New evidence of animal development that led to the Cambrian explosion

A study combining a new compilation of the fossil record with the most extensive molecular dataset to date pins the last common ancestor of all living animals to 800 million years ago.

Add the geochemical data of the era, and the package offers a new perspective on the Cambrian explosion.

The study is published in the November 25, 2011 issue of Science.

Previous research into early animal life — organisms composed of many cells with different functions — has suggested they emerged anywhere from 600 million to more than a billion years ago. But such analyses relied mostly on fossils or on earlier approaches to building molecular clocks — using data from living species to date the origins of their distant ancestors.

SFI Faculty Chair Doug Erwin, curator of Paleobiology at the Smithsonian Institution in Washington, D.C., and colleagues decided to probe further. By matching genetic sequences for more than 100 living animal species — including arthropods, echinoderms, and mollusks, along with the typical array of animals with backbones — to the fossil records, the team found that complex animals originated roughly 800 million years ago.

Further, the "developmental toolkit" required for producing basic structures and functions, such as a gut, was largely in place by the time the early ancestors of jellyfish and sea anemones emerged 700 million years ago.

There's still a long lag until the Cambrian explosion began 541 million years ago. Erwin and team proposes that for millions of years, sponges and simple animals slowly changed their environment: filtering carbon out of seawater and burying in the sediments after organisms died let oxygen levels increase, and burrowing in the soft seafloor aerated the sediment which created favorable places for still more animals to live. New species formed ecological relationships, enabling still more ecological engineering.

These environmental and ecological changes allowed animals to take advantage of the potential in the developmental toolkit that provided building blocks of animal diversity. Together, these processes created a positive feedback loop that led to the evolutionary explosion of the Cambrian.

"People have proposed explanations that involve changes in environment, the origin of the developmental toolkit, or ecological interactions," Erwin says. "By carefully integrating the fossil and molecular data, we were able to separate the timing of these events. This gives us a much better understanding of how the processes are linked."

Read the paper in Science (November 25, 2011)