There are several different theories about the origin of Indus Valley Civilization but none of them has been fully confirmed including the so-called Dravidian theory. The scholars mostly agree that Indus Civilization developed from agricultural society which used the Mesopotamian model of irrigation.
Like origin, collapse of the Indus Valley Civilization remains unknown but it was certainly not caused by a single factor. Mohenjo-daro has been devastated by floods for several times and it seems that the city was in decline already for some time before it finally ceased to exist. The decline of Indus Valley Civilization might be also a result of climate changes, exhausted soil, a disease and invasions by foreign peoples. There has been a lot of speculations whether the Indo-Aryans were responsible for the final blow to the Indus Valley Civilization. Archaeological excavations in Mohenjo-daro revealed unburied human remains on the streets of the city but there is no firm evidence that walled cities and citadels which were according to the books of Rigveda attacked by the Indo-Aryans were actually referring to the cities of the Indus Valley Civilization. Even more, the archaeological evidences imply that Indus Valley Civilization collapsed before the arrival of the Indo-Aryans, while the remote cities south from Gudjarat seem to survive the fall of both largest cities of the Indus Valley Civilization.