In this episode, a question that haunted Charles Darwin: if natural selection boils down to survival of the fittest, how do you explain why one creature might stick its neck out for another?

The standard view of evolution is that living things are shaped by cold-hearted competition. And there is no doubt that today's plants and animals carry the genetic legacy of ancestors who fought fiercely to survive and reproduce. But in this hour, we wonder whether there might also be a logic behind sharing, niceness, kindness ... or even, self-sacrifice. Is altruism an aberration, or just an elaborate guise for sneaky self-interest? Do we really live in a selfish, dog-eat-dog world? Or has evolution carved out a hidden code that rewards genuine cooperation?

For transcripts, see individual segment pages.