Marmosets develop their exceptional social skills from an early age
From altruism to culture, language to teaching, did shared childcare give rise to the things that make us human?
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PICTURE the scene: you鈥檙e having a Sunday afternoon dinner with family and friends. Dad is busy in the kitchen demonstrating how to chop the veg and granny is sitting in the corner telling a story to the kids. In a while you will all sit down to share good food and conversation.
Now imagine a similar scene involving our closest ape cousins. For all the sophisticated mental powers that have been attributed to chimpanzees in recent years, they wouldn鈥檛 do well in this situation. Pandemonium would reign in the kitchen, some chimp would take all the food and the only conversation would be screeching. Poor granny would be out in the cold and the smallest kids, if left unattended, might well be kidnapped and eaten.
What could explain these dramatic differences between us and our nearest relatives? Over the last few years, a handful of researchers have been building a case for the idea that cooperation arose from the development of a single behaviour: shared childcare. They claim that the care, nutrition and protection of youngsters by adults other than the mother bring about profound psychological changes in a species. In humanity, this paved the way for the enhanced cooperation and altruism that ultimately led to culture, language and technology.
There is no doubt that humans are extraordinarily social when compared with most of the animal kingdom. We are generally good at reading other people鈥檚 emotions and adapting our behaviour appropriately, we work well in teams on highly complex projects, and we sometimes even extend our kindness to perfect strangers. In one classic experiment by economist at the University of Zurich, Switzerland, people were given 拢20 and asked to share as much or as little of it as they liked with an anonymous partner whose identity would never be revealed and who would never know the giver鈥檚 identity. It makes sense to keep the lot but, against logic, most people gave away between 25 and 50 per cent of the money (.
This capacity for cooperation is thought to have been essential for the development of culture and technology, making it one of the defining changes in our evolution. So where did it come from?
There are many theories out there. Some researchers have suggested that we initially evolved to be nice to just our kin. That wouldn鈥檛 necessarily improve our own survival, but it would increase the chance that some of our genes, which we share with our relatives, reach another generation, protecting our genetic heritage. Altruism towards people outside the family, on the other hand, might have arisen from the expectation that they would then return the favour at a later point 鈥� a relationship that would increase the chances of survival for both. More recently, others have argued that our cooperative skills arose from the need to forge alliances during the earliest forms of warfare.
None of which can fully explain why humans are so different from our ape relatives. Chimps, for example, are known to initiate warlike raids into neighbouring communities, and they also form mutually beneficial friendships. Yet they rarely show kindness to the extent that you find in human behaviour.
Mean streaks
Indeed, chimps appear to have quite a mean streak. Experiments by of the University of California, Los Angeles, show that if caged chimps are given the opportunity to pull food treats within their reach, or within reach of a neighbouring chimp too, few choose the prosocial option. Silk found this held even if their peer was gesturing for food (). 鈥淭he bar was set really low,鈥� she says. 鈥淭hey didn鈥檛 have to give anything up in order to deliver rewards to others鈥� there was no reason not to be nice.鈥� There just seemed to be no motivation for them to help one another.
Not all non-human primates are so mean, however. When two of Fehr鈥檚 colleagues at the University of Zurich 鈥� 鈥� tested groups of marmosets using a similar set-up, for example, the monkeys almost always chose the altruistic option (.
Intrigued, van Schaik and Burkart began to wonder what might explain these acts of altruism. One similarity seemed to stand out: humans and marmosets are 鈥渃ooperative breeders鈥�. Much more than most other primates, the adults of a marmoset group willingly protect and actively feed each other鈥檚 young, usually without any prompting.
Chimps, in comparison, are independent breeders who will rarely help another鈥檚 family. They are not even particularly giving to their own infants. 鈥淚t鈥檚 not like human mothers preparing meals for their kids, offering them food and letting the kids eat first,鈥� says Silk. 鈥淚n chimps it鈥檚 more that mum tolerates the kids taking bits. They don鈥檛 usually get the best bits that mum would really like to have herself.鈥�
Van Schaik and Burkart began to suspect that the evolution of cooperative breeding might have paved the way for greater altruism more generally, and when they heard that anthropologist Sarah Hrdy of the University of California, Davis, had been thinking along similar lines, the trio decided to write a paper outlining their hypothesis ().
Their reasoning was simple enough: in groups that share childcare, the young need to be better at reading another鈥檚 behaviour, to charm the adults and to recognise when someone might harm them. 鈥淭he infants who are a little bit better at doing this are going to be those most likely to survive,鈥� says Hrdy. 鈥淥ver generations you get directional Darwinian selection favouring infants who are a little better at mind-reading and a bit more inclined to quest for engagement with others.鈥�
鈥淚n groups that share childcare, the young need to be better at reading another鈥檚 behaviour, to charm the adults and to recognise who might harm them鈥�
Van Schaik and Burkart point out that shared childcare would also promote greater altruism in the adults, since proactive giving means the young don鈥檛 have to call out in hunger to helpers, which could attract the attention of predators. This wouldn鈥檛 be an issue for chimps as the infant is exclusively cared for by one adult 鈥� the mother 鈥� who is always right there and can attend to her infant鈥檚 needs.
Observations of other cooperative breeders provide some support for their hypothesis. Elephants and some canid species, such as African wild dogs, for example, will often care for another鈥檚 young and they also show a great deal of cooperation within a group, sharing sources of food and water, and assisting injured or disabled group members. Domestic dogs, who are descended from cooperatively breeding wolves, and elephants also perform better on socio-cognitive tasks, such as imitation, compared to other animals of similar brain size. Dogs have even been known to 鈥渃onsole鈥� the victims of a fight.
The most convincing evidence remains with the primates, however. In late 2010, van Schaik and Burkart comprehensively reviewed all the available data on cognitive abilities in three groups of New World monkeys: the cooperatively breeding callitrichids, a group including marmosets and tamarins; capuchins, which occasionally share care of their young; and squirrel monkeys, which are independent breeders with little shared support. Although the callitrichids didn鈥檛 do well in regular cognitive tasks, they consistently outperformed the other two groups at tasks that required social skills such as imitation, social learning and gaze understanding. They also aced the tasks involving cooperation, doing even better than the famously clever capuchins (). Like marmosets, tamarins also fare well on more specific tests of altruism ().
Those who babysit, teach
The researchers think their hypothesis could go some way to explaining the roots of many more complex behaviours. 鈥淐ooperative breeding was the prequel to the main human feature film,鈥� says Hrdy, 鈥渁nd that is language, cumulative culture and high-level cooperation.鈥� For example, many cooperative breeders are also good at teaching. 鈥淎s well as donating food, [they] donate information,鈥� says van Schaik.
Besides humans, very few other species are known to instruct others. Among the non-human primates, the best teachers are not the more cerebral apes, but the cooperatively breeding callitrichids. Adult tamarins, for instance, will at first bring infants dead insects to eat, but once the youngsters are old enough, they present live prey. Eventually, the helper simply indicates where an insect is hidden, leaving the youngster to make the catch for itself. 鈥淭he helpers are highly sensitive to the ability of the infant,鈥� says Burkart.
The best-known teachers in the rest of the animal kingdom 鈥� meerkats, ants and birds called pied babblers 鈥� are all cooperative breeders too. 鈥淢any people think that for teaching, you have to be clever,鈥� says van Schaik, 鈥渂ut the simple rules meerkats use in teaching pups how to hunt scorpions, for example, shows that you don鈥檛 have to understand what you鈥檙e doing to be an efficient teacher.鈥� What you do need, he claims, is the motivation and skills that allow you to share information with others, which he believes come from cooperative breeding.
When you combine this motivation with the more advanced cognition of our ape brains, you begin to have the propagation of the complex skills that have marked human civilisation, van Schaik claims. This is important because influential innovations 鈥� like the invention of a new tool 鈥� come round very rarely, but with social learning, 鈥渢hey stick around and spread鈥�. The result, he says, is sophisticated culture and technology. A lot of research supports the idea that teaching is crucial for the evolution of culture (New 杏吧原创, 20 November 2010, p 38), but cooperative breeding helps to explain why humans are so particularly good at it compared with the other apes.
According to the researchers, the social mindset afforded by cooperative breeding may have also set the stage for the evolution of language. Some rudimentary verbal skills might have evolved earlier in other intelligent species, like chimps, says van Schaik, except they just don鈥檛 have the motivation to tell each other what鈥檚 on their minds. With shared care, we feel the need to share information, he says, and we make declarative statements like: 鈥淗ey it鈥檚 snowing out there, better come home early鈥�. 鈥淐himps and orang-utans, they don鈥檛 feel the need for that, but once you have cooperative breeding, the evolution of language is just expected,鈥� he says.
There remains the burning question of why humans ever turned to cooperative breeding to start with. The researchers already have some ideas. Though some speculation is required here, there is general agreement that by the early Pleistocene, around 2 million years ago, the African habitat of our ancestor Homo erectus was much more arid than before, and food was harder to come by.
With our high-maintenance bodies and big brains, these difficult conditions would have meant that food sharing was essential for the survival of the young, says Hrdy. The result may have been cooperative breeding. A move towards a diet that was richer in meat would have also played a role at around this time, since hunting is such an unpredictable source of food.
Murderous in-laws
Still, if cooperative breeding is so beneficial, why don鈥檛 chimps do it? Surely chimp mums could do with a bit of help, too. The problem here is that chimps can鈥檛 trust those around them. 鈥淚f I were a chimpanzee mother,鈥� says Hrdy, 鈥淚鈥檇 be surrounded by individuals that might want to eat my baby.鈥� This is because, unlike most other primate species, chimps live in patrilineal communities, in which the males of a community stick together and the females move on to other groups. Since the new mothers aren鈥檛 related to any of the individuals surrounding them, they can鈥檛 rely on the other females to look after their young.
Until recently, early human societies were thought to be patrilineal too 鈥� which should decrease the chances of cooperative breeding. However, Hrdy points out that existing hunter-gatherer societies are pretty flexible, with individuals moving backwards and forwards between groups. It鈥檚 plausible that our ancestors acted similarly. 鈥淪o the husband might come and join his wife鈥檚 group, live there for a while until one or two children are born,鈥� says Hrdy. That means the mother still has matrilineal support when she gives birth, which means she鈥檚 more likely to have trustworthy help with her child-rearing.
Such support still seems to have a noticeable effect today. At the meeting of the European Human Behaviour and Evolution Association in Poland last year, and of the London School of Economics reported that women in the UK are more likely to start a family if they have strong kin ties, particularly if the relatives live within 80 kilometres and are in regular contact.
So far, the cooperative breeding hypothesis has been warmly received by other researchers in the field, though not everyone is convinced. Silk, for example, has doubts. Although she agrees the reasoning is plausible, she points out that individuals in callitrichid groups are often related, whereas humans are altruistic with strangers who don鈥檛 share our genes. 鈥淗ow we got to be such generalised nice folks is the big question,鈥� she says. Van Schaik counters that ancestral humans probably lived with their relatives, or at least their mate and their mate鈥檚 relatives with whom they would have also had strong bonds, when cooperative breeding and prosociality first evolved. Other factors, such as concern for reputation and the benefits of reciprocal friendships, could have consolidated these altruistic tendencies into the prosociality we see in humans today.
There is plenty of research still to be done, though, and van Schaik and Burkart now plan to investigate the altruism of a much larger number of primate species. Should the idea pan out, there will nevertheless remain a niggling question for any of us enjoying the kind of jovial family meal that seems to emphasise the traits that distinguish us from other apes. Altruistic we may be, but why are we so reluctant to help with the washing up?
