At 2 a.m. on February 26, 1852, the Royal Navy troopship Birkenhead, which was carrying more than 600 people, including seven women and 13 children, struck a rock near Danger Point, two miles off the coast of South Africa. Almost immediately, the ship began to break up. Just three lifeboats could be launched. The men were ordered to stand on deck, and they did. The women and children (along with a few sailors) were put into the lifeboats and rowed away. Only then were the men told that they could try to save themselves by swimming to shore. Most drowned or were eaten by sharks. The heroism of the troops, standing on deck facing almost certain death while others escaped, became the stuff of legend. But the strange thing is, such heroics are not rare: Humans often risk their lives for strangers—think of the firemen going into the World Trade Center—or for people they know but are not related to.
Interview: Survival of the Kindest Olivia Judson discusses the evolutionary roots of altruism and fellow feeling
How does a propensity for self-sacrifice evolve? And what about the myriad lesser acts of daily kindness—helping a little old lady across the street, giving up a seat on the subway, returning a wallet that’s been lost? Are these impulses as primal as ferocity, lust, and greed? Or are they just a thin veneer over a savage nature? Answers come from creatures as diverse as amoebas and baboons, but the story starts in the county of Kent, in southern England.
Kent has been home to two great evolutionary biologists. In the 19th century, Charles Darwin lived for many years in the village of Downe. In the 20th, William Donald Hamilton grew up catching beetles and chasing butterflies over the rolling hills near Badgers Mount.
Hamilton was a tall man with a craggy face and the tops of a couple of fingers missing from a childhood accident— he blew himself up while making explosives. He died in 2000, at age 63, after an illness contracted while undertaking another risky endeavor: a trip to the Congo to collect chimpanzee feces. When I first met him, in Oxford in 1991, he had a terrific shock of white hair, rode a rickety bicycle at prodigious speed, and was preoccupied with the question of why sex is useful in evolutionary terms. (For my doctorate, I worked with him on this question.) But he began his career studying social behavior, and in the early ’60s he published a trio of now- classic papers in which he offered the first rigorous explanation of how generosity can evolve, and under what circumstances it is likely to emerge.
Hamilton didn’t call it generosity, though; he called it altruism. And the particular behaviors he sought to explain are acts of extreme self-sacrifice, such as when a bee dies to defend the hive, or when an animal spends its whole life helping others rear their children instead of having some of its own.
To see why these behaviors appear mysterious to biologists, consider how natural selection works. In every generation, some individuals leave more descendants than others. If the reason for their greater “reproductive success” is due to the particular genes they have, then natural selection has been operating.
Here’s an example: Suppose you’re a mosquito living on the French Mediterranean coast. Tourists don’t like mosquitoes, and the French authorities try to keep the tourists happy by spraying insecticide. Which means that on the coast, mosquitoes bearing a gene that confers insecticide resistance tend to leave many more descendants than those lacking it—and so today’s coastal mosquitoes are far more resistant to insecticide than those that live inland.
Extreme altruists, by definition, leave no descendants: They’re too busy helping others. So at first blush, a gene that promotes extreme altruism should quickly vanish from a population.
Hamilton’s solution to this problem was simple and elegant. He realized that a gene promoting extreme altruism could spread if the altruist helped its close relations. The reason is that your close relations have some of the same genes as you do. In humans and other mammals, full brothers and sisters have, on average, half the same genes. First cousins have, on average, an eighth of their genes in common. Among insects such as ants and bees, where the underlying genetics work differently, full sisters (but not brothers) typically have three-quarters of their genes in common.
Hamilton derived a formula—now known as Hamilton’s rule—for predicting whether the predisposition toward a given altruistic act is likely to evolve: rB>C. In plain language, this says that genes that promote the altruistic act will spread if the benefit (B) that the act bestows is high enough, and the genetic relationship (r) between the altruist and the beneficiary is close enough, to outweigh the act’s cost (C) to the altruist. Cost and benefit are both measured in nature’s currency: children. “Cheap” behaviors—such as when a small bird squawks from the bushes to announce it’s seen a cat or a hawk—can, and do, evolve easily, even though they often benefit nonrelatives. “Expensive” behaviors, such as working your whole life to rear someone else’s children, evolve only in the context of close kin.
Since Hamilton first proposed the idea, “kin selection” has proved tremendously powerful as a way to understand cooperative and self-sacrificial behavior in a huge menagerie of animals. Look at lions. Lionesses live with their sisters, cousins, and aunts; they hunt together and help each other with child care. Bands of males, meanwhile, are typically brothers and half-brothers. Large bands are better able to keep a pride of lionesses; thus even males who never mate with a female still spread some of their genes by helping their brothers defend the pride. Or take peacocks. Males often stand in groups when they display to females. This is because females are drawn to groups of displaying males; they ogle them, then pick the guy they like best to be their mate. Again, peacocks prefer to display with their brothers rather than with males they are not related to.
Kin selection operates even in mindless creatures such as amoebas. For instance, the soil-dwelling amoeba Dictyostelium purpureum. When times are good, members of this species live as single cells, reproducing asexually and feasting on bacteria. But when times get tough—when there’s a bacteria shortage—thousands of individuals join together into a single entity known as a slug. This glides off in search of more-suitable conditions. When it finds them, the slug transforms itself into a fruiting body that looks like a tiny mushroom; some of the amoebas become the stalk, others become spores. Those in the stalk will die; only the spores will go on to form the next amoeboid generation. Sure enough, amoebas with the same genes (in other words, clones) tend to join the same slugs: They avoid mixing with genetic strangers and sacrifice themselves only for their clones.
Kin selection also accounts for some of the nastier features of human behavior, such as the tendency stepparents have to favor their own children at the expense of their stepkids. But it’s not enough to explain the evolution of all aspects of social behavior, in humans or in other animals.