Nature via Nurture: Genes, experience and what makes us human

Gradually, Evered’s confidence returned – partly, no doubt, because Figan was by no means always with his brother: Faben was still friendly with Humphrey, and Figan, wisely, steered clear of the powerful male. Moreover, even when the brothers were together, Faben did not always help Figan: sometimes he just sat and watched.

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Though few realised it until later, Goodall’s anthropomorphism had driven a stake through the heart of human exceptionalism. Apes were revealed not as blundering, primitive automata, who were bad at being people, but as beings with social lives as complex and subtle as ours. Either human beings must be more instinctive, or animals must be more conscious than we had previously suspected. The similarities, not the differences, were what caught the attention.

Of course, the news that Goodall had narrowed the Cartesian gap travelled very slowly across the divide between animal and human sciences. Even though the very purpose of Goodall’s study, as conceived by her mentor the anthropologist Louis Leakey, was to shed light on the behaviour of ancient human ancestors, anthropologists and sociologists were trained to ignore animal findings as irrelevant. When Desmond Morris spelled out the similarities in his book The Naked Ape in 1967, he was generally dismissed as a sensationalist by most students of humankind.

Defining human uniqueness had been a cottage industry for philosophers for centuries. Aristotle said man was a political animal. Descartes said we were the only creature that could reason. Marx said we alone were capable of conscious choice. Now only by heroically narrow definitions of these concepts could Goodall’s chimps be excluded.

St Augustine said we were the only creature to have sex for pleasure rather than procreation. (A reformed libertine should know.) Chimpanzees begged to differ, and their southern relatives, bonobos, were soon to blow the definition to smithereens. Bonobos have sex to celebrate a good meal, to end an argument or to cement a friendship. Since much of this sex is homosexual or with juveniles, procreation cannot even be an accidental side effect.

Then we thought we were the only species to make and use tools. One of the first things Jane Goodall observed was chimpanzees fashioning stalks of grass to extract termites, or crushing sponges of leaves to get drinking water. Leakey telegraphed her ecstatically: ‘Now we must redefine tool, redefine man, or accept chimpanzees as humans.’

Next we told ourselves that we alone had culture: the ability to transmit acquired habits from one generation to the next by imitation. But what are we to make of the chimpanzees of the Tai forest in West Africa, which for many generations have taught their young to crack nuts using wooden hammers on a rock anvil? Or the killer whales that have utterly different hunting traditions, calling patterns and social systems according to which population they belong to?

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We had assumed we were the only animal to wage war and to kill our fellows. But in 1974 the chimps of Gombe (and subsequently most other colonies studied in Africa) put paid to that theory by raiding silently into the territory of neighbouring troops, ambushing the males and beating them to death.

We still believed we were the only animal with language. But then we discovered monkeys have a vocabulary for referring to different predators and birds, while apes and parrots are capable of learning quite large lexicons of symbols. So far there is nothing to suggest that any other animal can acquire a true grasp of grammar and syntax, though the jury is still out for dolphins.

Some scientists believe that chimpanzees do not have a ‘theory of mind’: that is, they cannot imagine what another chimpanzee is thinking. If so, for example, they could not act upon the knowledge that another individual holds a false belief. But experiments are ambiguous. Chimps regularly engage in deception. In one case, a baby chimp pretended that he was being attacked by an adolescent in order to get his mother to allow him to suckle from her nipple.

(#litres_trial_promo) It certainly looks as if they are capable of imagining how other chimps think.

More recently, the argument that only human beings have subjectivity has been revived. The author Kenan Malik argues that ‘humans simply are not like other animals and to assume we are is irrational…Animals are objects of natural forces, not potential subjects of their own destiny.’ Malik’s point is that because we, uniquely, possess consciousness and agency, so we alone can break out of the prison of our heads and go beyond a solipsistic view of the world. Yet I would argue that consciousness and agency are not confined to human beings, any more than instinct is confined to non-human animals. See almost any passage of Goodall’s books for evidence. Even baboons have recently performed well enough at computer discrimination tasks to show they are capable of abstract reasoning.

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This debate has been running for more than a century. In 1871 Darwin drew up a list of human peculiarities that had been claimed to form an impassable barrier between man and animals. He then demolished each peculiarity one by one. Though he believed only man had a fully developed moral sense, he devoted a whole chapter to the argument that a moral sense was present, in primitive form, in other animals. His conclusion was stark:

The difference in mind between man and the higher animals, great as it is, is certainly one of degree and not of kind. We have seen that the senses and intuitions, the various emotions and faculties, such as love, memory, attention, curiosity, imitation, reason, &c., of which man boasts, may be found in an incipient, or even sometimes in a well-developed condition, in the lower animals.

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Wherever you look there are similarities between our behaviour and that of animals, which cannot be simply swept under the Cartesian carpet. Yet, of course, it would be perverse to argue that people are no different from apes. The truth is we are different. We are more capable of self-awareness, of calculation and of altering our surroundings than any other animal. Clearly, in some sense, this sets us apart. We have built cities, travelled in space, worshipped gods and written poetry. Each of these things owes something to our animal instincts – shelter, adventure and love – but that rather misses the point of them. It is when we go beyond instinct that we seem most idiosyncratically human. Perhaps, as Darwin suggested, the difference is one of degree rather than kind; it is quantitative, not qualitative. We can count better than chimpanzees; we can reason better, think better, communicate better, emote better, perhaps even worship better. Our dreams are probably more vivid, our laughter more intense, our empathy more profound.

Yet that leads straight back to mentalism, equating an ape with an apprentice person. Modern mentalists have diligently tried to teach animals to ‘speak’. Washoe (a chimp), Koko (a gorilla), Kanzi (a bonobo) and Alex (a parrot) have all done remarkably well. They have learned hundreds of words, usually in the form of sign language, and have learned to combine them into primitive phrases. Yet, as Herbert Terrace pointed out after doing the same with a chimpanzee called Nim Chimpsky, all these experiments have taught us is how bad these animals are at language. They rarely even rival a two-year-old child, and they seem incapable of using syntax and grammar except by accident. As Stalin is reputed to have said of military force, quantity has a quality all its own. We are so much better at language than even the cleverest ape that it really could be called a difference of kind, not degree. That is not to say it does not have roots and homologies in animal communication, but then a bat’s wing has homology with a frog’s front foot, and a frog cannot fly. To concede that language is a qualitative difference does not imply that we can set human beings apart from nature, though. Trunks are unique to elephants. Spitting venom is unique to cobras. Uniqueness is not unique.

So which are we, similar to apes or different from apes? Both. The argument about human exceptionalism, today as in Victorian times, is mired in a simple confusion. People still insist that their opponents must take sides: either we are instinctive animals, or we are conscious beings, but we cannot be both. Yet both similarity and difference can be true at the same time. You do not have to abandon an ounce of human agency when you accept the kinship of our minds with those of apes.

(#litres_trial_promo) Neither similarity nor difference wins; they coexist. Let some scientists study the similarities while others study the differences. It is time we abandoned what the philosopher Mary Midgley has called ‘the strange segregation of humans from their kindred that has deformed much of enlightenment thought’.

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SEX AND ITS EFFECTS

There is one way in which behaviour seems to evolve differently from anatomy. In the case of anatomy, most similarities are the result of common descent, or what evolutionists call phylogenetic inertia. For example, human beings and chimpanzees both have five digits on each hand and foot. This is not because five is the perfect number for the lifestyle of both species, but because among the early amphibians, one happened to have five digits and most of its myriad descendants, from frogs to bats, have not altered the basic pattern. Some, like birds and horses, do have fewer digits, but none of the apes do.

The same is not true of social behaviour. By and large, ethologists have found very little phylogenetic inertia in social systems. Closely related species can have very different social organisation if they live in different habitats or eat different food. Distant relatives can have very similar social systems by convergent evolution if they inhabit similar ecological niches. Where two species show similar behaviour, it tells you less about their common ancestor and more about the pressures of the environment that shaped them.

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A good example is the sex life of the African apes. As primatologists delved further into the lives of apes, they found that alongside the similarities were some intriguing contrasts. These contrasts were thrown into sharper relief by the studies of George Schaller and Diane Fossey on gorillas, Birute Galdikas on orang-utans and the later studies of Takayoshi Kano on bonobos. In the zoo, a chimp looks a bit like a small gorilla. The skeletons of large chimpanzees have been confused with those of small gorillas. In the wild, however, there is a marked difference in their behaviour. It all starts with diet. Gorillas are herbivores, eating the stems and leaves of green plants such as nettles or reeds as well as some fruit. Chimpanzees are principally frugivores, seeking out fruit in trees, but adding ants, termites or monkey meat when they can. This difference in diet dictates a difference in social organisation. Plants are abundant but not very nutritious. To thrive on them, a gorilla must spend nearly all day eating and need not move very far. This makes a group of gorillas rather stable and easy to defend. This in turn has tempted male gorillas into evolving a polygamous mating strategy: each male can monopolise a small harem of females and their immature young, driving away other males.

Fruit, however, appears unpredictably in different places. Chimpanzees need to have large home ranges to be sure of finding a fruiting tree. But when a tree is found there is plenty of food to go round, so the animals can share their home range with many other chimps. But because of the large home range, these groups often split up temporarily. Consequently, for the male chimp, the polygamy strategy does not work. The only way to control access to such a large group of females is to share the job with other males. Hence the sexual favours of a troop of chimps are shared among an alliance of males. One becomes the ‘alpha’ male and takes a greater share of the matings, but he does not monopolise.

This difference in social behaviour, stemming from a difference in diet, was wholly unsuspected until the 1960s. And it was only in the 1980s that a remarkable consequence became clear. The difference has left its mark on the anatomy of the two ape species. For gorillas the reproductive rewards of owning a harem of females are so great that males which take great risks to get them have generally proved more fecund ancestors than males of a more cautious disposition. And one risk that is worth running is growing to a very large size – even though it takes a lot of food to run a big body. Consequently, an adult male gorilla weighs about twice as much as a female.

Among chimpanzees, males are not under such pressure to be big. For a start, being too big makes it harder to climb trees, and it means you have to spend more time eating. Better to be only a little larger than a female and to use cunning as well as strength to rise to the top of the hierarchy. Besides, there is no point in trying to suppress all sexual rivals, because you will sometimes need them as allies to defend the home range. However, because most females are mating with lots of males within the troop, the male chimps that most often became ancestors were in the past the ones that ejaculated often and voluminously. The competition between male chimps continues inside the female vagina in the form of sperm competition. Consequently, male chimpanzees have gigantic testicles and prodigious sexual stamina. As a proportion of body weight, chimpanzee testicles are 16 times greater than gorilla testicles. And a male chimp has sex approximately one hundred times as often as a male gorilla.

There is a further consequence. Infanticide is common in gorillas as it is in many primates. A bachelor male infiltrates a harem, grabs a baby and kills it. This has two effects on the baby’s mother (apart from causing her great, though transient, distress): first, by halting her lactation it brings her back into oestrus; second, it persuades her that she needs a new harem master who is better at protecting her babies. And who better to choose than the raider? So she leaves her mate and marries her baby’s killer. Infanticide brings genetic rewards to males, who thereby become more fecund ancestors than males that do not kill babies; hence most modern gorillas are descended from killers. Infanticide is a natural instinct in male gorillas.

But in chimps females have ‘invented’ a counter-strategy that largely averts infanticide: they share their sexual favours widely. The result is that any ambitious male, if he were to start his reign with a killing spree, might be killing some of his own babies. Males that hold back from baby-killing therefore leave more offspring behind. To confuse paternity by seducing many males into possible fatherhood, the females have evolved exaggerated sexual swellings on their pink bottoms to advertise their fertile periods.

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The testicle size of a chimp is a meaningless statistic on its own. It only makes sense by comparison with the gorilla testicles. That is the essence of the science of comparative anatomy. And having looked at two species of African ape in such a way, why not include a third? Anthropologists are fond of claiming an almost limitless diversity of behaviours in human cultures, but there is no human culture so extreme that it even begins to compare with the social system of either the chimpanzee or the gorilla. Not even the most polygamous human society is exclusively organised into harems that are passed from one male to another. Human harems are built up one by one, so that most males, even in societies that encourage polygamy, only have one wife. Likewise, despite various attempts to invent free-love communes, nobody has succeeded in achieving, let alone sustaining, a society in which every man has repeated brief affairs with every woman. The truth is, the human species has just as characteristic a mating system as any other: characterised by long pair bonds, usually monogamous, but occasionally polygamous, embedded in a large chimp-like troop or tribe. Likewise, however variable testicle size is among men, there is no man living whose testicles (as a proportion of body weight) are as small as a gorilla’s or as big as a chimpanzee’s. As a proportion of body weight, our testicles are nearly five times as large as gorillas’ and one-third the size of chimpanzees’. This is compatible with a monogamous species showing a degree of female infidelity. The difference between species is the shadow of the similarity within the species.

An intriguing explanation of the human pair bond once again focuses on food. The primatologist Richard Wrangham puts it down to cooking. With the taming of fire and its adoption for cooking – which is a form of predigestion of food – there came a reduced need for chewing. Suggestive evidence for the controlled use of fire now goes back to 1.6 million years ago, but circumstantial evidence hints that it may have happened even earlier. At around 1.9 million years ago the teeth of human ancestors shrank at the same time as the body size of females grew. This indicates a better diet more easily digested, which in turn sounds like cooking. But cooking requires you to gather food and bring it to the hearth, which would have provided ample opportunities for bullies to steal the fruits of others’ labour. Or, since males were at that time much bigger and stronger than females, for males to steal food from females. Accordingly, any female strategy that prevented such theft would have been selected, and the obvious one was for a single female to form a relationship with a single male to help her guard the food they both gathered. These increasingly monogamous males would then not be competing with each other so fiercely for every mating opportunity, which would result in their becoming smaller relative to females – and the sex difference in size began to shrink 1.9 million years ago.

(#litres_trial_promo) Later, the pair bond developed into something even deeper when ancestral human beings invented a sexual division of labour. Among all hunter-gatherers, men are usually more interested in and better at hunting; women are more interested in and better at gathering. The result is an ecological niche that combines the best of both worlds – the protein of meat and the reliability of plant food.

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But, of course, there are not three species of African ape; there are four. The bonobos that live to the south of the Congo river may look rather like chimpanzees, but they have been evolving apart for two million years, ever since the river split their ancestral range in two. Like chimps they eat fruit; like chimps they live in large home ranges shared by multi-male troops. It follows that their sex lives, and their testicle size, should be like those of chimpanzees. But, as if to teach us scientific humility, they are astonishingly different. In bonobos, females are usually able to dominate and intimidate males. They do this by forming coalitions and coming to each other’s aid. A male bonobo in trouble can count on his mother’s support more than he can count on that of his male friends. An adult female bonobo, supported by her best friends, can usually outrank any male.

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But why? The secret of the bonobo sisterhood lies in sex. The bond between two female best friends is cemented by frequent and intense bouts of ‘hoka-hoka’, which scientists unromantically translate into genito-genital rubbing. Under the benign rule of cooperative and loving sisterhoods, the society of the bonobo reads more like a feminist fantasy than something real. That it should come to be understood only in the 1980s, when male-biased science was under challenge, is an uncanny coincidence. (The mind boggles at how the Victorians would have described hoka-hoka.)

As predicted by feminist doctrine, male bonobos have reacted to the new female-dominated regime by evolving kinder, gentler natures. There is much less fighting and shouting, and so far murderous raids on members of other troops are unknown. Since female bonobos are even more sexually active than chimps and have sex nearly ten times as often (and a thousand times as often as gorillas), the ambitious male bonobo’s best strategy for attaining fatherhood is to save his energy for the bedchamber, not the boxing ring. I would like to be able to tell you that bonobo testicles are even bigger than chimpanzee testicles, but – although they are certainly very large – nobody has yet managed to weigh them.

(#litres_trial_promo) In her book Sexual Selections, Marlene Zuk describes how the timely discovery of bonobo sex lives has made them into the latest animal celebrities, supplanting the dolphins which had rather blotted their eco-friendly image by indulging in something that looks very like kidnapping and gang rape. Inevitably, sex therapists have begun trumpeting the ‘bonobo way’ of sex. Dr Susan Block (of the Dr Susan Block Institute for the Erotic Arts and Sciences in Beverly Hills) proclaims that these ‘horniest apes on earth’ are models for us all if we are to live in peace. ‘Liberate your inner bonobo,’ she urges. ‘You can’t very well fight a war while you’re having an orgasm.’ She pledges a share of the profits from her ‘ethical hedonism’ television and Internet shows to bonobo conservation.

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These are just our closest cousins. The apes of Asia – orang-utans and gibbons – have entirely different sex lives again. So do the many and various species of monkey, presenting a bewildering variety of social and sexual stratagems, each one suited to its habitat and food. Forty years of field primatology have confirmed that we are a unique species, completely unlike any other. There is no exact parallel to the human scheme. But in the animal kingdom, there is nothing exceptional in being unique. Every species is unique.

ENTER GENETICS

The argument about human exceptionalism, swaying between Darwinian similarity and Cartesian difference, shows no sign of ending. Each generation is doomed to fight the same old battles. If you arrive in the world in a time when people have strayed a bit far into anthropomorphic similarity, then you can find a fresh argument for how different animals and people are. If the air is full of difference, then you can champion the similarities. Philosophy is like this: eternally unsettled and only occasionally disturbed by new facts.

Then came an unexpected threat to this pleasant debate. A threat of a resolution. A threat of defining once and for all, at root, what the difference is between a person and a chimpanzee; what you would have to do to a chimpanzee to make it into a person.

It happened about the same time that Jane Goodall was undermining the exceptionalism of human behaviour. Almost completely forgotten until rediscovered in the 1960s was an extraordinary experiment done by a Californian named George Nuttall in 1901 while at Cambridge University. He noticed that the more closely related two species were, the more their blood produced the same immune reaction in a rabbit. He injected blood from, say, a monkey into a rabbit repeatedly for some weeks, then a few days after the last injection extracted serum from the rabbit’s blood. That serum, mixed with the blood of a monkey, caused it to thicken as the immune reaction set in. Mixed with the blood of a different animal, it thickened more according to how closely related the species were. By this means Nuttall established that human beings were more closely related to apes than they were to monkeys. This ought to have been obvious from the lack of a tail and other features, but it was still controversial at the time.

In 1967 at Berkeley, Vincent Sarich and Allan Wilson revived Nuttall’s biochemical techniques in a more sophisticated form and used them to construct a ‘molecular clock’ that measured the actual length of time since two species had shared a common ancestor. They concluded that human beings had shared a common ancestor with the great apes not 16 million years ago, as was then conventional wisdom, but only about five million years ago. Anthropologists, whose fossils implied a more ancient split, reacted with contempt. Sarich and Wilson stuck to their guns. In 1975, Wilson asked his student Marie-Claire King to repeat the exercise for DNA in order to find the genetic differences between human beings and apes. She came back disappointed. It was impossible to find differences, she said, because human and chimpanzee DNA was so astonishingly similar: close to 99 per cent of the DNA in a human being was identical to that in a chimpanzee. Wilson was thrilled: the similarity was more exciting than the difference.