Over the next 100 million years or so, new kinds of animals emerged. Some were recognizable members of living groups of animals, while others were so bizarre that paleontologists suspect they belonged to long-extinct lineages. And then, around 520 million years ago, the fossil record of animals starts to roar like a firehose switched from a trickle to full blast. Many of the oldest known members of living animal groups–including our own–appear during the Cambrian Period. But the Cambrian fossil record is also rife with forms only distantly related to animals on Earth today, some of which were so weird that the sight of a reconstruction of the creatures made scientists burst out laughing.

Many people have become familiar with this period of evolution through Steven Jay Gould’s 1989 influential book, Wonderful Life. In the 24 years since then, scientists have learned a lot more about the Cambrian. Two of the leading experts on the period, Doug Erwin of the Smithsonian Institution and James Valentine of Berkeley have collaborated on a new book, The Cambrian Explosion: The Construction of Animal Biodiversity, in which they synthesize evidence, both old and new, about this exceptional chapter in animal evolution.

The so-called Cambrian Explosion probably had many fuses. Erwin and Valentine explain how the Earth was undergoing drastic changes in the millions of years leading up to the flowering of the animal kingdom, with global ice ages and a burst of oxygen flooding the oceans. The stage was set for big, active creatures to evolve. As predators emerged, their prey became better defended with spikes and shields; the predators in turn became even deadlier. The animals changed their environment–burrowing animals, for example, pierced the sea floor with countless tunnels. As the environment changed, new kinds of animals evolved that could occupy new niches. The animal kingdom became both physically and ecologically complex.