The Cambrian explosion

The story

Darwin's problem

When Charles Darwin published On the Origin of Species, in 1859, one thing worried him. In the oldest rocks then known, those of the Cambrian period, fossils already appeared in large numbers and of many kinds: shells, crustaceans, worms. Before those rocks there seemed to be nothing. If animals descended from one another over long spans of time, where were their ancestors? Darwin admitted that this was a serious difficulty for his theory and wrote that an explanation would have to be found in the future. He was right: the answer has come, in part, with later discoveries.

The Cambrian and its “explosion”

The Cambrian begins about 539 million years ago and lasts until about 485 million years ago. In the first twenty million years or so, palaeontologists see in the fossils a very rapid change: animals appear with shells, skeletons and complex bodies, and many of the big groups of animals that exist today. People speak of the “Cambrian explosion”. But be careful: twenty million years is a very long time by our measures. It was not a day, nor a real explosion. It was a phase of very fast diversification, compared with the billions of years that had come before.

Part of the effect is also due to the fact that shells and bones are preserved better than soft bodies. Before the Cambrian there were already animals, as the Ediacara fossils and the traces of worms show, but without hard parts they did not leave much behind. When animals learned to build shells of calcium and other minerals, fossils became far more numerous. Scholars debate how much of the “explosion” is real and how much is due to this effect.

A sea full of monsters

The Cambrian seas were full of creatures that seem to come from someone's imagination. Trilobites, distant relatives of crabs and insects, had a body divided into three parts and a segmented shell; thousands of species have been described, and they lived for about 270 million years. Then there were animals like Anomalocaris, a predator usually 30 to 40 centimetres long (some species perhaps half a metre), with two big arms for grabbing and a circular mouth. Opabinia had five eyes and a trunk with a claw; Hallucigenia, a worm-like body with spines, was reconstructed upside down for more than ten years before people understood how it really was.

Many of these animals are known thanks to exceptional deposits, where even the soft parts were preserved. The most famous is the Burgess Shale, in the Canadian Rockies. It was discovered in 1909 by the American palaeontologist Charles Walcott, who collected thousands of fossils. Another is Chengjiang, in China, about 518 million years old, found in 1984 by the palaeontologist Hou Xianguang; it has even yielded the first fish-like vertebrates, such as Haikouichthys.

Eyes, fangs and armour

Something changed in the way animals lived. The first active predators appeared, and as a result prey needed defences: shells, spines, armour, the ability to burrow and hide. Complex eyes appeared: trilobites had compound eyes with lenses of calcite, a transparent mineral. Seeing makes it possible to hunt better and to escape predators, and with every improvement by the attacker, the defender must improve in turn. Scholars speak of an “arms race”.

Why did it happen?

Nobody knows for certain. There are many hypotheses and it was probably a combination. Perhaps the rise of oxygen in the seas made bigger and more active animals possible. Perhaps the chemistry of the oceans changed, with more calcium available for building shells. Perhaps the birth of new predators mattered. Perhaps some genes that control the development of the body, the Hox genes, had already evolved and made it possible to build bodies in many different ways. Climate, moving continents and sea levels may have counted as well. The scientific debate is very lively.

One thing is certain: after the Cambrian the animal world was never the same. Almost all the main groups of animals we know, from molluscs to vertebrates, crustaceans and worms, are found in these seas.