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Ant colonies are one of the wonders of nature - complex, organised and mysterious. This programme reveals the secret, underground world of the ant colony in a way that's never been seen before. At its heart is a massive, full-scale ant nest, specially-designed and built to allow cameras to see its inner workings. The nest is a new home for a million-strong colony of leafcutter ants from Trinidad.

Waterworlds - Secrets of Our Living Planet (4of4): the secrets of our watery habitats, fresh and salty.

The Magical Forest - Secrets of Our Living Planet (3of4): the annual miracle of the temperate forest: the destruction of its ecosystem in winter, followed by it rebuilding itself in spring.

The Secret of the Savannah (Secrets of Our Living Planet 2of4): how the world's most spectacular grasslands flourish, despite being short of one essential nutrient - nitrogen.

The Emerald Band (Secrets of Our Living Planet 1of4): a spellbinding account of how the very special conditions that exist in the rainforest have allowed vast colourful communities of animals and plants to evolve.

The Survivors

Brains are not essential to life. Many organisms, from algae to jellyfish, get along just fine without a central information-processing center. But there is no question that a brain gives many animals an edge. For in the struggle for survival, brawn often gives way to a brain that can outthink a competitor.

Choreographed Cooperation Life may be a contest in which only the fittest individuals survive, but cooperation has also played a key role in evolution. This episode takes a close look at the alliances that animals have forged — with others of their own kind and very different organisms — in a bid to stay alive. In fact, teamwork occurs everywhere, from flocks of birds and herds of wildebeest to colonies of ants and termites.

For billions of years, life on Earth has been engaged in its own eternal arms race. As predators became better hunters, their prey also evolved better defenses.

It takes a passionate look at the evolution of sex, which allows a species to pass its genes along from generation to generation. Sex is everywhere. Bees do it and birds do it — and so do lizards and bacteria. In its simplest form, sex is the process of mixing genes from two parents into a new offspring. This gene swapping ensures that each generation is just a bit different from the one that came before, and that each individual is a bit different from others.

This explains how both conflict and cooperation have helped shape the species that today populate our world. And it profiles the winners and losers in this epic evolutionary contest — from the massive dinosaurs who disappeared long ago to the humble bacteria that have survived for billions of years virtually unchanged.

PBS Origins, Episode 4, Back to the Beginning. Origins: Neil deGrasse Tyson traces Earth's geologic evolution.

Extraterrestrial life

The Earth's first primitive life forms.

The tumultuous first billion years of Planet Earth