What a piece of work is man! Everyone agrees on that much. But what exactly is it about Homo sapiens that makes us unique among animals, let alone apes, and when and how did our ancestors acquire that certain something? The past century has seen a profusion of theories. Some reveal as much about the time their proponents lived in as they do about human evolution.

Many of them have merit, but they share a bias: the idea that humanity can be defined by a single well-defined trait or group of traits and that a single stage in evolution was a crucial turning point on the inevitable road to Homo sapiens.

But our ancestors weren’t beta tests. They weren’t evolving toward something,