For most of the 20th century, the model of human origins was a tree: with the trunk dividing into branches, and then twigs. Each species of human relative (hominin) was a neat, single branch.
As an undergraduate, I was taught that Homo sapiens was one of these branches that emerged in Africa, spread across the world, and displaced every archaic human it encountered.
Neanderthals, Homo erectus, and other ancient relatives were evolutionary dead ends – unfortunate cousins who left no descendants. In the 30 years since I left university, those early lessons are now radically revised.
