What happened to the Indus Valley civilization and how did it end? Some people (among them, Gordon Childe) say that there was a sudden end to it due to an unexplained catastrophe.
The river Indus is well-known for its mighty floods which overwhelm and wash away cities and villages. Or a changing climate might lead to a progressive desiccation of the land and the encroachment of the desert over cultivated areas. The ruins of Mohenjo-daro are themselves evidence of layer upon layer of sand being deposited, raising the ground level of the city and compelling the inhabitants to build higher on the old foundations.
Some excavated houses have the appearance of two or three storied structures, and yet they represent a periodic raising of the walls to keep pace with the rising level. The province of Sind we know was rich and fertile in ancient times, but from mediaeval times onwards it has been largely desert.
It is probable, therefore, that these climatic changes had a marked effect on the people of those areas and their ways of living. And in any event climatic changes must have only affected a relatively small part of the area of this widespread urban civilization, which, as we have now reason to believe, spread right up to the Gangetic Valley, and possibly even beyond. We have really not sufficient data to judge. Sand, which probably overwhelmed and covered some of these ancient cities, also preserved them; while other cities and evidences of the old civilization gradually decayed and went to pieces in the course of ages. Perhaps future archaeological discoveries might disclose more links with later ages.
While there is a definite sense of continuity between the Indus Valley civilization and later periods, there is also a kind of break or a gap, not only in point of time but also in the kind of civilization that came next. This latter was probably more agricultural to begin with, though towns existed and there was some kind of city life also. This emphasis on the agricultural aspect may have been given to it by the newcomers, the Aryans who poured into India in successive waves from the north-west.
The Aryan migrations are supposed to have taken place about a thousand years after the Indus Valley period; and yet it is possible that there was no considerable gap and tribes and peoples came to India from the north-west from time to time, as they did in later ages, and became absorbed in India. We might say that the first great cultural synthesis and fusion took place between the incoming Aryans and the Dravidians, who were probably the representatives of the Indus Valley civilization. Out of this synthesis and fusion grew the Indian races and the basic Indian culture, which had distinctive elements of both. In the ages that followed there came many other races: Iranians, Greeks, Parthians, Bactrians, Scythians, Huns, Turks (before Islam), early Christians, Jews, Zoroastrians; they came, made a difference, and were absorbed. India was, according to Dodwell, 'infinitely absorbent like the ocean.' It is odd to think of India, with her caste system and exclusiveness, having this astonishing inclusive capacity to absorb foreign races and cultures. Perhaps it was due to this that she retained her vitality and rejuvenated herself from time to time.
The Moslems, when they came, were also powerfully affected by her. 'The foreigners (Muslim Turks),' says Vincent Smith, 'like their forerunners the Sakas and the Yueh-chi, universally yielded to the wonderful assimilative power of Hinduism, and rapidly became Hinduised.'
[ Excerpts from “The Discovery of India” first published in 1946 ]