Documentary telling the little-known story of how Darwin came to write his great masterpiece, On the Origin of Species, a book which explains the wonderful variety of the natural world as emerging out of death and the struggle of life. In the twenty years he took to develop a brilliant idea into a revolutionary book, Darwin went through a personal struggle every bit as turbulent as that of the natural world he observed. Fortunately, he left us an extraordinary record of his brilliant insights, observations of nature, and touching expressions of love and affection for those around him. He also wrote frank accounts of family tragedies, physical illnesses and moments of self-doubt, as he laboured towards publication of the book that would change the way we see the world.

New Zealand - 850 years ago New Zealand was the last major land mass to be discovered and colonised by humans. A mere 850 years ago, Polynesian seafarers arrived in a land with no terrestrial mammals. New Zealand was a land of birds, and its avian rulers were giants: huge herbivorous moas were hunted by Haast's eagle - the largest eagle the world has ever seen. But within the space of only 100-400 years, the eagle, all the moas and over 20 other species of birds were gone. Had mankind become the monster?

Attenborough's journey continues in Canada's Rocky Mountains, where fossils document an explosion in animal diversity never seen before or since. Travelling from there to North Africa, the rainforests of Australia and the east coast of Scotland, Attenborough discovers how animals evolved to conquer not only the oceans but also the land and air. These remote and fascinating creatures are brought to life as never before with the help of cutting-edge scientific technology and photorealistic visual effects. From the first large predators to the first legs on land, these were creatures that evolved the traits and tools that allow all animals, including ourselves, to survive to this day.

Sagan discusses the story of the Heike crab and artificial selection of crabs resembling samurai warriors, as an opening into a larger discussion of evolution through natural selection (and the pitfalls of intelligent design). Among the topics are the development of life on the Cosmic Calendar and the Cambrian explosion; the function of DNA in growth; genetic replication, repairs, and mutation; the common biochemistry of terrestrial organisms; the creation of the molecules of life in the Miller-Urey experiment; and speculation on alien life (such as life in Jupiter's clouds). In the Cosmos Update ten years later, Sagan remarks on RNA also controlling chemical reactions and reproducing itself and the different roles of comets (potentially carrying organic molecules or causing the Cretaceous–Paleogene extinction event).

The theory of Némesis, a star that orbits the Sun and causes catastrophic events is explored. Nemesis is a hypothetical red dwarf or brown dwarf, originally postulated in 1984 to be orbiting the Sun at a distance of about 95,000 AU (1.5 light-years) somewhat beyond the Oort cloud, to explain a perceived cycle of mass extinctions in the geological record, which seem to occur more often at intervals of 26 million years.

Long ago, two species of human beings coexisted on Earth until competition drove one of them to extinction. This program, set in the southwest of France 35,000 years ago, reconstructs the life in the Neanderthal world at the time Cro-Magnons first entered the scene. All aspects of Neanderthal clan life are examined, including tool- and weapon-making, hunting and gathering, health and healing, childbirth, rituals, and making fire. Footage of skeletal remains and the scholarly research of eminent paleontologist Chris Stringer and Oxford University's Paul Pettitt support the documentary.