In the modern world, we cooperate with and live side by side with strangers, who often look, act, and speak in ways very different to us. We work together on goals with culturally distant nations that span the globe. I'm recording this talk, but I could have given it to you in person. That's unusual in many respects. It's unusual from a cross-species perspective - comparing us to our closest primate cousins, a room full of strange chimps is a room full of dead chimps. It's unusual from a historical perspective - even a few hundred years ago, a stranger in our midst was a potential threat. And it's unusual from a geographic perspective - even today some places are safer and more cooperative than others. Cooperation varies in scale, intensity, and domain - some countries cooperate on healthcare, others on defence. Compounding the puzzle, the evolutionary mechanisms that explain cooperation undermine one another and can stabilize non-cooperative or even maladaptive behavior. I'll discuss the latest discoveries in the science of cultural evolution and human cooperation and how these might apply to the development of cooperative AI.