Why did almost every animal body plan appear in a geological instant?

Nobody knows for sure, and that is still an open question. The leading explanations fall into three families, environmental, developmental and ecological, and the best-known candidates are rising oxygen in the oceans and the evolution of eyes. What is clear is the event itself: in a window starting about 538.8 million years ago and lasting 13 to 25 million years, the fossil record shows the first appearances of practically every animal phylum. Before that, most life was simple: single cells or small multicellular organisms, sometimes organized into colonies.

The puzzle is old. In his 1859 book On the Origin of Species, Charles Darwin called the sudden appearance of trilobites, with no apparent ancestors, "undoubtedly of the gravest nature" among the difficulties for his theory, and later admitted he could give "no satisfactory answer" for the missing earlier fossils. In 1909, the American paleontologist Charles Walcott found Cambrian fossils in shale near Mount Burgess in Canada, and went back until his death in 1924, digging out more than 65,000 specimens.

Their strangeness took decades to sink in. In the 1970s, Harry Whittington and colleagues reanalysed the Burgess Shale fossils and concluded that several were as complex as any living animal, yet different from all of them: Marrella, an arthropod that fits no known arthropod class, and the five-eyed Opabinia, so odd that the audience laughed when Whittington presented his analysis.

As for the cause, oxygen levels correlate with the diversity of complex life, and in the Early Cambrian, jumps in biodiversity track the spread of oxygenated water on the seafloor. Andrew Parker proposed instead that eyesight changed the game: once predators could see prey from a distance, prey needed armor and spines. Many scientists doubt vision alone did it, since eyes may have evolved long before the Cambrian.