The mysterious fall of the largest of the world's earliest urban civilizations nearly 4,000 years ago in what is now India, Pakistan, Nepal and Bangladesh now appears to have a key culprit — ancient climate change, researchers say. Ancient Egypt and Mesopotamia may be the best known of the first great urban cultures, but the largest was the Indus or Harappan civilization. This culture once extended over more than 386,000 square miles (1 million square kilometers) across the plains of the Indus River from the Arabian Seato the Ganges, and at its peak may have accounted for 10 percent of...