The episode begins in the South American rainforest whose rich variety of life forms is used to illustrate the sheer number of different species. Since many are dependent on others for food or means of reproduction, David Attenborough argues that they couldn't all have appeared at once. He sets out to discover which came first, and the reasons for such diversity. He starts by explaining the theories of Charles Darwin and the process of natural selection, using the giant tortoises of the Galapagos Islands (where Darwin voyaged on HMS Beagle) as an example. Fossils provide evidence of the earliest life, and Attenborough travels a vertical mile into the Grand Canyon in search of them. By the time he reaches the Colorado River bed, the geological strata are 2,000 million years old â€' yet there are no fossils. However, the "right rocks" are found on the shores of Lake Superior in Canada, where wafer-thin slices of flint, called chert, reveal filaments of primitive algae. Also, the micro-organisms that flourish at Yellowstone Park in Wyoming appear to be identical to the Earth's oldest fossils. The evolution of single-celled creatures, from simple cyanophytes to more complex ciliates, and then from multi-celled sponges and jellyfish to the many variations of coral and its associated polyps, is discussed in detail. The fossilised remains of jellyfish are shown within the Flinders Ranges of Australia, and are estimated to be 652 million years old.

Film Duration: 54 min