In the fourth episode of this landmark series charting the story of human civilisation, Andrew Marr reaches the Middle Ages.After the collapse of the Roman Empire, Europe was little more than a muddy backwater. Vikings explored and pillaged from Northern Europe to North America. But they also laid the foundations of powerful new trading states - including Russia.This was also the Golden Age of Islam. And the knowledge of ancient civilisations from India, Persia and Greece were built upon by Islamic scholars in Baghdad's House of Wisdom.By exploring the conquests of Ghengis Khan, the adventures of Marco Polo and the extraordinary story of an African King - the wealthiest who ever lived - Marr finds out how Europe emerged from the so-called 'Dark Ages' and used influences from around the world to rise again with the Renaissance.