Episode Four: Violent Planet Over millions of years mankind has evolved from a puny, vulnerable creature to become a dominant force on the planet. Occasionally the planet bites back - with terrifying consequences that have shaped our evolution. 70,000 years ago humans spread across Africa and Southern Asia. But then a catastrophe strikes, taking us to the brink of extinction. The Toba mega-volcano erupts in Indonesia sending trillions of tonnes of ash into the atmosphere, blocking the sun. The prey we hunt become scarce. The human population drops from over a million to just a few thousand. Yet out of this extreme hardship we emerge stronger. But the planet has more tests to throw at us. 20,000 years ago large parts of the world are covered in Ice. Again we are forced to adapt, and for the first time we must create appropriate clothing. We turn a shard of bone into a needle - capable of stitching together close fitting clothes. Around 10,000 years ago the ice recedes. In the Middle East edible plants and grasses - like wheat - thrive. Humans take advantage of this excess, and establish permanent settlements. The warming world helps give birth to farming. But with farming and settlement, conflict arises between neighbouring groups. In the Mongolian Steppes a leader emerges: Genghis Khan. He leads the Mongols in a bloody rampage across the world, killing millions, forging a giant empire that connects the Far East to Europe. A long period of rain, floods and famine have rendered Europeans weak. Down the Mongol highways travels an even more terrifying killer than Genghis Khan: The Black Death. Millions die. When the plague subsides, survivors find themselves facing a new world. There are fewer people, more land, food and jobs. In this new Europe, Venice becomes rich from trade and banking. The Renaissance blossoms in an explosion of innovation, culture and commerce. The search for new trade routes inspires wealthy Europeans to explore, to travel ... and they discover the New World. Today, with all our technological advantage, we are still as vulnerable to the immense power of the planet as we ever were. Tsunamis, earthquakes, volcanoes, climate change, all remind us how fragile and tenuous our existence can be.