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Richard Dawkins examines the growing suspicion the public has for science-based medicine, despite its track record of successes like the germ theory of disease, vaccines, antibiotics and increased lifespan. He notes a fifth of British children are currently not immunised against measles, mumps and rubella, attributing it to fears arising from a highly controversial report linking the vaccine with autism. Dawkins criticizes the growing field of alternative medicine which does not pass the same objective and statistical rigour as scientifically derived treatments using controlled double-blind studies. Without verifiable evidence, alternative therapies must rely on biased anecdotes and word of mouth to perpetuate. Dawkins observes these treatments have fanciful rationales and rituals behind them, with many alternative treatments employing pseudoscientific jargon such as "energy", "vibration" or "quantum theory" to give themselves greater credence to patients.

The third episode follows tigers, harpy eagles, chimpanzees, army ants and other predators as they rise to the challenge of hunting within the forest - a dense, confusing, three-dimensional world, one in which even finding prey is a maddening task. The prize for succeeding at nature's great game of hide-and-seek is one worth winning. Forests cover one third of the land surface, and concealed within are over half of the species on Earth.

Intelligence and adaptability allow primates to tackle the many challenges of life, and this is what makes our closest relatives so successful. This resourcefulness has enabled primates to conquer an incredible diversity of habitat. Hamadryas baboons live on the open plains of Ethiopia in groups up to 400 strong. Strength in numbers gives them some protection from potential predators. But, should their path cross with other baboon troops, it can lead to all-out battle, as males try to steal females from one another, and even settle old scores. Japanese macaques are the most northerly-dwelling primates and they experience completely different challenges. Some beat the freezing conditions by having access to a thermal spa in the middle of winter. But this privilege is only for those born of the right female bloodline. For western lowland gorillas, it's the male silverback that leads his family group in the rich forests of the Congo basin. He advertises his status to all with a powerful chest-beating display. Most primates are forest dwellers, and one of the strangest is the tarsier – the only purely carnivorous primate. As it hunts for insects the tarsier leaps from tree to tree in the dead of night, using its huge forward-facing eyes to safely judge each jump. Good communication is essential for success in primate society.

Sagan reflects on the future of humanity and the question of 'who speaks for Earth?' when meeting extraterrestrials. He discusses the very different meetings of the Tlingit people and explorer Jean-François de La Perouse with the destruction of the Aztecs by Spanish conquistadors, the looming threat of nuclear warfare, and the threats shown by destruction of the Library of Alexandria and the murder of Hypatia. The episode ends with an overview of the beginning of the universe, the evolution of life, and the accomplishments of humanity and makes a plea to mankind to cherish life and continue its journey in the cosmos. The Cosmos Update notes the preliminary reconnaissance of planets with spacecraft, the fall of the Berlin Wall and the end of apartheid in South Africa, and measures towards the reduction of nuclear weapons.

the final episode deals with plants that live in hostile environments. Attenborough visits Ellesmere Island, north of the Arctic Circle, to demonstrate that even in a place that is unconducive to life, it can be found. Algae and lichens grow in or on rock, and during summer, when the ice melts, flowers are much more apparent. However, they must remain close to the ground to stay out of the chilling wind. In the Tasmanian mountains, plants conserve heat by growing into 'cushions' that act as solar panels, with as many as a million individual shoots grouped together as one. Others, such as the lobelia in Mount Kenya, have a 'fur coat' of dense hairs on their leaves. The saguaro cactus in the Sonoran Desert flourishes because of its ability to retain vast amounts of water, which can't be lost through leaves because it has none. Many desert dwellers benefit from an accelerated life cycle, blooming rapidly within weeks after rainfall. Conversely, Mount Roraima is one of the wettest places on Earth. It is a huge sandstone plateau with high waterfalls and nutrients are continuously washed away, so plants have to adapt their diet if they are to survive. A bladderwort is shown invading a bromeliad. Inhabitants of lakes have other problems to contend with: those that dominate the surface will proliferate, and the Amazon water lily provides an apt illustration. Attenborough ends the series with an entreaty for the conservation of plant species.

Anthropologist Wade Davis and river advocate Robert F. Kennedy Jr journey down the Colorado River on a two-week expedition to highlight water conservation issues. Traveling by rafts, kayaks and wooden dories, they are accompanied by their daughters and guided by Shana Watahomigie, a Native American National Park ranger. Filmed with a 350-pound 3D camera, it involved the cooperation of three Indian nations, the National Park Service, film sponsor Teva’s team of kayakers and more than a dozen experienced river guides. The film explores America's drought and freshwater shortages, the impact on the river of damming, and human water supply needs. Grand Canyon Adventure: River at Risk was directed by Greg MacGillivray and narrated by Robert Redford.