The last common ancestor of humans, chimpanzees and bonobos wasn鈥檛 especially prone to violence, according to a study attempting to reconstruct the evolution of warlike behaviour among apes. Hostility between groups of this ancestor may have been tempered by bonds between unrelated females, the study suggests 鈥� but researchers warn that its conclusions are highly speculative.
The question of whether violence is integral to human nature has been debated for centuries. Biologists have tried to answer it by looking at chimpanzees, which, along with bonobos, are our closest relatives.
All four subspecies of chimpanzee engage in gruesome violence between groups. Some biologists have noted parallels between this behaviour and human warfare, suggesting that the common ancestor of humans and chimpanzees must have been violent and aggressive.
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鈥淭he problem with such an approach is that it focuses on only one of our closest relatives while leaving out the other closest relative, bonobos,鈥� says at the University of Bristol, UK. 鈥淒ifferent groups of bonobos usually get on very well and one of the ways they resolve conflict is by having sex.鈥�
To get a broader view of the question, Opie and his colleagues collected data from 301 primate species on 13 traits linked to lethal violence, such as infanticide and male alliances. They traced the distribution of these behaviours across the evolutionary tree of primates and performed a statistical analysis to identify which behavioural traits were relevant to intergroup violence.
They found that encounters between groups in the common ancestor of humans, chimpanzees and bonobos probably weren鈥檛 always hostile, as was previously thought.
In chimpanzees, only females leave their social groups when they reach maturity. Opie and his colleagues think that this makes intergroup violence more likely, because groups are dominated by males with strong bonds. But, by inferring ancestral behaviour based on the dispersal patterns in living primate species, they found that the females and males of our common ancestor were equally likely to leave their social group after puberty.
鈥淚f anything, it may have been more like gorillas and modern humans, who can vary the dispersal patterns depending on environmental or habitat factors,鈥� he says. 鈥淭hese two findings seem to undermine some of the arguments based on observations of intergroup interactions and dispersal patterns in chimpanzees.鈥�
The three other traits that the researchers learned were important in influencing violence in primates were male coalitions, males being dominant over females and males being exclusively responsible for defending the group 鈥� features that are also seen today in chimpanzees.
Opie and his colleagues found that the most recent ancestor we share with chimpanzees was probably the first ape that showed cooperation among unrelated females. They conclude that violence in males was kept at bay by such female alliances and the males having to cooperate to form coalitions.
But other researchers aren鈥檛 entirely convinced by the findings. 鈥淚t is a very interesting, thorough and thought-provoking study,鈥� says at Harvard University. 鈥淗owever, such broad comparative analyses are only as good as the accuracy of the primate studies. For example, we do not know how widespread male dispersal in bonobos is and there seem to be substantial population differences in chimpanzees in female coalition-building and participation in territory defence.鈥�
The method of using evolutionary trees to reconstruct what ancestral species were like is highly speculative for behavioural traits, says at the University of Minnesota. 鈥淚n the absence of a fossil record for this species, we really don鈥檛 have much to go on,鈥� he says. 鈥淓ven with a fossil record, inferring the social behaviour of extinct species poses tremendous challenges.鈥�
The study鈥檚 conclusions are plausible, but highly speculative, says , also at Harvard University. 鈥淲hen it comes to the last common ancestor between humans and chimps, their inferences essentially are driven by just three data points: humans, chimps and bonobos.鈥�
Surbeck thinks aspects of aggressive behaviour are inherent to any social-living species. 鈥淚 think that comparisons with our closest living relatives show us that there is an inherent propensity for violence in the form of infanticide, male sexual violence against females and even killing. However, social and environmental and, in the case of humans, cultural factors determine whether those traits are selected for and manifest within a species or context,鈥� he says.
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Article amended on 3 July 2023
We corrected the wording of Michael Wilson鈥檚 quote.
