Are the wealthy just born in the right place at the right time? Are the poor victims of a system designed to keep them down? Or do physics and biology determine who is rich and who is poor? Throughout history, distribution of wealth is governed by hidden forces: DNA, environmental stress, patterns of human migration and even the laws of thermodynamics! Nature seems to demand winners and losers in life. But does this mean greed is king, and the rich can take what they want? Or is cooperation – between microorganisms, monkeys, and humans – more essential to survival of a species?

In the final challenge of the game of life, raising offspring is the ultimate prize. Continuing the line through the next generation, is the next best thing to immortality. But it’s far from easy... Some parents must risk their own lives for their offspring.

In this final episode we complete our journey, travelling back from the March equinox to the end of June. Kate Humble is in the Arctic at a place where spring arrives with a bang, whilst Helen Czerski chases a tornado to show how the earth's angle of tilt creates the most extreme weather on earth.

We live our lives pursuing happiness 'out there' as if it is a commodity. We have become slaves to our own desires and craving. Happiness isn't something that can be pursued or purchased like a cheap suit. This is Maya, illusion, the endless play of form. In the Buddhist tradition, Samsara, or the endless cycle of suffering is perpetuated by the craving of pleasure and aversion to pain. Freud referred to this as the "pleasure principle." Everything we do is an attempt to create pleasure, to gain something that we want, or to push away something that is undesirable that we don't want. Even a simple organism like the paramecium does this. It is called response to stimulus. Unlike a paramecium, humans have more choice. We are free to think, and that is the heart of the problem. It is the thinking about what we want that has gotten out of control. The dilemma of modern society is that we seek to understand the world, not in terms of archaic inner consciousness, but by quantifying and qualifying what we perceive to be the external world by using scientific means and thought. Thinking has only led to more thinking and more questions. We seek to know the innermost forces which create the world and guide its course. But we conceive of this essence as outside of ourselves, not as a living thing, intrinsic to our own nature. It was the famous psychiatrist Carl Jung who said, "one who looks outside dreams, one who looks inside awakes." It is not wrong to desire to be awake, to be happy. What is wrong is to look for happiness outside when it can only be found inside.

In ‘Why Do I Need You?’ Dr. David Eagleman explores how the human brain relies on other brains to thrive and survive. Our fundamentally social nature can hold the key to our sucdess as a species. Our brains are so fundamentally wired to interact that we are something more like a single vast super-organism. In this age of digital connection, we desperately need to understand how human brains interact if we want our civilization to have a future, if we want to avoid fanaticism and to embrace cooperation. This neural interdependence begins at birth. Dr. David Eagleman invites a group of babies to a puppet show to showcase their ability to discern who is trustworthy, and who isn’t.

The third episode is devoted to the ways in which plants reproduce. Pollen and a stigma are the two components needed for fertilisation. Most plants carry both these within their flowers and rely on animals to transport the pollen from one to the stigma of another. To do this, they attract their couriers with colour, scent and nectar. It isn't just birds that help pollination: some mammals and reptiles also do so. However, it is mostly insects that are recruited to carry out the task. To ensure that pollen is not wasted by being delivered to the wrong flower, some species of plant have developed exclusive relationships with their visitors, and the gentian and its attendant carpenter bees is one example. Since pollen can be expensive to produce in terms of calories, some plants, such as orchids, ration it by means of pollinia and a strategically placed landing platform. Other orchids offer no reward for pollination, but instead mislead their guests by mimicking their markings and aroma, thus enticing males to 'mate' with them (Pseudocopulation). The most extreme fertilisation method is one of imprisonment, and one plant that uses it is the dead horse arum. It is often found near gull colonies, and mimics the appearance and smell of rotting flesh. Blow-flies are attracted to it, and are forced to stay the night before being allowed to depart in the morning, laden with pollen. Finally, Attenborough introduces the world's largest inflorescence: that of the titan arum.