"I would lay down my life for two brothers or eight cousins," said the British geneticist J.B.S. Haldane, neatly summarizing the theory of "kin selection": To predict how much one person will sacrifice for another, it says, just look at the percentage of genes they inherited from a common ancestor. The odds were 50-50 that any given gene in Haldane's chromosomes was also inherited by a brother; but for a cousin, those odds are only 12.5 percent.
There's no doubt kin selection, as worked out by William D. Hamilton in the 1960's, has some relevance to people. For instance, as Martin Daly and Margo Wilson have shown, in North America a child's risk of abuse is seven times greater from a step-parent than from a genetic parent.
On the other hand, it's equally obvious that family-feeling isn't usually governed by literal relatedness: People will spend less on themselves and give the money to earthquake victims; they'll take in a cat or a dog; if they are soldiers, they will die to save their comrades.
And, of course, people adopt unrelated children and raise them as their own. Daly and Wilson were able to collect their statistics on children raised by non-kin precisely because adoption is so common.
People often describe these sacrifices with the language of kinship ("we're all God's children, so we must help," or "our dog is part of the family" or "I fight with my brothers-in-arms"). But the family involved is metaphorical, not genetic. Meanwhile, of course, people often treat actual relatives as if they were strangers—preferring a "brother" in Christ, say, over a literal brother who rejects religion.
Humanity's independence from the rules of kin selection poses a big problem for evolutionary psychologists, and some have proposed walling off the human species from the rest of Nature: Our extraordinary altruism to non-relatives was supposed to be a defining trait of Homo sapiens.
No more. Adoption of non-relatives, it turns out, is also practiced by chimpanzees.
In this paper in the journal PLoS One, the primatologists Christophe Boesch, Camille Bolé, Nadin Eckhardt and Hedwige Boesch report 18 different cases in which an chimp baby was adopted after its mother died. And even as it blows away received ideas about altruism in animals, the paper also messes with conventional wisdom about gender: Half the adoptive parents were male. (Three were brothers of their adoptee, three weren't related, and in two cases, the researchers didn't know if adopter and adoptee were kin. In one instance, the adopter turned out to be the child's biological father, though chimp males have no way of knowing this.)
Finally, another surprise: As John Hawks noticed, the paper reports that adopted orphans weren't any more likely to survive two years than were those left to fend for themselves, even though the adoptive "parents" did the same the self-sacrificing things as biological parents: Carrying infants, sleeping with them, and giving them food. That suggests that for baby chimps, there is no substitute for a mother. Which challenges the assumption, at the heart of many theories of altruism, that helping behavior actually helps.