Temperamentally, bonobos and chimps are poles apart. Humankind, however, has something from both of its kinsmen, says Frans de Waal

There is no greater contrast between humankind's two closest relatives than how they behave when tempted by good food. If forest chimpanzees are given a pile of bananas, they will fight over it. The males become violent and the females have no choice other than to back off. Male bonobos are also the first to approach a tasty meal. But instead of being aggressive, they hurry and look over their shoulders as they collect as much food as they can. The "weaker sex" then drives them off, before the group has lots of sex and happily shares the food.

Bonobos are genetically as close to us as chimpanzees. Hence both ape species shed light on the ancestor we all share. But the question as to whether we most resemble the violent chimpanzee or the sexy bonobo is a bit like asking whether a surface is best measured by its length or width. We obviously have something of both. Worse is to consider one pole at the expense of the other when making statements about human nature.

Nevertheless, this is what Western culture has been doing for centuries by depicting our competitive side as somehow more authentic than our social one. But if people are as selfish as is assumed, how do they form societies? The traditional view is that of a contract among our ancestors, who decided to live together "by covenant only, which is artificial", as Thomas Hobbes put it. We are seen as loners: smart enough to pool resources yet lacking any true attraction to each other.

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The old Roman proverb Homo homini lupus - "man is wolf to man" - captures this asocial vision, which continues to inspire law, economics and political science. The problem isn't just that this misrepresents us, but that it also insults one of the most gregarious and loyal co-operators in the animal kingdom - so loyal, in fact, that our ancestors wisely domesticated them. Wolves survive by bringing down prey larger than themselves through teamwork. The hunters then regurgitate meat for the nursing mothers, the young and sometimes the sick and old who stayed behind. A wolf who would let narrow individual interests prevail would soon find himself alone chasing mice.

Apes know the same solidarity, and even chimpanzees are not always violent.

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In Ivory Coast, wild chimps have been seen taking care of group members wounded by leopards, licking away blood and preventing flies from coming near the wounds. If injured companions can't keep up, the others slow down for them. All of this makes perfect sense given that chimpanzees - like wolves and people - live in groups for a reason. We would not be where we are today had our ancestors been socially aloof.

How fundamental bonding is to primates was demonstrated by Dawn Prince-Hughes, an American with Asperger's syndrome. She found inner peace while taking care of gorillas in a zoo - or perhaps it was the apes who took care of her. In Songs of the Gorilla Nation , Prince-Hughes tells how people would unnerve her with their direct stares and direct questions to which they wanted immediate answers. The gorillas, on the other hand, gave her space and conveyed a comforting calm. Most of all, they were patient.

Gorillas are "oblique" characters in that they rarely engage in face-to-face contact. They also lack the white sclera around the iris that makes the human gaze such an unsettling signal. Moreover, apes rarely stare the way we do: they glance. They have incredible peripheral vision and follow much around them from the corners of their eyes.

The way the gorillas empathised with Prince-Hughes by "looking without looking and understanding without speaking", as she put it, took place via postures and body mimicry - the ancient language of connection. Congo, the colony's mighty silverback, was the most sensitive and comforting. The male gorilla, despite his ferocious "King Kong" reputation, is a born protector.

That it takes an autistic person - someone considered deficient in interpersonal skills - to pick up on the primacy of ape bonding, and the kinship we sense with bodies more hairy than but similar to ours, is remarkable. That Prince-Hughes was pulled out of her solitude by gorillas rather than chimpanzees or bonobos makes sense in view of gorilla temperament. These apes are not nearly as extrovert as their cousins.

Consider an illuminating incident at a Swiss zoo. One night, its chimps managed to remove the skylight of their housing and escape to the rooftops, some of them travelling throughout the city, jumping from house to house.

It took days to round them all up.

The episode gave a local animal rights group the brilliant idea of "liberating" gorillas in the same zoo. Without giving much thought to what might be best for the animals, they climbed onto the ape house and removed a skylight above their quarters. But even though the gorillas had many hours in which to escape, they didn't. The following morning, the caretakers found the apes staring with amazement into the air, fascinated by the gaping hole above them. None of them was curious enough to climb out. This, in a nutshell, is the temperamental difference between chimps and gorillas.

By stressing primate bonding, my message is not that we lack competitive and aggressive tendencies. We have those in abundance. But in nature, everything is balanced around an optimum. In the same way that warm-blooded animals had to be equipped with ways to cool off, such as outsized ears, every social tendency is offset by another that runs counter to it. Our societies are never completely peaceful, never completely competitive, never ruled by sheer selfishness and never perfectly moral. We're full of contradictions. We know competition as well as co-operation, selfishness as well as sociability, strife as well as harmony. Human nature is inherently multidimensional. Being more systematically brutal than chimps and more loving and empathic than bonobos, we are by far the most bipolar ape.

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On top of this inherent duality comes the role of intelligence. We habitually weigh the pros and cons of our actions before carrying them out.

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This may sound too obvious to mention, but it is radically different from how biologists used to present things. In the 1960s, every noticeable tendency of the human species was labelled an "instinct". This downplayed the role of learning and experience, as if we are programmed to act this way or that. One might think that we have become more careful, but the same sort of thinking is still with us today. Evolutionary psychologists compare the human brain to a Swiss Army knife to which evolution has one by one added "modules" for everything from face recognition and tool use to childcare and friendship. Unfortunately, no one knows exactly what a brain module is, and evidence for their existence is no more tangible than that for instincts.

We undeniably have inborn predispositions, yet I don't see us as blind actors following nature's genetic script. The same holds for our fellow primates. Let me explain this with the example of Yeroen, a male chimpanzee at Arnhem Zoo, in the Netherlands, who injured his hand in a fight. Yeroen was building a coalition with the up-and-coming Nikkie, but in the scuffles leading to their political partnership, Nikkie had bitten him. It wasn't a deep wound. Nevertheless, Yeroen limped heavily. After a couple of days, however, we got the impression that he limped mainly if Nikkie was around.

I found this hard to believe, so we conducted systematic observations. Each time we saw Yeroen walk with a limp, we recorded Nikkie's whereabouts. It emerged that the younger chimp's field of vision was crucial. Yeroen would walk past Nikkie from a point in front of him to a point behind him. While he was in Nikkie's view he would hobble pitifully. But once out of sight, he would walk perfectly normally again.

Yeroen seemed to be faking a limp so that his partner would go easy on him.

Hurting one's buddy is never a smart move, and Yeroen seemed to be pointing this out to Nikkie by exaggerating his pain. Putting on a front is something humans do all the time - the couple trying to look happy in public to hide a strained marriage, the people laughing at their boss's unfunny jokes. Keeping up appearances is something we share with the apes.

This is one reason I have trouble with the theory of animals as blind actors. They ponder the many options before them and, depending on the circumstances, decide what to do. In the laboratory, apes are usually tested on abstract problems such as finding rewards pointed out by experimenters.If they fail, as they sometimes do, the conclusion is that we're smarter than they are. But in the social domain, in which apes deal with those they've known all their lives, they seem about as intelligent as we are.

A crude way to test this would be to put a human in a chimp colony to see how he or she fared. This is obviously unrealistic, as a chimp's strength far outstrips any human's. But imagine we could find someone strong enough to stand up to an adult ape. Since one's standing in the community is decided by social cliques and mutual support, the challenge for the human would be to win over friends without being too assertive or submissive.

Otherwise, one would end up at the bottom of the pecking order, or worse.

There would be absolutely no point trying to hide fear or hostility, because human body language is an open book to chimps. My prediction is that an ape colony would prove no easier to negotiate than an average collection of people at work.

Comparisons among humans, chimps and bonobos thus go well beyond shared "instincts" or "modules", however defined. All three species face similar social dilemmas and need to overcome similar contradictions in trying to achieve status or in finding mates and resources. They need to compete without upsetting the group dynamic on which survival depends. In doing so, they apply their full brain power and their full range of emotions to find solutions. Sometimes they limp when they want to. At other times they fight and then reconcile with a kiss as if they can't decide between hostility and tenderness. True, our species looks farther ahead and weighs more options than the apes, but this hardly seems a fundamental difference.

Even if we wield the better chess computer, we're still all playing chess.

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Frans de Waal is professor of primate behaviour at Emory University and director of the Living Links Center at the Yerkes National Primate Research Center in the US. His book Our Inner Ape: The Best and Worst of Human Nature is published on November 1 by Granta Books, ?17.99. De Waal will be giving a lecture to launch his book on November 1 at University College London. This is a free public event but places must be reserved. Please contact Victoria Herridge at v.herridge@ucl.ac.uk .