It's 3.5 million years ago and in East Africa a remarkable species of ape roams the land. Australopithecus afarensis has taken the first tentative steps towards humanity by standing and walking on two legs. The rift valley was forming, and the rain forests dying as Africa dried out - turning the landscape into a mosaic of scattered trees and grass. In this new environment afarensis found it more efficient to move about on two legs rather than four.

Francisco de Orellana failed to find El Dorado, but discovered the Amazon. Early in 1541, a rumor swept Quito that beyond the mountains, there lay a land richer than Mexico, or even Peru - a land of gold. The ruler of this land was so rich that he covered himself with gold dust every day and washed it off every evening. He was "the golden man", El Dorado. They had tree goals: to find La Canela, the land of cinnamon: to assess new lands for colonization; and to find El Dorado. On march 1541, They marched eastwards with more that 200 Spanish troops and thousands of native servants.

The second programme deals with the mechanics of flight. Getting into the air is by far the most exhausting of a bird's activities, and Sir Attenborough observes shearwaters in Japan that have taken to climbing trees to give them a good jumping-off point. The albatross is so large that it can only launch itself after a run-up to create a flow of air over its wings. A combination of aerodynamics and upward air currents (or thermals), together with the act of flapping or gliding is what keeps a bird aloft. Landing requires less energy but a greater degree of skill, particularly for a big bird, such as a swan. Weight is kept to a minimum by having a beak made of keratin instead of bone, a light frame, and a coat of feathers, which is maintained fastidiously. The peregrine falcon holds the record for being fastest in the air, diving at speeds of over 300 km/h. Conversely, the barn owl owes its predatory success to flying slowly, while the kestrel spots its quarry by hovering. However, the true specialists in this regard are the hummingbirds, whose wings beat at the rate of 25 times a second. The habits of migratory birds are explored. After stocking up with food during the brief summer of the north, such species will set off on huge journeys southwards. Some, such as snow geese, travel continuously, using both the stars and the sun for navigation. They are contrasted with hawks and vultures, which glide overland on warm air, and therefore have to stop overnight.

The next instalment focuses on dietary needs and how different species have evolved beaks to suit their individual requirements. The latter come in a multitude of forms. Blue tits and goldfinches have beaks akin to tweezers, with which to extract seeds, while the hawfinch's razor-like bill can deal with a cherry-stone. However, the crossbill is the only finch that can twist its mandibles in opposite directions. Jays store acorns for winter by burying them in the ground, whereas woodpeckers can keep up to 60,000 of them in one tree trunk. Sap is also desirable, and there are a variety of methods used to obtain it. The hoatzin is the only specialised leaf-eater, and accordingly has a digestive system more akin to that of cattle. Plants recruit birds to aid pollination, and offer nectar as a reward. Hummingbirds eat little else, and the sword-bill's beak is the longest of any bird in relation to its body. Insects are also highly prized, and Galapagos finches are shown to possess some ingenuity as they not only strip bark, but also use 'tools' to reach their prey. Crows are hailed as being among the most intelligent birds, and one is shown using a twig to spear a grub within a fallen log. The robin is an opportunist, and Sir Attenborough observes one seizing morsels as he digs a patch of earth. In South America, a cattle tyrant sits atop an obliging capybara and uses its vantage point to spot passing food that may be dislodged by its grazing partner.

Fourth episode examines how plants either share environments harmoniously or compete for dominance within them. Attenborough highlights the 1987 hurricane and the devastation it caused. However, for some species, it was that opportunity for which they had lain dormant for many years. The space left by uprooted trees is soon filled by others who move relatively swiftly towards the light. The oak is one of the strongest and longest-lived, and other, lesser plants nearby must wait until the spring to flourish before the light above is extinguished by leaves. Tropical forests are green throughout the year, so brute force is needed for a successful climb to the top of the canopy: the rattan is an example that has the longest stem of any plant. As its name suggests, the strangler fig 'throttles' its host by growing around it and cutting off essential water and light. Some can take advantage of a fallen tree by setting down roots on the now horizontal trunk and getting nutriment from the surrounding moss and the fungi on the dead bark. The mountain ash (eucalyptus regnans) grows so tall, that regeneration becomes a considerable problem. It is easily flammable, so its solution is to shed its seeds during a forest fire and sacrifice itself. It therefore relies on the periodic near-destruction of its surroundings in order to survive. Attenborough observes that catastrophes such as fire and drought, while initially detrimental to wildlife, eventually allow for deserted habitats to be reborn.

The fifth programme explores the alliances formed between the animal and plant worlds. Attenborough dives into Australia's Great Barrier Reef and contrasts the nocturnal feeding of coral, on microscopic creatures, with its daytime diet of algae. Some acacias are protected by ants, which will defend their refuge from any predator. Besides accommodation, the guards are rewarded with nectar and, from certain species, protein for their larvae as well. Fungi feed on plants but can also provide essential nutriment to saplings (Mycorrhiza). The connection is never broken throughout a tree's life and a quarter of the sugars and starches produced in its leaves is channelled back to its fungal partners. Meanwhile, fungi that feed on dead wood leave a hollow trunk, which also benefits the tree. Orchids enjoy a similar affiliation. Lichens are the product of a relationship between fungi and a photosynthetic associate, usually algae. They are extremely slow-growing, and a graveyard is the perfect location to discover their exact longevity. Mistletoe is a hemiparasite that obtains its moisture from a host tree, while using own leaves to manufacture food. Its seeds are deposited on another by the mistletoe tyrannulet, following digestion of the fruit. The dodder (Cuscuta) is also parasitic, generally favouring nettles, and siphons its nourishment through periodic 'plugs' along its stem. The rafflesia has no stem or leaves and only emerges from its host in order to bloom — and it produces the largest single flower: one metre across.