There is no general agreement regarding the causes of the breakdown of Harappan urban society. Broadly speaking, the principal theories thus far proposed fall under four headings. The first is gradual environmental change, such as a shift in climatic patterns and consequent agricultural disaster, perhaps resulting from excessive environmental stress caused by population growth and overexploitation of resources. Second, some scholars have postulated more-precipitous environmental changes, such as tectonic events leading to the flooding of Mohenjo-daro, the drying up of the Sarawati River, or other such calamities. Third, it is conceivable that human activities, such as invasions of tribespeople from the hills to the west of the Indus valley, perhaps even Indo-Aryans, contributed to the breakdown of Indus external trade links or more directly disrupted the cities. The fourth theory posits the occurrence of an epidemic or a similar agent of devastation. It appears likely that some complex of natural forces compromised the fabric of society and that subsequent human intervention hastened its complete breakdown.

The Post-Urban Period in northwestern India

It is still far from certain at what date the urban society broke down. The decline probably occurred in several stages, perhaps over a century or more; the period between about 2000 and 1750 bce is a reasonable estimation. The collapse of the urban system does not necessarily imply a complete breakdown in the lifestyle of the population in all parts of the Indus region, but it seems to have involved the end of whatever system of social and political control had preceded it. After that date the cities, as such, and many of their distinctively urban traits—the use of writing and of seals and a number of the specialized urban crafts—disappear. The succeeding era, which lasted until about 750 bce, may be considered as Post-Harappan or, perhaps better, as “Post-Urban.”

In Pakistan’s Sind province the Post-Urban phase is recognizable in the Jhukar culture at Chanhu-daro and other sites. There certain copper or bronze weapons and tools appear to be of “foreign” type and may be compared to examples from farther west (Iran and Central Asia); a different but parallel change is seen at Pirak, not far from Mehrgarh. In the Kachchh and Saurashtra regions there appears to have been a steady increase in the number of settlements, but all are small and none can compare with such undoubtedly Harappan cities as Dholavira. In this region, however, the distinctive foreign metal elements are less prominent.

An intriguing development occurs along the Saraswati valley: there the early Post-Urban stage is associated with the pottery known from the Cemetery H at Harappa. This coincides with a major reduction in both the number and size of settlements, suggesting a deterioration in the environment. In the eastern Punjab too there is a disappearance of the larger, urban sites but no comparable reduction in the number of smaller settlements. This is also true of the settlements farther east in the Ganges-Yamuna valleys. It is probably correct to conclude that, in each of these areas during the Post-Urban Period, material culture exhibited some tendency to develop regional variations, sometimes showing continuations of features already present during the Pre-Urban and Urban phases.

The appearance of Indo-Aryan speakers

Scholars have traditionally agreed that a people speaking Old Indo-Aryan dialects of the Indo-Iranian branch of the Indo-European language family arrived in the Indian subcontinent during the late 3rd and 2nd millennia bce. These newcomers purportedly came from the steppes to the north and east of the Caspian Sea, moving first southward into the southern parts of Central Asia and from there fanning out across the Iranian plateau and spreading throughout northern India, disrupting the established sedentary culture and driving its Dravidian-speaking inhabitants of the Indus civilization southward. The movement itself remains hypothetical, but evidence from cemeteries at Sibri and south of Mehrgarh, near the mouth of the Bolan Pass, shows striking parallels—including foreign copper and bronze tools and weapons and typical pottery forms—with that from cemeteries of the Sapalli-Tepe group in Tajikistan and Uzbekistan. This correspondence suggests a date of about 2000 bce for the presence of these people on the borders of the Indus system.

However, it is even more difficult to identify traces that may be associated with the movement of Indo-Aryan speakers into the central Indus plains or to determine whether the occasional copper or bronze weapons of foreign type found in late contexts at Mohenjo-daro or Chanhu-daro are evidence of their presence there. Moreover, even if Indo-Aryans actually conquered some of the Indus cities and established hegemony over the local population, it has to be explained why they appear to have given up many of their distinctive material products while presumably retaining their distinctive speech.

One hypothesis is that between about 2000 and 1500 bce not an invasion but a continuing spread of Indo-Aryan speakers occurred, carrying them much farther into India, to the east and south, and coinciding with a growing cultural interaction between the native population and the new arrivals. From these processes a new cultural synthesis emerged, giving rise by the end of the 2nd millennium to the conscious expressions of Aryan ethnicity found in the Rigveda, particularly in the later hymns (see Early Vedic Period).

A more recent and controversial theory put forward by such scholars as American Jim G. Shaffer and Indian B.B. Lal suggests that Aryan civilization did not migrate to the subcontinent but was an original ethnic and linguistic element of pre-Vedic India. This theory would explain the dearth of physical signs of any putative Aryan conquest and is supported by the high degree of physical continuity between Harappan and Post-Harappan society.

The late 2nd millennium and the reemergence of urbanism

Toward the end of the 2nd millennium there appears to have been a further deterioration in the environment throughout the Indus system. Many of the Post-Urban settlements seem to have been abandoned, and traces are found of temporary settlements that were probably associated with nomadic pastoral groups and distinguished by the poverty of their material culture. Along the Saraswati there is further evidence of the drying up of the Derawar oasis, with a further decline in the number and size of settlements. As yet, these events are not properly dated, but they may tentatively be assigned to a period from about 1200–800 bce. In Saurashtra a similar if less extreme decline in the number of settlements is also evident. Even much farther south, in Maharashtra, the opening of the 1st millennium seems to have coincided with a period of desiccation, in which the flourishing agricultural settlements at sites such as Inamgaon declined; temporary encampments of pastoral nomads indicate a general deterioration in the standard of living.

To the north, in Punjab, Haryana, and the upper Gangetic plain, such deterioration is less apparent, perhaps because the proximity of the Himalayas produced a higher level of rainfall. It is in this area that a new tendency emerges—the expansion of settlements associated with the pottery known as Painted Gray Ware. This characteristic ceramic accompanied a spread of settlements toward the east into the upper Ganges-Yamuna valleys and constitutes a distinguishing feature of the process of development that, by the second quarter of the 1st millennium bce, gave rise to the first cities of the Ganges system. (The previous wave of urbanization appears not to have penetrated below the upper Gangetic plain.)

Another factor that coincided with, if not actually contributed to, the new process of change is the beginning and spread of iron working. The earliest dated occurrence of iron is probably that from about 1200 to 1100 bce at Pirak in the Kachchhi region. Comparably early dates are suggested at other widely scattered sites, but it probably took many years for the use of iron in almost all types of toolmaking to become common in all regions. During this period an increasingly marked contrast may be observed between the growing number of cities across the north and the relatively less-developed settlement pattern of peninsular India, where a mixture of small-scale agriculture and pastoralism coincided with the appearance of the various types of “Megalithic” graves and monuments.

Peninsular India in the aftermath of the Indus civilization (c. 2000–1000 bce)

It was stated above that the earliest known settlements in peninsular India appeared early in the 3rd millennium and showed either a mixed agricultural or strongly pastoral character. From about 2000 bce there appears to have been a general expansion of these settlements. It is sometimes suggested that this expansion may have been in some way a result of the end of the Indus civilization and that large numbers of “Harappans” migrated to the south. There is little solid evidence to support this view, and it appears rather that the development was primarily indigenous.What is particularly noteworthy is the way in which regional cultural variants occurred throughout peninsular India and often seem to be ancestral to the major cultural regions known from later historical times. In Maharashtra the excavations at Inamgaon have provided the clearest picture so far of the developments and changes that took place in one of these regions. There can be seen the variety of crops and domestic animals, the changing house types, suggestions of tribal chiefdoms, limited craft specialization, and trade. Copper and bronze artifacts, though still relatively scarce, appear alongside stone blades and axes. This mixed technology continued until the time when iron became common. Farther south, in Karnataka and Tamil Nadu, there is similar evidence, although the staple crop appears to have been millet, and wheat and barley are absent.