The Animal Kingdom's Otherworldly Ancestors

We’re all descended from sea sponges.

At one time, the squishy invertebrates constituted most of the animal life on Earth, but about half a billion years ago, something remarkable happened: an evolutionary explosion known as the Cambrian Period.

Rising oxygen levels in the ocean’s shallows and a morphing genome took animal evolution on a bizarre course, all in a period of 20 to 30 million years. The period marks the establishment of animals on Earth, said Jim Valentine, UC Berkeley professor emeritus of paleontology and paleobiology.

“The fauna has a very weird look to it,” Valentine said. “It looks like these things are from another planet. Some seem eerily familiar because they do belong to major categories of animals that are still with us.”

Some of the critters’ appearances are indeed strange, some border on terrifying, but others bear a strong resemblance to modern jellyfish or sea anemones. The wide swath of life that arose in the Cambrian Period is the subject of a new book by Valentine and Doug Erwin of the Smithsonian Institute, titled “The Cambrian Explosion: The Construction of Animal Biodiversity.”

“What could be more interesting than the taking over of the biosphere by animals half a billion years ago for a scientist? That’s just the greatest problem you can think of if you’re a biological scientist,” Valentine said with a laugh.

The book pulls in research from a number of disciplines to explain what made the rise of animals possible. What allowed for this expansion of biodiversity?

“That’s a $54,000 question,” Valentine said.

One of the key elements to the Cambrian Explosion was a flexible genome that allowed animals to take on larger and more complex forms, Valentine said. Most scientists believe oxygenation of shallow waters allowed the tiny slug- and worm-like creatures to grow. These millimeter-sized bugs lived along mats of microorganisms, but as they grew, they began tearing up the mats, burrowing in them and scratching the surface. At some point, some started eating each other.

Cambrian fossils are found in about 30 areas worldwide, Valentine said, but sites in South China’s Yunan Province and the White Sea in Northern Russia offer the best specimen because of their ideal conditions that preserve not only shells and skeletons, but much of the soft parts such as eyes and limbs. This made the level of detail in Quade Paul‘s illustrations possible.

“The detail is fantastic, so there’s little doubt about the morphological inferences you can make from the fossils,” Valentine said.

The modern world, the world of animals, was in a sense born in the Cambrian Period, Valentine said. While life wouldn’t move on shore for many millions of years, the same principles that permitted the Cambrian Explosion would eventually allow for the evolution of terrestrial life: insects, dinosaurs, apes and us.