proceeds, the fitness landscape changes. Your best evolutionary strategy, in other words, depends on who else is evolving along with you. No species is a Robinson Crusoe alone on an island. And when what you should do depends on what others are doing, game theory is the name of the game.

Recognizing this ever-shifting evolution landscape is the key to explaining how cooperative behavior comes about. In particular, it helps to explain the vastly more elaborate cooperation exhibited by humans compared with other animals.

It’s not that nonhuman animals never cooperate. Look at ants, for instance. But such social insect societies can easily be explained by evolution’s basis in genetic inheritance. The ants in an ant colony are all closely related. By cooperating they enhance the prospect that their shared genes will be passed along to future colonies.

Similar reasoning should explain some human cooperation— that between relatives. As Maynard Smith’s teacher J. B. S. Haldane once remarked, it would make sense to dive into a river to save two drowning siblings or eight drowning cousins. (On average, you share one-half of a sibling’s genes, one-eighth of a cousin’s.) But human cooperation is not limited to planning family reunion picnics. Somehow, humans evolved to cooperate with strangers.

When I visited Martin Nowak, he emphasized that such nonkin cooperation was one of the defining differences between humans and the rest of the planet’s species. The other was language. “I think humans are really distinct from animals in two different ways,” he said. “One is that they have a language which allows us to talk about everything. No other animal species has evolved such a system of unlimited communication. Animals can talk about a lot of things and signal about a lot of things to each other, but it seems that they are limited to a certain finite number of things that they can actually tell each other.”

Humans, though, have a “combinatorial” language, a mix-and-match system of sounds that can describe any number of circum-