Taking One for the Team

29 August 2008 (All day)

THE KOBAL COLLECTION/Warner Bros./Legendary Pictures

Gung ho! A tribal hero's bravery--and genes--can live on even after death in battle.

Bravery and belligerence make for a vaunted warrior, but they also increase the chances of getting killed. Now, researchers have built a mathematical formula that explains how such self-sacrificing traits nevertheless could evolve because they tend to live on in the widespread descendents of fallen heroes. The findings could advance our understanding of how genes are passed along even if they don't improve an individual's survival chances.

Combatants often give up their lives in defense of their cause. From an evolutionary perspective, cowardice at first seems like a smarter strategy: If you run away and live, you’ll have more offspring than the gallant warrior who fights to the death. So why do men follow the drumbeat, generation after generation, to die in war? Researchers speculate that extreme altruism has a genetic basis, though they haven’t been able to isolate specific genes. They also don't know how such genes might have been passed on to succeeding generations in early humans.

In a paper accepted by the Proceedings of the Royal Society B, evolutionary biologists Laurent Lehmann and Marcus Feldman of Stanford University in Palo Alto, California, describe a mathematical formula they derived to track the spread of hypothetical genes for belligerence and bravery in small groups of humans that engage in frequent combat. They installed the genes randomly within group members and allowed the groups to interact.

The model assumed that a sizeable fraction of hard-core warriors would die by the sword, but it also assumed a reward for those who didn't: they got to mate with females from the conquered tribe. "Belligerence and bravery are substantially driven by the benefits of conquest," Lehmann says. So in the scenario, those who managed to mate before dying passed on genes for belligerence and bravery to their descendents.

Researchers usually assumed such traits could evolve and persist only in very small groups, but the formula showed that the process continues even in tribes of up to 50 males and 50 females. So tribal warfare "may explain the evolution of altruism in larger groups," Lehmann says. "Bravery turns out to be catching."

"The work has implications for society," says evolutionary biologist Andy Gardner of the University of Edinburgh in the U.K. "By looking at our evolutionary origins, and the selective forces shaping our social and antisocial behaviors, we might better understand why we are so inclined towards war."

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