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.
Young animals prepare for their first winter away from their parents, humpback whales return to Alaska's rich feeding grounds and salmon return to spawn in the rivers where they were hatched. But the returning salmon have to negotiate a path past hungry brown bears fattening themselves before they hibernate.
Take a journey through majestic primeval forests of the Urals. These protected sites are known for their extremes, where the fine side by side, and giant deer, and giant leviathan, weighing half a ton, and tiny animals. This also, in search of food, make their seasonal raiding Eurasian elk. Unlike other types of elk Eurasian elk live alone, so for the pregnant female elk stress of meeting with other animals usually leads to a fight. As soon as the snow begins to melt in the woods, baring the ground, a lone wolf set out in search of food, not disdaining the frozen remains of animals
Our body is a true time machine. It is a mirror of the history of the living world. If an engineer today had to fabricate the ideal human, he would most likely not design us the way we are currently made. He would go to the most logical, the most efficient, the most rational; and when you look closely, this is not always what characterizes us.