by Alex Boese
But what about the sheer unlikeness that a pig and a chimp would ever mate? McCarthy didn’t believe this was an issue either, as long as a pig had been the father and a chimp the mother. Anatomically, only this combination would work, and behaviourally as well, because a male pig wouldn’t have been fussy about what it mated with. Zoologists note that it’s quite common for many animals to attempt to mate with what are euphemistically referred to as ‘biologically inappropriate objects’. The female chimpanzee, McCarthy speculated, might have cowered submissively because she felt threatened. This was the romantic scenario he envisioned as the origin of the human species.
McCarthy imagined that this baby pig–chimp, once born, had been raised by chimps. When it reached maturity, it then mated with other chimps, as did its descendants. As the generations passed, this interbreeding caused the hybrid line to become progressively more chimp-like, which explained, McCarthy said, why humans today are far more primate than porcine. Most of the pig has been bred out of us. He argued that this same process would have concealed much of the genetic evidence of our pig ancestry, making it an extremely challenging task to detect it.
McCarthy self-published his pig–chimp hypothesis online in 2013. Realistically, that was the only way he was going to get it out in front of the public. No academic journal was going to touch the thing. Within weeks, the Daily Mail got wind of it and ran an article plastered with the headline, ‘Humans evolved after a female chimpanzee mated with a pig’. This gained McCarthy a worldwide audience and instant notoriety.
Mainstream scientists went apoplectic. Several angrily suggested that, if McCarthy seriously believed pigs and primates could produce an offspring, he should try to impregnate a pig, preferably with his own sperm, and report back the results. McCarthy demurred.
His detractors had plenty of more specific criticisms. Many of them focused in on his list of ways that humans differed from chimps, blasting it as being cherry-picked to support his odd thesis. They noted that it omitted the two most obvious differences, which are that we walk on two legs and have big brains, neither of which are pig-like traits. The various similarities, they explained, while intriguing, were a result of convergent evolution, which is the phenomenon of unrelated species evolving similar features as a result of having faced similar selective pressures.
Critics also insisted that, despite what McCarthy claimed, there definitely were barriers to reproduction on a cellular level that would prevent pig–chimp hybridization. The pig and primate lineages split eighty million years ago. Too many differences had accumulated during that time to possibly be bridged. It was doubtful a pig sperm would even recognize a chimpanzee egg to be able to fertilize it. There was certainly no known example in animal biology of hybridization across such a vast taxonomic gap.
Then there was the lack of genetic evidence. Even if the descendants of the supposed pig–chimp hybrid had been interbreeding with chimps for many generations, as McCarthy contended, telltale clues of pig heritage should have remained in our DNA. Both the pig and human genomes had, by 2013, already been sequenced in their entirety, but analysis of the two had failed to reveal any obvious similarities. Pig sequences in our DNA were the kind of thing researchers would have noticed. As far as McCarthy’s critics were concerned, this was the final and most decisive nail in the coffin of the pig–chimp hybrid.
The saga wasn’t quite over, though. In 2015, there was a flurry of excitement among its fans (yes, the hypothesis boasts a fan base) when researchers revealed the discovery of an unexpected similarity between pig and human genetic elements called SINEs, or short interspersed elements. Did this mean there was genetic support for the pig–chimp hypothesis, after all? Was McCarthy about to be vindicated?
Not quite. McCarthy himself cautioned that the finding hardly proved his hypothesis, describing it as ‘just one run in a nine-inning game’. Without far more substantial evidence, his hypothesis was doomed to remain relegated to the outer limits of the academic fringe.
But even if the theory is, as most scientists assume, wildly wrong, it does raise the provocative question of what the limits of hybridization actually are. How far apart can two species be and still produce an offspring?
You’ll search biological literature in vain for a precise answer. There’s a general consensus that eighty million years of taxonomic distance (as lies between pig and human) is far too big a gap to bridge, but what’s the exact cut-off point? Forty million years? Four million? Two million? It seems to depend on what you’re trying to match.
Lions and tigers, whose lineages split around four million years ago, manage to produce offspring. Most biologists don’t think that humans and chimpanzees, however, who diverged six million years ago, could hybridize. Though, really, there hasn’t been much effort to test this assumption. But it was confirmed in 2012 that our ancestors did interbreed with Neanderthals, from whom we diverged about 500,000 years ago, which makes many of us Neanderthal–human hybrids.
This data suggests that only relatively recently separated species can hybridize, but there are outliers that complicate this picture, such as guinea fowl and chickens, which can produce fertile baby guinhens even though their lineages diverged fifty-four million years ago. If you look at hybridization among plants, all bets are off.
And, with genetic engineering, almost anything may now be possible. Scientists are hybridizing species that would never come across each other in nature. As it turns out, one species that researchers are particularly interested in is the pig. Many pig organs genuinely are similar to human organs, as McCarthy pointed out. This has led to great interest in the possibility of transplanting pig organs into people. If this were possible, it could solve the organ-shortage crisis.
Serious problems stand in the way of this, not the least of which is how to stop human immune systems from rejecting pig organs. One possible solution, though, which has already had billions of dollars devoted to it, is to breed pigs that are more human, on a cellular level. Researchers at the Salk Institute and the University of California are actively working to achieve this and they’ve already succeeded at creating specimens of human–pig chimeras.
Which is to say that, even if a pig–chimp hybrid didn’t come into existence six million years ago, unusual creatures that are mixtures of the two do now exist in laboratories. They just happen to be the creation of science, not nature.
What if hallucinogenic drugs made us human?
When ingested, the mushroom Psilocybe cubensis begins to cause noticeable effects after about twenty minutes. These can vary greatly from person to person, but it’s common for the physical ones to include pupil dilation and an increased heart rate. The psychological ones may involve dizziness, confusion, visual hallucinations, distortions of space and time and a profound feeling of being at one with the cosmos.
These effects are only temporary. For most, they wear off after four to six hours, but they can leave a powerful lasting impression. The author and ethnobotanist Terence McKenna was particularly moved by his experiences. So much so that he began to suspect Psilocybe cubensis might have played a more far-reaching role in the history of our species than anyone had previously suspected. What if, he wondered, the mushroom was the reason for the emergence of human intelligence?
McKenna detailed this speculation in his 1992 book Food of the Gods. He envisioned our distant ancestors munching on mind-altering mushrooms and having their brains literally expanded, over successive generations, by the subsequent psychedelic experience. He called this his stoned-ape theory of human evolution.
The central mystery touched upon by the stoned-ape theory is the remarkable evolutionary development of the human brain. Two million years ago, the brains of our hominid ancestors were only a third of the size of the present-day average and were just a little larger than the brains of modern chimpanzees. Then they started growing, rapidly. As far as we know, it’s the only time in the history of evolution that a species has experienced such rapid brain growth, and
the end result was that humans gained brains that were larger, relative to body size, than those of any other creature on Earth. What could have caused this extraordinary growth?
Palaeoanthropologists have proposed a number of possibilities, such as tool use, language, communal hunting and even our sociality as a species. The problem is that it’s been impossible to come up with a definitive answer because there’s so little evidence to go on. Brains don’t fossilize, and skulls, which do fossilize, can only reveal so much about the grey matter they housed. Given this meagre amount of material to work with, consensus has eluded researchers. Which is why a crack has remained open for a more unorthodox possibility, and along came McKenna’s stoned-ape theory.
Although McKenna is often described as an ethnobotanist, which sounds scientific, he didn’t have any formal training. He was a self-taught visionary and intellectual. After a conventional childhood in a small town in Colorado, McKenna headed to the University of California, Berkeley, in the mid-1960s and fell in with the counterculture. He then took off to travel the world, eventually ending up in the Amazon jungle, where he first experienced psychedelic mushrooms. It changed his life. He returned home to the United States and, in 1976, co-authored Psilocybin: Magic Mushroom Grower’s Guide with his brother Dennis. It sold over 100,000 copies, and he subsequently developed a career as a lecturer and writer, making it his mission in life to evangelize on behalf of psychedelic drugs. His stoned-ape theory was, in a way, the culmination of this effort. It was his attempt to offer a naturalistic, scientific case for how magic mushrooms could have supercharged the evolution of the human brain.
According to McKenna, humanity’s psychedelic brain evolution began several million years ago (he was vague on exact dates) when proto-humans wandered out of the steaming jungles of Africa onto the dry grasslands. Our ancestors, at this stage, as he tells it, were rather low achievers. They scavenged for food wherever they could find it, often following the herds of wild cattle that migrated across the savannah.
Then, one fateful day, one of these proto-humans made a serendipitous discovery. As he trailed behind a herd, weaving in between piles of manure, he spotted a mushroom growing in a dung heap. He reached down, plucked out the fungi and popped it in his mouth. The result was an experience of startling novelty, because this wasn’t just any old mushroom. It was a magic mushroom, Psilocybe cubensis, containing the potent hallucinogen psilocybin. This first accidental psychonaut had discovered the ‘visionary fungi of the African grasslands’. Soon, all his companions were seeking out these dung-growing mushrooms, transforming themselves into the stoned apes of the theory.
McKenna believed that these mushrooms didn’t merely excite our ancestors with pleasurable sensations, they also provided them with adaptive evolutionary advantages. He referred to studies by the psychiatrist Roland Fischer suggesting that, at low levels, the mushrooms improved visual acuity, particularly edge detection, which aided hunting. They served as chemical binoculars. At slightly higher levels, the fungi increased sexual arousal, encouraging those who ate them to mate more often and produce more offspring than abstainers. At these quantities, the mushrooms also mellowed the fierce individuality of the male hunters, calming them down and helping to promote communal caring for the young.
At even higher levels, the mushrooms yielded ‘full-blown shamanic ecstasy’. Here, the brain development really came into play. One of the known effects of psilocybin, noted McKenna, is that it causes the senses to overlap and blend in peculiar ways. It seems to reorganize the brain’s information-processing capabilities. McKenna argued that this perceptual rewiring might have broken down mental barriers, catalysing the development of imagination, self-reflection, symbolic thought and, perhaps most importantly, language. And, if it did so, if it encouraged early humans to vocalize and interpret the sounds coming out of their mouths in new ways, it could, over many millennia, have led to an increase in brain size. As McKenna put it, we may ‘literally have eaten our way to higher consciousness.’
In McKenna’s chronology, the period of active human–mushroom symbiosis lasted for almost two million years, from the era of our distant ancestor Homo habilis right up to the dawn of civilization. He deemed this to have been a golden age in our history, the era of what he called a ‘partnership society’, when humans developed their full brainpower, nourished by the wisdom of the fungi. This phase ended 12,000 years ago, when the mushrooms grew scarce because of climate change, and our ancestors settled down to adopt agriculture.
However, they missed the mushrooms, and sought other drugs to take their place. What they found was alcohol. For McKenna, this represented a tragic fall from grace, our banishment from the Garden of Eden, because alcohol, of which he was no fan, promoted aggression and hierarchy, giving rise to a ‘dominator culture’ that has ruled for the past twelve millennia.
McKenna’s history of our species ultimately turned into a tale of lost innocence, leading to the present day, in which, by his diagnosis, we live alienated from nature and from one another. The cure he prescribed was to embrace once more the shamanic wisdom of the mushroom, thereby reuniting with nature and regaining the paradise of the partnership society we lost.
Major publications reviewed Food of the Gods, including the Los Angeles Times and the Washington Post, as well as scientific journals such as Nature and American Scientist. McKenna couldn’t claim that he was ignored. The good news for him was that many critics praised his gift for language. The bad news was that almost everyone panned his scientific claims. They just didn’t buy his thesis that drugs had made us human.
A recurring criticism was that he only marginally engaged with the existing scientific literature. He romped through various disciplines – anthropology, archaeology, psychology, mycology – using whatever facts suited his purpose, but he didn’t delve deeply into any one subject area. The book focused on sweeping ideas rather than the minutiae of scholarship.
Another complaint was that his social and political views interfered with his science. He made no attempt at all to be scientifically impartial. Instead, he openly lobbied for liberalizing the drug policies of the industrialized world, insisting that psychoactive drugs could play a positive role in society and shouldn’t be criminalized. The result, complained critics, was that his book ended up reading more like pro-drug propaganda than science.
More seriously, critics accused him of misrepresenting research. For example, the study by Roland Fischer that he referenced, claiming it had shown that psilocybin improved visual acuity, actually said no such thing. It had shown that psilocybin altered vision, but it made no suggestion of an improvement. The ‘chemical binoculars’ that McKenna imagined aiding Palaeolithic hunters didn’t actually exist.
Then there was the sheer outrageousness of his argument. The entire premise seemed totally outlandish and, frankly, silly. The scientific community, therefore, reached its verdict. No one could deny that McKenna had a way with words and a gift for coming up with provocative ideas, but ultimately it was deemed that he lacked the scholarly rigour to produce a compelling, credible argument. The charitable interpretation was that the stoned-ape theory was an entertaining fable dreamed up by someone overenthusiastic about the positive value of psychedelics. The less charitable interpretation was that it was pseudoscientific foolishness.
So, McKenna definitely failed to sell his theory to mainstream science, but was this a case where the overenthusiasm and inexperience of the messenger got in the way of the message? Does the stoned-ape theory deserve to be taken seriously despite its rejection? A handful of researchers think so.
These supporters tend to come from the field of psychedelic-drug research, which means they’re more inclined than most to attach great significance to psychedelics. Nevertheless, they’re convinced that hallucinogenic mushrooms could have played a role in human evolution and, since McKenna died in 2000, they’ve tried to keep the stoned-ape theory alive in his absence.
One of these fans is the mycologis
t Paul Stamets, considered to be a leading authority on mushrooms and psychedelics. In April 2017, at an academic conference in California, he declared his belief that the stoned-ape theory was ‘right on’, to which the audience responded with enthusiastic applause. Another fan is Terence McKenna’s brother, Dennis – hardly an unbiased source, though unlike Terence he has full-blown scientific credentials, with a doctorate in botanical sciences from the University of British Columbia.
Their argument for taking the theory seriously partially focuses on the growing scientific appreciation of the power of psychedelics. New research continues to reveal the dramatic effect these drugs have on the brain. Recent studies using functional magnetic resonance imaging (fMRI) have shown that they stimulate deep connections between parts of the brain that wouldn’t normally be in communication, and the effects seem to be extremely long-lasting. Many users report that the experience of taking them is permanently life-changing.
There’s also the fact that these powerful psychoactive substances were certainly present in the environment inhabited by our ancestors. Psilocybe cubensis is indigenous to the tropics and subtropics, where it grows in the dung of a variety of species, including elephants, zebra, antelopes, buffalo and cows. A curious proto-human would have needed only to pick a mushroom up and pop it in his mouth to experience its effect.
To the stoned-ape advocates it seems only natural to connect the dots between these two things and conclude that hallucinogens could plausibly have played a role in the human brain’s sudden development.
The objection from critics is that, no matter how powerful and readily available these drugs might have been, there’s no apparent reason why they would have influenced human evolution. The theory’s advocates respond by returning to McKenna’s argument that the drugs might have aided the acquisition of language.