Paleontologists have found traces of animal life dating back at least 635 million years. Those earliest animals may have been like today’s sponges, rooted to the sea floor and filtering food particles from the water. 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.

But the diversity of the Cambrian had another source: the DNA of the animals themselves. Animals evolved genetic programs for turning a single egg into a complex body. These programs turned out to be supremely evolvable–with relatively minor mutations, they could give rise to new forms.

The book is also accompanied by some remarkable paintings by Quade Paul. I’ve reproduced some of them here as a slide show, and below.

View Images Myllokunmingia may be the oldest known vertebrate, with a skull made of cartilage and other hallmarks of vertebrates (like us). Copyright Quade Paul

View Images Herpetogaster may be related to living starfish and acorn worms

View Images Banffia is baffling; it’s not clear yet what its closest living relatives are. Copyright Quade Paul

View Images Diania had spiked, segmented legs that show some similarities to those of insects and other arthropods. Some scientists see that as a sign of close kinship. Copyright Quade Paul

View Images Hurdia was a primitive cousin of insects and other arthropods. Copyright Quade Paul

View Images Anomalocaris was a giant of the Cambrian, reaching over a meter long. Copyright Quade Paul

View Images Fuxianhuia is a close relative of living arthropods such as insects. Copyright Quade Paul

View Images Odontogriphus had a circular mouth ringed with teeth. Copyright Quade Paul

View Images Wiwaxia may have been related to today’s mollusks. Copyright Quade Paul

View Images Maotianoascus and Ctenorhabdotus were early relatives of today’s jellyfish. Copyright Quade Paul

View Images Orthozanclus, a relative of mollusks. Copyright Quade Paul

View Images Pikaia was a relative of vertebrates. Copyright Quade Paul Pikaia description

View Images Opabinia had five eyes and a single appendage extending from its head. Copyright Quade Paul

All the pictures in this post are copyright 2013 Quade Paul.