The world of invertebrates exists in a web of relationships with plants and other animals. Unique footage of the world's smallest insect (a fairy wasp only quarter of a millimetre long) shows it flying underwater to find the eggs of water beetles in which to lay its own brood. Some ants 'farm' the trees that give them shelter, creating areas known as 'Devil's gardens'. To make sure these grow without competition, they kill off other seedlings in the surrounding vegetation. The blister beetle's larvae huddle together on the end of a piece of grass and mimic a female bee. When a male bee tries to mate with the 'female', the larvae grab on to his belly. Confused, he flies away and searches for a real female. When he eventually finds her and mates with her, the beetle larvae hurriedly swap from his front on to her back, and hence get carried back to her nest where they eat her pollen supplies.

Invertebrates don't always operate alone. True society was the last feature to evolve in invertebrates, as recently as the time of Tyrannosaurus. In the last programme see the tensions below the surface in some of the great social structures built by insects, and witness the carnage when an ant colony and a termite colony wage war.

Mammals from tiny shrews, giant anteaters, armoured armadillo to bats have specialised in eating insects. Some have evolved into complex and effective hunters, even pursued their prey into the skies

Survival of the fittest: Raw competition? Intense cooperation? Both are essential. Interactions between and within species are among the most powerful evolutionary forces on Earth, and understanding them may be a key to our own survival.

In ‘Who will we be?’ Dr. David Eagleman journeys into the future, and asks what’s next for the human brain, and for our species. We stand at a major turning point, one where we might take control of our own development. We face a future of uncharted possibilities in which our relationship with our own body, our relationship with the world, the very basic nature of who we are is set to be transformed. For thousands of generations, humans have lived the same life cycle over and over. We are born, we control a fragile body, we experience a limited reality, and we die. But science and technology are giving us tools to transcend that evolutionary story. Our brains don't have to remain as we have inherited them. We are capable of extending our reality, of inhabiting new bodies, and possibly shedding our physical forms altogether. And we are discovering the tools to shape our own destiny. Who we become is up to us.

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.