When Eugene McCarthy looks in the mirror, certain traits jump out at him. His body fat. His mostly hairless skin, and the fine lines that run over it. His nose, protruding from his face, all “fleshy and wiggly.”

These traits, along with dozens of others — thick tooth enamel, female orgasm, and long hind limbs, to name a few — are dots McCarthy, a former genetics researcher at the University of Georgia, connected long ago. Since the early ‘80s, he has believed that humans are the result of an errant sexual encounter between our closest relative, the chimpanzee, and the animal with which we seemingly share all aforementioned traits: the pig.

Pigs and chimps, “both of which are very sexual animals,” have probably gotten busy more than once over geological history, McCarthy figures. On one such occasion, that union may have resulted in a fertile pig-chimp mix. This animal (a chig? a pimp?) may have then reproduced with a regular chimp. Fast forward multiple generations, and perhaps something resembling a human emerged.

It’s a far-out idea that has gotten its share of ridicule since McCarthy published it on his website in 2013. Many geneticists and evolutionary biologists dismiss his claims as wild speculation at worst, and a curious thought exercise at best.

McCarthy does not have genetic evidence to support his hypothesis, and without that, he won’t win the hearts and minds of most scientists. Still, his story prompts the question: Which ideas are entertained in the realm of mainstream science, and which get kicked to the fringes?

The white coyotes of Newfoundland

It was once thought that two species, by definition, could not produce fertile offspring. But many plants and animals can break that rule if they diverged from a common ancestor recently enough.

In 2001, a golden retriever allegedly ran off with a coyote during coyote breeding season in Newfoundland, Canada. Two years later, reports of coyotes mysteriously covered in white fur started surfacing.

Researchers sequenced the genes of a few white coyotes and discovered the animals all carried two copies of a gene variant affecting pigmentation. The same variant produces light hairs in — you guessed it — golden retrievers.

It may be that the eloping golden retriever and coyote interbred, producing hybrid pups, which then mated with full coyotes to produce what’s called “backcrossed” offspring. Whenever two coyotes with a copy of the variant mate, there’s a possibility they’ll create a snowy pup.

A taxidermied white Newfoundland coyote at Salmonier Nature Park.

When two organisms of different species or subspecies breed, it’s called hybridization. In addition to introducing new traits to a population, hybridization can sometimes generate new species. There’s the clymene dolphin, a hybrid of spinner and striped dolphins; the great skua, a seabird thought to be a hybrid between two other skuas; and many hybrid plant species.

Over the last couple decades, in part because of genetic and molecular advances that allow for greater detection of hybridization, researchers have started to pay more attention to hybrids and the extent to which they drive diversification of life on Earth. By some estimates, 10 to 30 percent of multicellular plant and animal species hybridize regularly.

McCarthy has developed his own version of evolution that replaces natural selection (“it’s mostly noise,” he says) with so-called “stabilization processes,” in which singular events create sudden changes in life form that stabilize over a few generations and then persist mostly unchanged until extinction. Hybridization is the most important of his stabilization processes.

This concept is the foundation not just of McCarthy’s hypothesis about human origins, but a full-blown scientific framework that he believes can change the course of modern biology — if only the dogmatic Darwinists in the scientific establishment would get out of the way.

The parent riddle

McCarthy was in his 20s, running a small construction company, when it first occurred to him that humans might be products of hybridization. His background was not in biology — he had a bachelor’s degree in mathematics — but he was reading Darwin’s On the Origin of Species for fun.

In Chapter 8, “Hybridism,” Darwin discussed how a hybrid can be fertile if the reproductive systems of its parents are similar enough. “That surprised me,” McCarthy said. “I thought before that hybrids would always be sterile, like mules.”

With more research, McCarthy realized what he’d been taught in high school biology was wrong. Fertile hybrids in fact existed in nature; not just in the plants, but in animals too. He wondered if hybridization could create new species — and if so, could it have created humans?

The more he thought about it, the more it made sense. Take the fact that humans are less fertile than many other mammals, he said — this could be a signature of our hybrid past.

Once McCarthy saw the pig in humans, he couldn’t unsee it.

McCarthy learned of a method naturalists sometimes use to guess the parents of an unknown hybrid. First, identify an animal that seems really similar, and assume that to be one parent. Then, list the ways in which the hybrid differs from that supposed parent; that list should describe the other parent.

He tried the strategy on humans, assuming chimpanzees — which most biologists believe descended from an ancestor shared with humans millions of years ago — to be one parent. Nearly all of the roughly 100 non-chimp traits he listed pointed back to pigs, including striking similarities in the kidneys, vocal cords, heart valves, and face and neck muscles of the two species.

“You’ve got to ask yourself, ‘Is that all just a coincidence?’” he said. “I try my best not to believe in anything, but, to me, it’s almost hard not to believe this.”

Most biologists would not agree. The genetic differences between chimps and pigs are just too wide to produce viable offspring, said Rike Stelkens, an assistant professor of zoology at Stockholm University who has spent her career studying the role of hybridization in evolution. “It would simply be impossible to span such large distances without completely messing up fundamental regulatory and developmental functions.”

McCarthy’s hypothesis, she continued, is “entirely ludicrous.”

Mainstreaming a theory

Once McCarthy saw the pig in humans, he couldn’t unsee it. He wanted to dig deeper, but first, he needed more training. In 1989, he started a master’s program in genetics at the University of Georgia, eventually earning a Ph.D in 2003. Between getting his degrees, and for several years after, he worked in the genetics department, teaching classes and contributing to research in various labs.

He tried discussing his ideas with professors and colleagues, to lukewarm reception. “There were not so many geneticists who had any expertise or interest in hybridization back then,” he said. “I think probably if they had thought of me as someone interested in hybridization they might not have even admitted me.”

McCarthy’s ideas totally went against the grain, said Susanne Warrenfeltz, a biochemist who became friends with McCarthy during her time as a postdoctoral researcher at the university. “Professors at that time were born and bred on Darwinian evolution,” she said. “That’s what they staked their careers on.”

Of course, many great theories started out as ludicrous once, including evolution itself. Before the work of Darwin and others, most scientists did not believe that different living things could share common ancestors.

In the 1960s, Thomas Kuhn, a philosopher of science, proposed that science needs an “essential tension” between radicalism and conservatism, in which so-called “normal science” continues along one path until interrupted by a paradigm shift.

When Nicolaus Copernicus first proposed heliocentrism — the idea that planets revolved around the sun rather than the Earth — he was not taken seriously.

“If you criticize Darwin, it’s like saying there is no Jesus in a Baptist church.”

Over time, however, a planetary system built around Earth became increasingly unfit for explaining observed phenomena. Johannes Kepler and Galileo Galilei helped shift the tide toward heliocentrism, and Isaac Newton finally solidified it as the dominant framework more than a century after Copernicus first published his ideas.

Revolutionary ideas like Copernicus’s are necessary, Kuhn wrote. Such ideas — evolution, the Big Bang theory, plate tectonics, and many other modern scientific truths — started out on the margins. However, he said, these ideas should be resisted until stepwise evidence vetted by scientific consensus moves them into the mainstream.

“Most of the time, science has to proceed under adherence to a dominant paradigm, or it just wouldn’t get anywhere,” said Harry Collins, a sociologist of science at Cardiff University in Britain. “But it also has to keep open to the possibility that some crazy idea might be right.”

The evolution of the theory of evolution

The standard view of evolution, often called “the modern synthesis” or neo-Darwinism, says evolution occurs gradually, and primarily through natural selection. Traits vary within populations, this view holds, partly because of random genetic mutations. This variation may allow certain individuals to better survive, reproduce, and pass on their traits; in a population of giraffes with long and short necks, for example, long-necked ones eat more leaves and produce more offspring until all giraffes have long necks.

In the 19th and early 20th centuries, natural selection was just one of many hypotheses for the primary mechanism of evolution. Other contenders included Lamarckism, which stated that organisms can pass on traits acquired through their lifetime (an idea that’s resurfacing through epigenetics, the study of how lifestyle and environment can produce biological changes capable of being inherited); orthogenesis, which stated that organisms evolve in an internally programmed direction; and saltation, which stated that evolution occurs in large, abrupt jumps.

Eventually, advances in genetics solidified natural selection as the chief driver of evolution. But McCarthy believes saltation deserves revisiting, through the lens of his stabilization processes. For support, he points to a curious and much-debated observation from paleontologists: the fossil record tends to show long periods of invariability, followed by bursts of change, rather than many incremental forms between species.

There are various hypotheses about why the fossil record is so punctuated, and the mainstream ones are centered around natural selection. Nevertheless, it’s true that evolution does not just occur in the straightforward manner many of us were taught in high school biology, where “survival of the fittest” leads to the emergence of new species.

Decades of research have painted a much more nuanced picture of evolution — one that involves many mechanisms alongside natural selection.

Take symbiogenesis, the theory that cells with nuclei evolved by forming symbiotic relationships with once-independent bacteria. First articulated in the early 1900s, it was developed into a full idea by Lynn Margulis, the late biologist, in the 1960s. Her first manuscript on the topic was rejected by 15 journals before being published in 1967, and one grant reviewer purportedly told her, “Your research is crap, do not bother to apply again.”

It wasn’t until the 1980s, when scientists showed that the genomes of mitochondria and chloroplasts could be traced to bacterial origin, that Margulis’s ideas were legitimized.

And symbiogenesis is just one of many principles in genetics that were scorned at first. Genetic drift, epigenetics, transposons, and the Baldwin effect, which says that organisms’ abilities to learn new behaviors can shape evolution, are a few others.

Science needs an “essential tension” between radicalism and conservatism

Despite all the advancements in the understanding of evolution, McCarthy thinks some of the rhetoric around neo-Darwinism amounts to dogma. “If you criticize Darwin, it’s like saying there is no Jesus in a Baptist church,” he said.

Ironically, perhaps in response to creationism, notions that go against the grain in evolutionary science can be met with backlash of a “quasi-religious tone,” said Harry Collins of Cardiff University. “I believe in the theory of evolution, but I feel very uncomfortable with the way it’s defended by people like Richard Dawkins.” Dawkins, an English evolutionary biologist, atheist Twitter crusader, and author of The Selfish Gene, is arguably the loudest champion of neo-Darwinism.

Christine Janis, a professor emerita of evolutionary biology at Brown University, said she thinks scientists like Dawkins do “more harm than good” with their militant secularism. But she doesn’t believe evolutionary biology is more dogmatic than any other field. “It’s really hard for any of us to get published, and rightly so — bold claims require bold evidence,” she said.

The outsider scientist

McCarthy left academia in 2007, frustrated because he wanted to work on his own ideas instead of assisting other researchers with their statistical and computational needs. He would later chronicle his frustrations in a satirical novel, The Department, based loosely around former colleagues.

Throughout this time, McCarthy had been building up his knowledge of hybridization. In 2006, he published Handbook of Avian Hybrids of the World, a 600-page reference on hybridization in birds, through Oxford University Press. By the time he left the university, he had a 400-page manuscript for another book, titled On the Origins of New Life Forms. He opened with a quote from the French philosopher and logician Pierre Abélard: “By doubting we come to questioning, and by questioning we perceive the truth.”

Not long after leaving the University of Georgia, McCarthy signed a contract with Oxford University Press to publish On the Origins of New Life Forms. After receiving mixed assessment from reviewers, however, the press decided not to print it.

McCarthy was faced with the decision to submit his manuscript elsewhere, or publish it on his own. He chose the latter, deciding that “peer review was unnecessary for a work of this type,” he said. “After all, On the Origin of Species didn’t go through peer review.”

Gene McCarthy

His reasoning is representative of outsider or fringe scientists, who tend to “think every idea should be assessed on its merits — nothing should be dismissed because it comes from an informal source,” said Andrew Bartlett, a sociologist of science at the University of Sheffield in Britain.

The hallmark of outsider scientists is that “they’ve all created a new science from scratch,” said Margaret Wertheim, a science writer and author of the book Physics on the Fringe. “There’s always some particular insight, and they see that structure, form, or principle at work everywhere. A lot of the world becomes focused through this one idea, which they have literally become enchanted with.”

Both Wertheim and Bartlett have spent a lot of time with outsider physicists, perhaps the most well-organized fringe science community. They noticed a common profile: male engineers, often retired, a decent number of whom have PhDs in engineering. In many cases, they feel modern physics has become overly complicated, incomprehensible, or “hijacked by the elite” — so they come up with their own explanations, according to Wertheim.

“You’ve got concepts like relative spacetime, wave particle duality, and other things that just feel alienating to a tremendous number of people,” she said. “Their feeling is, ‘Look as an intelligent person, I believe the universe works in a way that should be comprehensible to a reasonable, well-educated person.’”

“There’s this romantic idea of science — of a single man, figuring out the universe with his reason alone.”

Along with that comes a scorn for how science has become institutionalized, Bartlett said. “There’s this romantic idea of science — of a single man, figuring out the universe with his reason alone.”

To be sure, modern science, with its “publish or perish” culture, is filled with problems, one of which is that it can be closed off to new ideas.

The grievances fringe scientists have against science are “often exaggerated versions of complaints that already exist in science,” Bartlett said. “But whereas fringe scientists would throw the baby out with the bathwater, most mainstream scientists would look for ways to improve the system.”

One challenge with outsider scientists — and with the public at large — is that they are often out-of-touch with how incremental most science is, Bartlett added. Though people love a story of the ostracized maverick like Galileo, those of us who are not scientists have to trust the consensus of experts, he said. It’s not that the minority opinion should be banished from science — it’s just that members of the public are not in a position to judge who’s right or wrong.

“We have to think, ‘Here’s a body of people with tremendous expertise, who have devoted their lives to understanding and building on the work of others,’” Bartlett said. “It’s for them to argue between each other and present us with the best answer they can at that moment.”

Of course, whether the public actually trusts the courts of science is another matter, and today’s movements against climate change and vaccination suggest there is serious room for improvement.

Part of the solution, Wertheim believes, is acknowledging that science can interact in nuanced ways with other beliefs, values, or ways of finding meaning in life.

“There’s a growing tendency to present science as something that has all the answers to everything,” she said, when of course it doesn’t. “It’s no wonder people kick back.”

Outsider scientists just want the same thing as insider scientists, and all humans for that matter, she said: To borrow a phrase from the theoretical biologist Stuart Kauffman, they want to “feel at home in the universe.”

Absence of evidence

The most damning refutation of McCarthy’s hypothesis is “the absence of any pig or pig-related genes in the human genome,” according to Roger Butlin, a professor of evolutionary biology at the University of Sheffield in Britain. Instead, the human genome is “entirely consistent” with the explanation that humans are great apes, most recently sharing an ancestor with living chimps and bonobos, he said.

Butlin is part of a growing community of researchers publishing on how hybridization can indeed contribute to the formation of species. Some of these studies look at something called introgression, the movement of genes or traits from one species to another through hybridization followed by backcrossing. Introgression is what happened with Newfoundland’s white coyotes, and it’s also thought to be responsible for much of the genetic variance in Darwin’s finches, which helped him develop his theory of natural selection. There’s even clear evidence of introgression in humans — not from pigs, but from archaic humans like Neanderthals.

Though he does not like to phrase it as such, McCarthy’s hypothesis basically suggests that the introgression of genes from pigs to chimps gave rise to humans. However, hybrids that can give rise to new species “are typically between closely-related” organisms such as different subspecies of fruit flies or mice, Butlin said — there is no evidence species as disparate as chimps and pigs can procreate.

The main reason many hybrids are infertile is because their parents have a different number of chromosomes, the structures that carry genes. When an animal produces its own egg or sperm cells, its maternal and paternal chromosomes need to pair up, swap and redistribute genetic information, then split again. If these chromosomes are incompatible, this process usually can’t occur, rendering the animal sterile.

Pigs and chimps have 38 and 48 chromosomes, respectively, which is a large difference. McCarthy points out that other animals with chromosome differences have been known to produce fertile offspring, such as sheep and goats (54 and 60 chromosomes) or zebras and donkeys (44 and 62 chromosomes).

This is because, on rare occasions, analogous sections of chromosomes manage to link up, even if they differ in number. One chromosome may pair with two similar chromosomes, for instance. For this to happen, however, the chromosomes of both parents still have to be alike enough in structure and pattern to “pair and then divide correctly,” Butlin said. In both of McCarthy’s examples, the parents are more closely related — and therefore more likely to have similar chromosomes — than pigs and chimps.

“After all, On the Origin of Species didn’t go through peer review.”

PZ Myers, a biology professor at the University of Minnesota, Morris, added that it’s unlikely a pig-chimp embryo would form correctly. The species “develop at such different rates and such different times that any embryo carrying both sets of genes would be getting very confusingly mixed signals,” he said.

Reproductive feasibility aside, there are problems with McCarthy’s list of traits linking humans and pigs, said Christine Janis of Brown University. Most are traits we also share with other terrestrial mammals as well as pigs, many (such as hairlessness and subcutaneous fat) are correlated rather than distinct, and some (like alcoholism and snuggling) seem outright arbitrary, she said.

Lastly, the history doesn’t check out, according to Melissa Wilson Sayres, an assistant professor of genomics and evolution at Arizona State University. In addition to there being no evidence of ancient humans resembling pigs anywhere in the fossil record, hairless pigs arose in Asia and Europe, while chimpanzees lived in Africa — putting them nowhere near each other for a tryst at the time of human origins.

A much nicer world

If McCarthy did crave more recognition from mainstream experts (he doesn’t, he insists), his best bet would be to look for a signature unique to pigs in the DNA of humans but not other apes, said John McDonald, a biology professor at Georgia Tech and a former advisor of McCarthy’s at the University of Georgia.

A few years ago, McCarthy tried to do just that. He and a friend wrote a computer program to search the human genome for traces of pig hybridization. But the task was too computationally intensive. “It would have taken a lifetime to process the data on the small computers we had access to,” he said.

With no institutional affiliations or collaborators, McCarthy is cut off from the equipment and funding he needs to actually test his ideas. But he still holds out hope that one day his ideas will get enough support that experiments can go forward. “Anyone who takes the time to actually look at my arguments and evidence usually at least wonders if they can be true,” he said. “I’m optimistic that this will eventually get demonstrated, If I can persuade the scientific community to put some oomph behind it.”

McCarthy believes altruism, not competition, is the way of the world. “Darwin’s biggest fan was Hitler,” he said.

Nowadays, McCarthy spends his time collecting mentions of mammalian hybrids he finds in newspapers, tabloids, old science reports, YouTube videos, or any other source, hoping to show that distant crosses between mammals are more common than people think. Lately, he’s been binging on reports of human-chicken hybrids.

He says he’s happy with the way things are. He has a family. Between donations and advertisements on his website and editing dissertations for students, he’s able to piece together an income.

“I pursue my research out of curiosity. Out of a certain, niggling obsessiveness about getting things right,” he said. “Those motives have nothing to do with what ‘the broader scientific community’ thinks about me.”

There’s also another reason McCarthy remains so attached to his ideas: He believes altruism, not competition, is the way of the world. With neo-Darwinism and natural selection, competition is a fact of life, and that logic can be used to justify war, conflict, and ethnic cleansing (“Darwin’s biggest fan was Hitler,” he said).

“That’s probably the main thing that keeps me going on this. I’m so repulsed by the other idea of just, ‘Take everyone and conquer, or outcompete, them.’”

Centering hybridization as the basis of evolution and progress is “just much nicer,” he said.

“And I also think it happens to be true.”

Corrections: An earlier version of this story misstated John McDonald's affiliation. He was formerly at the University of Georgia, but is now at Georgia Tech. An earlier version also misstated the year Eugene McCarthy published On the Origins of New Life Forms; it was 2013, not 2008.