The Sumerians were the first known people to settle in Mesopotamia over 7,000 years ago, between the Tigris and Euphrates rivers (modern day Iraq). Sumer was often called the cradle of civilization. By the 4th millennium BC, it had established an advanced system writing, spectacular arts and architecture, astronomy and mathematics. The origin of the Sumerians remains a mystery till this day.
10 Civilizations That Disappeared Under Mysterious Circumstances. Indus Valley Civilization One of the great civilizations of the ancient world is called simply the Indus or Harappan civilization. Thousands of years ago, it may have boasted up to 5 million people, almost 10 percent of the world's population, spread over a region that encompassed parts of today's India, Pakistan, Iran and Afghanistan.