A controversial theory holds that aggression was a driving force in human evolution. The “Killer Ape” hypothesis argues that the human ancestors who thrived were those better adapted for fighting. For example, Carrier said, modern humans can form fists, which our closest primate relatives cannot. This particular hand configuration may have evolved primarily for better manual dexterity — but it also could have come in handy as a club. Likewise, when human ancestors started walking on two legs, their face bones also evolved to be stronger and less delicate. This could be due to diet, Carrier said, but male face bones are more robust than female face bones, a sign that male-male competition could be at play. In other words, thick facial bones could be a defense against the fist, a weapon that would have become available once human ancestors became bipedal.

If intelligence evolves in the context of social competition, and aggression is the natural outcome of competition, it’s hard to imagine that clever aliens could also be kind. Is this the end for hopes of sweet little E.T.?

Maybe not. The social competition model doesn’t work without cooperation, after all. Humans fight, wage war and sometimes murder each other. But humans also form coalitions, care for each other and even build coalitions of coalitions, such as nation-states.