Darwin’s theory predicts that new life forms evolve gradually from old ones in a constantly branching, spreading tree of life. Those brave new Cambrian creatures must therefore have had Precambrian predecessors, similar but not quite as fancy and sophisticated. They could not have all blown out suddenly, like a bunch of geysers. Each must have had a closely related predecessor, which must have had its own predecessors: Darwinian evolution is gradual, step-by-step. All those predecessors must have come together, further back, into a series of branches leading down to the (long ago) trunk.

But those predecessors of the Cambrian creatures are missing. Darwin himself was disturbed by their absence from the fossil record. He believed they would turn up eventually. Some of his contemporaries (such as the eminent Harvard biologist Louis Agassiz) held that the fossil record was clear enough already, and showed that Darwin’s theory was wrong. Perhaps only a few sites had been searched for fossils, but they had been searched straight down. The Cambrian explosion had been unearthed, and beneath those Cambrian creatures their Precambrian predecessors should have been waiting‐and weren’t. In fact, the fossil record as a whole lacked the upward-branching structure Darwin predicted.