Answer:
The earliest known civilization of Indian sub-continent is Harappan civilization or Indus Valley Civilization. Most of the sites discovered were around the river Indus. the next period of Indian History is comprised of Aryan civilization. It is believed that they came into waves either from Central Asia on the basis of linguistic grounds. Thus the area of Indus River valley was fertile and habitable land and more attracted to anyone want to enter India. A recent study suggests that there were connections between late Harrapan and early Aryan cultures thus crossing Indus seems to be vital for Early Aryans.