This week's programmes in the history of the world looks at the growing sophistication of humans around the globe, between 5000 and 2000 BC. Mesopotamia had created the royal city of Ur, the Indus valley boasted the city of Harappa and the great early civilisation of Egypt was beginning to spread along the Nile. New trade links were being forged and new forms of leadership and power were created. And, to cope with the increasing sophistication of trade and commerce, humans had invented writing. In today's programme, Neil MacGregor describes a small clay tablet that was made in Mesopotamia about 5000 years ago and is covered with sums and writing about local beer rationing. The philosopher John Searle describes what the invention of writing does for the human mind and Britain's top civil servant, Gus O'Donnell, considers the tablet as an example of possibly the earliest bureaucracy