The first episode tells how invertebrates became the first creatures of any kind to colonise dry land. Their forerunners were shelled and segmented sea creatures that existed 400 million years ago. Some of them ventured out of the water to lay their eggs in safety, and Attenborough compares those first steps with today's mass spawning of horseshoe crabs off the Atlantic coast of North America.
Following extraordinary rumours of tigers living in the mountains of Bhutan, the expedition shifts to high altitude. Cameraman Gordon Buchanan captures remarkable footage of a snow leopard cub from over 5,000 metres in the air. Along the Tibetan border, explorer Steve Backshall treks to the mystical Tiger Mountain, where he has a very close encounter with the world's most elusive predator.
Africa's wildest river is home to the most spectacular wildlife. Hippos fight for territory while herds of elephant, water buffalo and zebra depend on it for life. In the wet season the rains burst the riverbanks and everyone, including people, must move whilst fish swim through the villages. In the dry season the creatures fight over the few pools of water while predators prowl. At its heart it plunges over Victoria Falls and into wild ravines before draining into the Indian Ocean, where storm clouds cycle the water back into the heart of Africa.
For most of the Earth's history, life consisted of the simplest organisms; but then something happened that would give rise to staggering diversity, and, ultimately, life as complex as that which we see today. Scientists are still struggling to figure out just what that was.