In the second episode, Nathan develops a rehearsal for Angela, a woman considering motherhood. Nathan hires child actors to simulate adopting and caring for a baby and sets her up in a rented farmhouse in rural Oregon. Due to Oregon child protection laws, Nathan's team must covertly switch out the baby every four hours and replace it with a robot baby at night. Seeking a simulated husband, Angela dates Robbin, a numerology-obsessed man who wants to have sex with Angela despite her devout Christian beliefs against premarital sex. When Robbin quits the project due to the robot baby's incessant crying, Nathan inserts himself into the experiment as Angela's non-romantic co-parent.

Moving on to the Late Eocene period 36 million years ago and mammals have prospered and are now the largest creatures on land and sea. This is an era of animals like andrewsarchus, the biggest mammal carnivore ever to walk on land, and the brontotheres, small-brained herbivores. It is in the sea, however, that the most monstrous mammals of all can be found. We follow the fate of a female basilosaurus, a huge serpent-like early whale, but nothing like the gentle filter feeding whales of the 21st century. Four times the length of the great white shark, with jaws to match, she is every inch a killer.

25 million years ago the biggest land mammals of all time, the indricotheres, stalked the Earth. Up to seven metres tall and weighing 15 tonnes, adults were too big to be eaten by any predator of the time. Only in the first few years of its life was indricotheres vulnerable. The programme follows the fate of a calf from his traumatic birth to see whether it can survive droughts, killer hogs and hyaenodon - a predator the size of a rhino with jaws that could crush a rock.

Diarmaid MacCulloch's own life story makes him a symbol of a distinctive feature about Western Christianity - scepticism, a tendency to doubt which has transformed both Western culture and Christianity. In the final programme in the series, he asks where that change came from. He challenges the simplistic notion that faith in Christianity has steadily ebbed away before the relentless advance of science, reason and progress, and shows instead how the tide of faith perversely flows back in. Despite the attacks of Newton, Voltaire, the French Revolutionaries and Darwin, Christianity has shown a remarkable resilience. The greatest damage to Christianity was actually inflicted to its moral credibility by the two great wars of the 20th century and by its entanglement with Fascism and Nazism. And yet it is during crisis that the Church has rediscovered deep and enduring truths about itself, which may even be a clue to its future.

Travelling forward in time to 30,000 years ago, it's the middle of an ice age. The landscape is dominated by mighty mammoths, living side-by-side with woolly rhinos, giant deer and two separate species of human. The programme follows the fate of a herd of mammoths in their annual struggle against the harsh ice-age conditions. Every summer they spend on the grassy plains of what will one day become the bottom of the North Sea, but every winter they are forced to head for the less exposed valleys further south. It is a journey fraught with danger: mammoths can get trapped in frozen bogs, and the herd must run the gauntlet of hunters like cave lions and the deadly Neanderthals.

In the far south west of Japan, there is a chain of islands stretching towards the tropics - a place where all life is influenced by the power of the sea, and where volcanoes and typhoons are forces to be reckoned with. The journey begins at an island at the top of the chain and travels south, revealing unexpected stories of isolation, unique wildlife and unsolved mysteries.