War, what is it good for? A lot, it could turn out.

Lethal warfare drove the evolution of altruistic behaviour among ancient humans, claims a new study based on archaeological records and mathematical simulations.

If correct, the new model solves a long-standing puzzle in human evolution: how did our species transition from creatures interested in little more than passing down their own genes to societies of (generally) law-abiding (mostly) monogamists?

No one knows for sure when these changes happened, but climactic swings that occurred between approximately 10,000 to 150,000 years ago in the late Pleistocene period may have pushed once-isolated bands of hunter-gatherers into more frequent contact with one another, says Samuel Bowles, an evolutionary biologist at the Santa Fe Institute in New Mexico and the University of Siena, Italy, who led the study. “I think that’s just a recipe for high-level conflict.”


Tribes at war

By warfare, Bowles isn’t talking about highly organised contests between nation-states and their armies. Rather, this period of warfare was probably characterised by ongoing skirmishes between neighbouring populations.

“We’re talking about groups of men who got out in twos or threes or fives,” he says. “They didn’t have a chain of command and it’s hard to see how they could force people to fight.”

For this reason, altruistic intent on the part of each warrior is key. Each person would do better to stay home than to put their life on the line for their neighbours – yet they still went out and risked their lives, Bowles says.

To assess whether or not people with a random genetic predisposition to altruism could flourish via armed conflicts, Bowles culled archaeological and ethnographic data on the lethality of ancient warfare and plugged them into an evolutionary model of population change.

Cost of clashes

In ancient graves excavated previously, Bowles found that up to 46 per cent of the skeletons from 15 different locations around the world showed signs of a violent death. More recently, war inflicted 30 per cent of deaths among the Ache, a hunter-gatherer population from Eastern Paraguay, 17 per cent among the Hiwi, who live in Venezuela and Colombia, while just 4 per cent among the Anbara in northern Australia.

On average, warfare caused 14 per cent of the total deaths in ancient and more recent hunter-gatherers populations.

The cost of losing an armed conflict as a group is high enough to balance out the individual risks of warfare, especially if a population is relatively inbred, Bowles’ model concludes. Since evolution acts on genes, it makes more sense to make more sacrifices for a related neighbour than an unrelated one.

Since Bowles had no way of knowing how inbred Pleistocene populations were, he compared contemporary hunter-gatherers such as African pygmies and native Siberians. Individuals in these populations were closely related enough to justify going to war, he found.

Inbreeding

“There’s no doubt that his is a controversial view,” says Ruth Mace, an evolutionary anthropologist at University College London. Inbreeding between the victors and any surviving losers would dilute, not concentrate, altruistic genes, she says.

Bowles modelled this possibility in a previous paper and found that even with a measure of inbreeding, altruists still win out. However, he agrees that it would slow the evolution of altruism through warfare. “A much better way to spread the genes is to kill everybody,” he says.

Mark van Vugt, a psychologist at the University of Kent at Canterbury, UK, notes that warriors could act in their own self-interest, not for the good of the group.

“Studies on the Amazonian Ya̧nomamö people show that these warriors do get a greater share of resources, they get more women, they sire more offspring,” he says. “How do you explain that there are individual benefits for these warriors? There shouldn’t be.”

Still, van Vugt thinks Bowle’s model is on the right track. Studies show that people divided into arbitrarily chosen groups – say heads and tails – behave altruistically to members of their group, but are more hostile toward non-members.

“Together we provide different pieces of the puzzle. If they fit together, they are starting to make sense,” van Vugt says.

Journal reference: Science (DOI: 10.1126/science.1168112)