The Earth formed 4.6 billion years ago, and a billion years later only primitive life inhabited its oceans. Single-celled bacteria were the planet's dominant life form for the next three billion years.

Then in a burst of evolutionary novelties, beginning about 600 million years ago, complex multicellular animals appear in the fossil record. Much of what we know about the so-called "Cambrian Explosion" comes from a unique fossil deposit found in the Canadian Rockies. The Burgess Shale was discovered in 1909 by Charles Doolittle Walcott, then Secretary of the Smithsonian Institution, and ranks as one of the 20th century's most significant paleontological discoveries.

Though many look strange enough for a science fiction film, the amazing array of marine animals discovered within the Burgess Shale includes the ancestors of virtually all modern animals. Other organisms are unlike anything alive today. Sanctacaris swam just above the ocean floor seeking food much the way modern horseshoe crabs do today. Anomalocaris-a gigantic beast for its time at three feet long-roamed the seas, preying on smaller creatures. And little Pikaia, a slightly flattened worm-like animal, harbored a special trait: it had the beginnings of a backbone and is the ancestor to all vertebrates. All of these animals evolved during the most dramatic burst of innovation in the history of evolution, when the blueprint for all the major groups of animals was laid down in a blink of geological time.

What did these extraordinary creatures of this biological "big bang" look like? How did so many body designs appear in such a short period of time? How are these sometimes bizarre-looking animals related to contemporary species? The exhibition, The Burgess Shale: Evolution's Big Bang, explores current theories about the "Cambrian Explosion," and presents highlights from the story of early life.