Archaeological excavations starting in the 1840s have revealed human settlements dating to 10,000 BCE in Mesopotamia and indicate that the fertile conditions of the land between two rivers [see Fertile Crescent] allowed an ancient hunter-gatherer people to settle in the land, domesticate animals, and turn their attention to agriculture. Trade soon followed and with prosperity came urbanization and the birth of the city. It is generally thought that writing was also invented due to trade out of the necessity for long-distance communication and for keeping more careful track of accounts.