The oldest fossilised remains of complex animals appear suddenly in the fossil record, and as if from nowhere, in rocks that are 538 million years old.

The very oldest of these are simple fossilised marks (called Treptichnus) made by something worm-like with a head and a tail. A host of other animals appear rapidly, ancestors of the diverse animal groups we know today: ancient crab-like arthropods, shelled molluscs and the forebears of starfish and sea urchins.

The rapid arrival of animals so different to each other (and their absence in even slightly older rocks) was a headache for Charles Darwin because it seemed to go against his idea of gradual evolution – and it has confused scientists ever since. However, a recent paper may provide a solution.

In 1859, Darwin wrote in On the Origin of Species: “If my theory be true … during these vast … periods of time, the world swarmed with living creatures. To the question why we do not find records of these vast primordial periods, I can give no satisfactory answer.”

Today, scientists are in disagreement about when these ancient animals evolved. The problem stems from a late 20th-century invention called the molecular........

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