Our lives are filled with loops that hurt us, heal us, make us laugh, and, sometimes, leave us wanting more. This hour, Radiolab investigates the strange things that emerge when something happens, then happens again, and again, and again, and again, and again, and … well, again.

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?

A journey to the edge of human limits. On this hour of Radiolab, we test physical endurance with a bike race that makes the Tour de France look like child’s play, and mental capacity with a mind-stretching memory competition.

Regardless of how you feel about numbers, chances are you rely on them every day. This hour, Radiolab ponders the nature of numbers, and wonders what life might look like without them.

The pursuit of knowledge leads sometimes to answers, often to failure, and almost invariably to more questions. In this hour of Radiolab, stories of love and loss in the name of science.

The uneasy marriage of biology and engineering raises big questions about the nature of life. In this hour, Radiolab journeys to the first billion years of life on Earth, looks at how modern engineers tinker with living things, and meets a woman who could have been two people.

What happens when there is no leader? Starlings, bees, and ants manage just fine. In fact, they form amazingly complicated societies. This hour of Radiolab: a look at the bottom-up logic of cities, Google, and even our brains.