Did wars make us the species we are today? Credit: Wikimedia Commons

Explanations of the evolution of human behaviour often invoke crucial biological changes and revolutionary cultural innovations. Now two papers in Science instead put demography — the size, density and distribution of populations — centre stage.

Samuel Bowles, a behavioural scientist at the Santa Fe Institute in New Mexico, tackles the puzzle of how humans acquired such unrivalled altruistic behaviour towards unrelated individuals — tendencies that allowed humans to cooperate as groups and, ultimately, to colonize the planet. The answer, paradoxically, could turn on war1.

Adam Powell and his colleagues at University College London propose that demography could account for the emergence of modern human behaviour — sophisticated tools, art, bodily ornamentation and other culturally transmitted expressions of symbolic thought2.

What war is good for

When everyone in a group is altruistic, says Bowles, the group does better as a whole. However, such groups are vulnerable to invasion and exploitation by free riders who selfishly accept the altruists' benevolence while giving nothing back. Selfishness pays off, and finally replaces altruism.

But there are ways out. Earlier theoretical work showed that within-group altruism can co-evolve with between-group conflict — warfare3. When the threat of being wiped out by another group is high, the costs incurred by individuals practising group-beneficial altruism can be offset by increasing the likelihood that the group, including altruists, survives.

Working together as a group has allowed humans to colonize the planet. Credit: Wikimedia Commons

Bowles draws on demographic data from the archaeological and ethnographic records, which, he says, indicate that intergroup conflict would have been common among our hunter-gather ancestors, and estimates that it accounted for roughly 14% of all deaths — much higher than the mortality rate seen in wars of recent history.

Under these conditions, Bowles shows that even costly group-beneficial altruism and cooperation could be favoured. "It's possible that a genuinely altruistic human nature could have evolved, and that it depended in part on the tendency to engage in inter-group conflict," says Bowles.

A notable feature of Bowles's model is that it is based on genetically transmitted altruism and, more controversially, genetic group selection — selection for traits that are passed on because they benefit the group, even at a cost to individuals. Many biologists think this is, in practice, an unworkable process in human groups because they are not genetically distant or differentiated enough from each other for selection of group traits to occur.

I'm not saying this is how it happened but it's a possibility to take seriously. Samuel Bowles , Santa Fe Institute

But Bowles thinks genetic data from hunter-gather groups shows that they meet this criterion, and are compatible with genetic group selection. "I'm not saying this is how it happened," he says, "but it's a possibility to take seriously."

Peter Richerson of the University of California, Davis, has his doubts. "There isn't anything wrong with the model," he says. "I just think he accepts too high a figure for genetic differentiation." In any case, Bowles's model applies with even more force to Richerson's favoured approach — cultural group selection — as cultural differences between different groups typically far exceed genetic differences.

Crowds and culture

Meanwhile, Powell and his colleagues examined the role of demography on the emergence of modern human behaviour. "The tendency has been to look for a genetic magic bullet that suddenly made humans flower into modern culture," says archaeologist Stephen Shennan, a co-author on the paper. Their work challenges this inference, and argues that demography, particularly population density and migration, can be the key determinant of when and where modern behaviour develops.

Archaeological evidence suggests that the first symbolic art and ornamentation may have arisen in Africa 70,000–90,000 years ago, yet disappeared until the beginning of the Late Stone Age around 40,000-50,000 years ago.

Demography could help explain the emergence of symbolic thought. Credit: Wikimedia Commons / HTO

It is unlikely that the acquisition and loss of some cognitive capacity can explain such transient bursts of modernity. But could demography? Small populations can easily lose cultural knowledge and skills — as has happened in Tasmania over the past 8,000 years it has been an island4.

For modern behaviour and culture to emerge, and persist, populations need to reach and maintain a certain density, so the authors looked at historical population densities and migration.

They also draw on genetic analyses that have used mitochondrial DNA to estimate humans populations at different times and in different regions of the world — such as Africa around 100,000 years ago, and Europe 45,000 years ago, roughly when evidence of modernity first turns up.

These rough genetic estimates suggest comparable population densities in both regions at the relevant dates, which meet the demographic requirements for supporting modern behaviour.

Chris Stringer, a palaeontologist at the Natural History Museum, London, says that the study by Powell and his coworkers is a "very nice bit of work", which formalizes ideas he has also expressed. And although he is not convinced it is the whole story, "it's a better model than a biological 'switch' that turned on modern behaviour", he says.

However, Richard Klein, an archaeologist at Stanford University in California, says the study relies too heavily on modelling, and overlooks relevant archaeological evidence, such as shellfish and tortoise remains that he thinks track human population density.

These species were hunted for food, with large animals preferentially sought as prey. Higher population density meant greater demand for food, and so smaller animals would be hunted too, reducing the average size of shellfish and tortoise remains. Analyses of such remains, says Klein, "indicate no difference in human population size between sites that Powell et al. think imply modern behaviour and ones they think don't".