David Attenborough tackles the subject of the origin of life on Earth. He investigates the evidence from the earliest fossils, which suggest that complex animals first appeared in the oceans around 500 million years ago, an event known as the Cambrian Explosion. The naturalist travels around the world, using some of the latest fossil discoveries and their nearest equivalents amongst living species to reveal what life may have been like at that time. Visual effects and computer animation are used to reconstruct and animate the extinct life forms.