The history of humanity - as told through one hundred objects from the British Museum in London - is back in South America. This week Neil MacGregor, the museum's director, is with objects from around the world between 1450 and 1600. This is the time of huge European expansion thanks to the new developments in ship building. Today he is with an object made by the Aztecs of present day Mexico. He describes the Aztec world and the Spanish conquest of this culture, through a double-headed serpent made from tiny pieces of turquoise - one of the stars of the British Museum. The Aztec specialist Adriane Diaz Enciso discusses the role of the snake in Aztec belief while the conservator Rebecca Stacey describes the scientific detective work that the object has prompted. Producer: Anthony Denselow