Between 8000–3500 BCE there was an exponential growth in population and the establishment of the first established settlements and trading routes in Mesopotamia, Asia Minor and Eurasia. Farming and the use of irrigation increased food resources, primarily in valley regions around major rivers, such as the Nile, Euphrates, Tigris, Indus and Huang-Ho. Here, semi-nomadic groups settled and the first towns, cities and great civilizations grew. In Mesopotamia, where the Tigris and Euphrates meet (‘the Fertile Crescent’) Eridu and Uruk were the first Sumerian city-states and there is evidence that, by 3500 BCE, they had discovered metal-working and had developed cuneiform, the world’s earliest writing systems. The Indus Valley civilization grew up around the Indus, with archaeological excavations in Harappa, Mohenjo-Daro and Lothal showing that by 3500 BCE they were major urban centres. An-yang became the capital of the later Shang dynasty, while Memphis became capital of the Egyptian Old Kingdom c. 2686 BCE.