Life’s extraordinary menagerie sprang up from the simplest of roots — chance chemical connections in the chaotic broth of early Earth. The forces of evolution then spent billions of years pushing life up the ladder of complexity. Along the way, life underwent a few major transitions, often when individuals surrendered some of their own self-interest to become part of a higher-functioning group. In this series of articles, we explore a handful of these major evolutionary leaps.

A word on dates: All the scientists we surveyed to create our timeline stressed that any dates for such long-ago events are approximate and provisional and the subject of considerable debate.