by Alex Boese
The strongest support for the theory, however, continues to be rooted in the issue of human uniqueness. It draws upon this very deep-seated intuition that we’re different from other animals and particularly from other primates. There’s just something odd about us, and this oddness, many feel, must have been the result of some extraordinary event in our evolution.
Anthropologists caution that this belief in our exceptionalism is misguided, that we’re not as unique as we like to think we are. It’s only our vanity making us believe that we are. Perhaps so, but sometimes it’s hard not to be drawn in by the idea. You might particularly get this feeling if you ever spend the day at the beach, where you can witness our species splashing in the waves in all its Speedo-wearing glory. Faced with such a sight, the notion that we might really be the descendants of a bunch of soggy apes may not seem entirely unlikely after all.
Weird became true: the out-of-Africa theory
Humans live all around the world: in Africa, Asia, the Arctic, the jungles of South America and even remote islands in the Pacific. Our ability to adapt to survive in almost all of the Earth’s environments is, in fact, one of our defining traits as a species. But where did we originate?
The accepted view among anthropologists, based upon a multitude of fossil evidence, is that our birthplace was Africa. The first of this evidence to emerge was the so-called Taung Child skull, discovered in South Africa in 1924 by Raymond Dart. It’s regarded as one of the most important fossil finds of the twentieth century, but this wasn’t always the case. When Dart initially presented it to the scientific community, their reaction was utterly dismissive. Leading British anthropologists waved it aside as inconsequential and rejected his argument that humans had originated in Africa. It took over a quarter of a century for them to change their minds.
Dart, as it turned out, hadn’t actually wanted to be in South Africa. It was only by a quirk of fate that he happened to be there with the appropriate anatomical training to recognize the significance of the skull when it came to his attention. He was Australian by birth and had studied anatomy at Oxford. When he learned that the newly founded University of the Witwatersrand in Johannesburg needed someone to fill the position of Chair of Anatomy, he had no interest, fearing that going there would be like being sent into academic exile. He only applied for the job and accepted the position after some arm-twisting from his adviser.
Nevertheless, it turned out to be a great career move because it sent him directly on a path to make the fossil find of the century, which happened a mere two years after he arrived in South Africa. As far as anthropological finds go, it was probably one of the least strenuous ever, because it involved no searching or digging on his part. He never even had to leave his office. The skull was delivered directly to him as part of a group of fossils collected in a nearby mine called the Buxton Limeworks. He had earlier made arrangements with the mine to ship him anything interesting they found in the course of their work. When he received the box they eventually sent over, he opened it up and there was the skull, lying on the very top.
Dart immediately recognized its importance. The story goes that he was getting dressed to attend a friend’s wedding when the box arrived, and his wife had to drag him away almost forcibly from it to get to the ceremony on time.
The skull was tiny, belonging to a child that must have lived hundreds of thousands of years ago, but its anatomy intriguingly combined features of both ape and human. Its brain was very small, like that of an ape, whereas its teeth were human-like. Dart could also see that its brain had human features, and he was able to determine that it must have walked upright, because faint markings revealed that its skull had been positioned upright on the spinal column, as opposed to hanging forward like that of an ape.
He believed that the creature, when it was alive, would have looked more ape-like than human, so he called it Australopithecus, meaning ‘southern ape’. Its presence in Africa, he knew, was highly significant, because no such transitional fossils between apes and humans had yet been found there.
Dart promptly sent a report of his findings to the journal Nature, where it was published in 1925. But, to his dismay, instead of being hailed as a great find, it met with scathing criticism. Arthur Keith, a distinguished British anthropologist, led the critics. He rejected the claim that the skull represented a transitional form between apes and humans as ‘preposterous’ – a word one doesn’t use lightly in the science community when discussing a colleague’s work. He declared that the skull was most likely that of a juvenile gorilla.
Part of this criticism was legitimate. The skeletons of primate infants tend to look somewhat similar. It’s only with age that the differences between the species become distinct. So, the fact that it was the skull of a child made it difficult to identify precisely what it was. It was also a challenge to establish its geological age. The exact location of its discovery wasn’t known, due to its delivery in a box, so Dart had to date it by anatomical clues. And Dart himself, at a mere thirty-two years old, was regarded as young and inexperienced.
A somewhat less legitimate reason for the criticism was that anthropologists simply didn’t think humans had come from Africa. No less an authority than Charles Darwin himself had actually made the case for our African origin in the 1870s, noting that it made sense for humans to have originated where the other great apes lived, but by the 1920s this was no longer a popular belief. This was partly because Asia was then in vogue as the likely place of our origin, on account of early-human fossils that had recently been found there. Mostly, though, it was plain old racism. Elitist European scholars weren’t inclined to acknowledge Africa as their ancestral home.
And then, as a final blow against the Taung Child, there was the matter of the Piltdown Man. This was an early-human fossil that had been found in Piltdown, England, in 1912, consisting of a skull that was large, like that of a modern-day human, but coupled with an ape-like jaw. The significance of this pairing was that it confirmed what many anthropologists had long suspected – that, in our evolutionary history, the first distinctively human feature to develop must have been a large brain. This made sense to them because they reasoned that intelligence is our most singular feature, so brain growth must have preceded all other anatomical changes.
The Taung Child, however, flew in the face of this reasoning. It combined a small brain with indications of bipedalism, suggesting our ancestors were walking upright long before they acquired large brains. In other words, the two fossils were telling different stories about our past. So, which to believe?
Given a choice between a big-brained British ‘missing link’ and a small-brained African one, British scholars came down firmly on the side of the ‘first Englishman’. Keith was one of the strongest supporters of the importance of the Piltdown Man, which is why he so quickly concluded that the Taung Child was a gorilla, not an early human.
Dart grew depressed by the rejection of his find. His marriage fell apart, and for a while he gave up further anthropological research. He was lucky enough, however, to live to see his vindication, as new fossil evidence steadily accumulated that supported his interpretation of the Taung skull. These finds included the discovery of additional Australopithecine skeletons in Africa by Dart’s colleague, Robert Broom.
By the late 1940s, the tide of intellectual opinion had swung decisively in Dart’s favour. All the fossils being found were telling the same story of bipedalism first and brain development much later, and the oldest hominid fossils were consistently being found in Africa. It must have given Dart a sense of immense satisfaction when even his arch-nemesis Keith publicly recanted, writing in a 1947 letter to Nature that, ‘I am now convinced … that Prof. Dart was right and that I was wrong.’
The final block of resistance crumbled away in the early 1950s with the stunning revelation that the Piltdown Man had been a crude forgery. A team of researchers at the British Museum had decided to examine it more closely, puzzled by how different it was from all th
e other fossil evidence. They discovered that its surface had been artificially stained – a clear sign of tampering. There’s still enormous controversy about who the forger might have been. The leading candidate is Charles Dawson, the lawyer and amateur fossil hunter who had found it.
In hindsight, it should have been obvious that it was a fake. As early as 1915, the American scholar Gerrit Miller had noted that the mechanics of chewing made it impossible for the jaw and skull to belong together. British anthropologists had ignored his warning. The Piltdown fossil told them what they wanted to believe, so they disregarded the contrary evidence. Its unmasking as a fraud cleared the way for the full acceptance of the Taung skull and, by implication, the African origin of humanity.
What if we’re descended from a pig–chimp hybrid?
In 1859, Charles Darwin published On the Origin of Species in which he challenged the old belief that species never changed by arguing that they actually evolved over time through a process of natural selection. Darwin realized this idea would shock people, and that it might particularly disturb religious sensibilities, so he carefully avoided delving into the implications of his theory for the origin of humans. He focused instead on species such as dogs and finches, and he saved the discussion of human evolution for twelve years later, in his book The Descent of Man.
Victorian readers, however, weren’t fooled. They immediately put two and two together and figured out, as his critics put it, that Darwin was claiming man must be descended from the apes. This definitely riled up religious tensions. Did it ever! To this day, the descent of humans from apes remains a hot-button issue, with some people still simply refusing to believe it.
One can then only imagine the reaction if the hypothesis of the geneticist Eugene McCarthy were ever to gain mainstream scientific acceptance. He significantly cranks up the potential outrage factor, arguing that humans may not only be descended from apes, but from pigs as well. More specifically, he speculates that around six million years ago, a mating event may have occurred between a female chimpanzee and a male pig (or rather, between what would have been the ancestors of the present-day species), and that the resulting offspring was the progenitor of the human lineage.
McCarthy earned his Ph.D. in evolutionary genetics from the University of Georgia in 2003. Three years later, Oxford University Press published his Handbook of Avian Hybrids of the World, which was an encyclopaedic listing of thousands of reported crosses between different bird species, occurring in both wild and captive settings. It earned high marks from reviewers, and it suggested that McCarthy had a promising career ahead of him.
Which is to say that he initially seemed to be heading down a traditional path, and, if he had continued in that direction, he probably would by now be comfortably ensconced as a professor at some university, regularly churning out scholarly articles and enjoying the respect of his peers. But McCarthy veered sharply away from that course. He began entertaining strange and radical thoughts – the kind of speculations that eventually made him a pariah within academia.
There may have been a hint of the rebel already in his decision to specialize in the subject of hybridization. The term describes the phenomenon of two different species mating and producing an offspring. The most well-known example is the mule, which is the result of a cross between a male donkey and a female horse.
Hybridization is of great interest to plant breeders who want to produce new varieties of fruit and vegetables with potentially beneficial attributes. You might occasionally happen upon the results of such research in the produce aisle at the supermarket. Perhaps you’ve seen broccoflower (a hybrid of broccoli and cauliflower) or plumcots (plums plus apricots).
Crossing plants is one thing, though, but the idea of mixing animal species, particularly those more widely separated than donkeys and horses, is quite another. It causes all kinds of cultural red flags to go up. Imagine a cross of a dog and a cow, or of a goat and a horse, or of a human and anything else. Such mixtures evoke fascination and horror. The possibility of their existence seems monstrous and unnatural. They clash with our belief in the proper order of things.
Mythology is full of examples of disturbing hybrids. King Minos of Crete was said to have kept a Minotaur (a half-bull, half-human creature) in a labyrinth beneath his castle. A medieval legend speaks of the existence of a Vegetable Lamb of Tartary (a plant that grew sheep as its fruit). Perhaps the most famous hybrid of all is found in Christian tradition, with the Devil often depicted as half goat and half human.
These were the strange kinds of crosses that McCarthy began to contemplate. He wondered how far the phenomenon of hybridization could be pushed. How far apart could species be and still produce a viable offspring?
This led him to think about the role hybridization might play in evolution. The standard model is that evolution occurs through the steady accumulation of genetic mutations. Selective pressures in the environment then determine which mutations are preserved and which are not. When two populations of a species are geographically isolated from each other, this natural process causes them to evolve in gradually diverging ways until eventually they’re no longer able to reproduce with each other. They become distinct species. The finches that Charles Darwin observed on the Galapagos Islands are a well-known example of this. Cut off from the mainland, they had undergone separate speciation.
McCarthy suspected that the reproductive barriers between species, even far-flung ones, weren’t as rigid as almost all scientists assumed. If this was the case, then hybridization might occasionally jump-start evolution. A hybrid mating could suddenly introduce a whole new set of genes into a population, causing its development to take a dramatic new course.
If McCarthy had confined these evolutionary speculations to bird or plant species, they would have been controversial, but only among academics. Instead, he zeroed right in on the most inflammatory question possible: might hybridization have played a role in human evolution?
McCarthy didn’t imagine every species to be a result of hybridization. He did, however, think it played a far greater role than most biologists credited it with, and he suspected that humans in particular might have been a product of the process. What led him to this conclusion was our low fertility rate in comparison with other mammals. Biological research, he pointed out, has found that, in a typical human male, up to 18.4 per cent of spermatozoa may be abnormally shaped and dysfunctional, compared to just 0.2 per cent in chimpanzees. Such a high rate of defective sperm has conventionally been attributed to clothing that affects scrotal temperature, but McCarthy noted that it’s also a common trait of hybrids.
If humans are a hybrid, the next question was what two species might have parented us. McCarthy explained that, when biologists suspect a creature is a hybrid, there’s a test they use to determine its parentage. First, they decide what species the creature is most like, and they assume that to have been one of the parents. Then, they make a list of all the characteristics that differentiate the suspected hybrid from its one known parent. With this list in hand, they try to match up the traits with another species. If they find a good fit, they figure they’ve probably found the other parent.
McCarthy went ahead and applied this test to Homo sapiens. We’re clearly a lot like chimpanzees, so he assumed that species (the late Miocene era ancestor of it) was one of our parents. Then he combed through the scientific literature to produce a comprehensive list of ways that humans differ from chimps.
The resulting inventory included items such as our naked skin, the fact that we sweat copiously, our thick layer of subcutaneous fat, our protruding noses and our lightly pigmented eyes. Anatomical differences beneath the surface included our vocal cords and our kidneys. The latter have a uniquely shaped internal cavity, described as being ‘multi-pyramidal’ because it has numerous inward projections. There were also a variety of behavioural differences. Humans like swimming, snuggling and drinking alcohol, and chimps don’t particularly care for any of these activities.
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Having produced this list, McCarthy asked himself what creature displayed all these characteristics. He found only one candidate. It was Sus scrofa, the ordinary pig. In fact, the match was uncannily good …
A pig! McCarthy confessed that he himself was initially reluctant to take the idea seriously. It was only when he discovered that human and pig vocal cords actually look quite similar that he grew convinced there might be something to the crazy idea forming in his head. Once the thought took hold, he kept seeing more and more parallels between humans and pigs.
But wouldn’t a pig–chimp hybrid be biologically impossible? Aren’t there mechanisms on the cellular level that would prevent a viable offspring between such far-flung species? Wouldn’t differing parental chromosome counts, for instance, prevent hybridization? And, even if an offspring did form, aren’t hybrids usually sterile?
McCarthy cited these objections as common misconceptions about hybridization. He acknowledged that hybridization between far-flung species was improbable, but he insisted that it wasn’t impossible. Nature might find a way to make it happen, and supposed reproductive barriers such as differing chromosome counts would do little to stop it. There are plenty of species with wildly differing numbers of chromosomes that manage to hybridize. Zebras, with forty-four chromosomes, and donkeys, with sixty-two, together produce baby ‘zeedonks’ just fine. What’s more, hybrid offspring are often fertile. In his own work on bird hybrids, he had found that they were eight times more prevalent than sterile ones. It was true that a reduction in fertility was common, but this was exactly what had led him to suspect humans could be hybrids in the first place.