warping and the trees are breathing. But in the days after a high dose, studies find lasting gains in mindfulness, openness, and present moment awareness. This isn’t the same thing as sharper attention or better perception, but it’s something.
When it comes to medium doses of psilocybin making more babies, McKenna offered no evidence for this, and no study since has shown psilocybin works as an aphrodisiac.
His third and primary claim is the one I can kind of get behind. According to McKenna, high doses sped up evolution by dissolving the boundaries of the self entirely, producing ecstatic states, strange vocalizations, and a group cohesion he believed sparked language, ritual, and religion. This does get at something real. High doses really do dissolve the self, and people who go through that together really do come out bonded; modern trials document both. Whether an experience like that could ignite language is a different kind of claim, one no experiment can reach.
Psilocybin is ancient, some 65 million years old, arising right around the time of the meteor that ended the dinosaurs. No, psilocybin did not come from space. But in the aftermath of the impact, with a world of dead wood to decompose, fungi thrived. It did not evolve for us, though. The genus’s ancestral home was rotting wood, where the molecule likely first served as a defense against insects competing for the same food. Psilocybe moved onto animal droppings much later in its history, and genetic work has shown that Psilocybe cubensis, the species in McKenna’s story, likely spread around the world with domesticated cattle, within the last ten thousand years. The mushroom of the savanna scene, in other words, was probably not there. Could our ancestors have met some other psilocybin species, somewhere else? Nothing says they could not have. Nothing says they did.
And when did we evolve? The human brain roughly tripled in size, from about 450 cubic centimeters in our prehuman australopith ancestors to about 1,350 cubic centimeters in us, and that expansion played out between roughly two million and two hundred thousand years ago. Stamets tells it differently. “What is really important for you to understand is that there was a sudden doubling of the human brain 200,000 years ago,” he said in that 2017 talk, as reported by Inverse. “From an evolutionary point of view, that’s an extraordinary expansion. And there is no explanation for this sudden increase in the human brain.” But even Inverse, covering the talk sympathetically, noted that the picture is messier than that, and that the doubling anthropologists actually describe belongs to Homo erectus, unfolding between roughly two million and seven hundred thousand years ago. And when researchers analyzed the fossil record quantitatively, they found the increase was gradual, accumulating within species like Homo habilis and Homo erectus over millions of years. The fossils show no sudden doubling at two hundred thousand years. There is no spike for a mushroom to account for. And the growth was mostly finished long before any datable evidence of the expanded consciousness that sets us apart. Language leaves no fossils, so no one can say when it began; what we can date, symbolic art and religion, only shows up in the record around forty thousand years ago.
And the truth is, the field is not short on alternative explanations for our big brains. Richer diets and cooking freed up the energy a bigger brain demands. Larger, more complicated social groups demanded better social skills. And once culture existed, it selected for brains that could carry more of it. Each of these has its own literature and its own arguments, but notice what they have in common: the leading answers to what made us human are already social ones. When Inverse asked Michael Graziano, a Princeton professor of psychology and neuroscience who studies consciousness, about the stoned ape theory, he had never heard of it; but he landed in the same social place, arguing that consciousness itself may have evolved because social intelligence paid. “It is possible that consciousness emerged partly to monitor, understand, and predict other creatures, and then we turned the same skill inward, monitoring and modeling ourselves,” he told Inverse. And I cannot help noticing that the skill Graziano describes, modeling others and turning the model inward, runs on the brain’s predictive machinery, the very machinery psilocybin loosens. So maybe it played a role in this way. Hold that thought.
Even so, modern research has shown that psychedelics really do remodel the brain. A single dose promotes the growth of new connections between neurons in mice, and those changes can persist for weeks. A dose can even change which genes brain cells are using, and those changes last too. This deserves a careful pause, because it sounds like the mutation McKenna wanted, and it is not. A mutation is a change to the DNA sequence itself; that is the raw material evolution ultimately runs on, and psilocybin has never been shown to cause one. What psilocybin changes is epigenetic: chemical tags that sit on top of the DNA and turn genes up or down while leaving the sequence untouched. So ruling out mutation does not rule out epigenetics. They are two different kinds of change, and the epigenetic one is where the interesting question lives.
The interesting question is inheritance: could tags like that pass to her children? This has actually been tested, in mice, and the honest answer is: sometimes, narrowly. In one well-studied strain, a mother’s diet changes the tags on a single gene and her pups’ coat color shifts with it. A father’s stress or diet can leave faint signatures in his offspring. And in a famous and still debated study, a trained fear of a smell seemed to pass to pups through tags in sperm. So epigenetic inheritance is real. But notice how narrow the tested cases are: coat color, metabolism, a conditioned fear, and the effects often fade within a generation or two. Nothing like a skill, a memory, or an insight has ever been shown to transfer. And for psilocybin specifically, the inheritance step is not just unproven, it is untested: the epigenetic changes we have found are in neurons, and no one has looked at whether the drug changes eggs or sperm at all.
That leaves the path that does hold up: culture. What if the mushroom-eater gains some knowledge, skill, fear, or adaptation, has babies, and simply shows them? When one Japanese macaque learned to wash sweet potatoes, the skill spread through her whole troop and down to the next generation, without a single gene changing; the young picked it up by watching her. So if the mushroom left anything in our lineage, it was most likely passed on the way the potato washing did: through culture first, and then, if the new way of living paid off for long enough, through selection slowly favoring the brains best suited to it.
I am not the first to land here. In 2021, two researchers argued in Frontiers in Psychology that ritual psychedelic use may have joined laughter, music, storytelling, and religion in nudging our lineage toward prosociality. To be clear, what those authors call “most certainly false” is McKenna’s strong version, the claim that psilocybin by itself made us human. Their own, humbler idea they hold openly, as speculation. Mine is the same kind, and I want to be as plain about that as I have asked everyone else to be.
Start with what our ancestors were actually doing while their brains grew. From about 1.76 million years ago, they were shaping teardrop stone handaxes and copying them faithfully for over a million years, which means the young were learning the craft, generation after generation, from the old. They were hunting animals far too large to take alone. At Dmanisi, around 1.8 million years ago, a man survived for years without teeth, which some paleoanthropologists read as the earliest evidence of a band caring for its weak. And by around four hundred thousand years ago, they were gathering around hearths. The signature behaviors of the expansion window were not sharper eyes or more offspring. They were learning from each other.
Now line that up with what psychedelics demonstrably do. In humans, they increase emotional empathy, openness, and cognitive flexibility. And it is not just humans reporting on themselves; other animals learn differently on these molecules too. Rabbits given LSD learn a conditioned response faster. Mice given low-dose psilocybin unlearn a trained fear faster. And psychedelics reopen the critical period for social reward learning in mice, the window in which a brain most easily learns from others. So imagine an ancestor, somewhere in that long stretch, eating whatever psilocybin species grew where she lived, and coming back a little more open, a little more tuned to the others around her, and maybe a little more innovative in her thinking. Maybe she thought... What happens if I carry the ember? What happens if I keep the sharp stone? Her band copies what works. Culture carries it forward, and if the new way of living pays off for long enough, selection slowly favors the brains best suited to it. No mushroom has ever been found at any hominin site, so this is a story about what is possible, not what is shown.
It is a smaller story than the one Stamets asked his audience to bring back. But it is one that works with what we know about our human evolution.
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