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.

Andrew Marr tells the story of the first empires which laid the foundations for the modern world. From the Assyrians to Alexander the Great, conquerors rampaged across the Middle East and vicious wars were fought all the way from China to the Mediterranean. But this time of chaos and destruction also brought enormous progress and inspired human development. In the Middle East, the Phoenicians invented the alphabet, and one of the most powerful ideas in world history emerged: the belief in just one God. In India, the Buddha offered a radical alternative to empire building - a way of living that had no place for violence or hierarchy and was open to everyone. Great thinkers from Socrates to Confucius proposed new ideas about how to rule more wisely and live in a better society. And in Greece, democracy was born - the greatest political experiment of all. But within just a few years, its future would be under threat from invasion by an empire in the east...

It is time to see if personalised dieting will work in normal life. The volunteers have been given one of three diets to follow - based on their genes, their hormones and their psychology. But now they are back at home, trying to stick to their personalised diets with all the stresses and temptations of real life. Dr Chris van Tulleken and Professor Tanya Byron discover how our genetic makeup can make temptation difficult to resist, how understanding the brain reveals what makes us comfort eat and what science can tell us about why we make disastrous food choices.

We float along the river of time. But does that river have a source? Where did time come from? Some believe time and space are one thing, and the Big Bang started the cosmic clock. Others believe the universe existed for almost half a million "years" before light could move and time began. Still others say time is older than our universe. But what if time itself is an illusion? Incredible new experiments may hold the answer. One groundbreaking experiment gives us the power to punch holes in time…and another may create a machine that operates outside time’s boundaries!

Competition to win a partner …has created some of the most extraordinary beauty and life-threatening violence anywhere in nature. Dazzling colours, elaborate dances and powerful weapons have all evolved to attract and defend a mate. The stakes could not be higher: without a mate, the journey of life ends here... sometimes literally.

The Pythagorian philosopher Plato hinted enigmatically that there was a golden key that unified all of the mysteries of the universe. It is this golden key that we will return to time and again throughout our exploration. The golden key is the intelligence of the logos, the source of the primordial om. One could say that it is the mind of God. With our limited senses we are observing only the outer manifestation of the hidden mechanics of self similarity. The source of this divine symmetry is the greatest mystery of our existence.