Various theories regarding the decline of the Indus civilization are given below:
Theories
1. Aryan Invasion Theory:
Generation of Archaeologists have come out from the schooling of an Aryan invasion theory’ to explain the decline of Indus civilization. It was believed that the ‘Aryans’ coming by way of Afghanistan invaded north-west India and overthrew the walled cities. Based on several Indian literary sources this event was believed to have taken place around 1500 BC.
Indirect evidences of sorts found in the two sites at that time (Mohenjo-Daro and Harappa) were linked with incidents and characters described in the Rig-Veda. Thus, Indra who is described as the destroyer of forts (Purandara) is shown as having destroyed the fortified Harappan cities in order to earn that name.
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Layers of ash and evidence of fire were likened to the description of Indras destroying Dravidian castles by putting them ablaze.
If by the term ‘Aryan’ one were to understand the culture which has harnessed horses to chariots and those who spoke Indo-European language besides having fire worship, etc. we can scarcely hope to demonstrate these characters from archaeological ruins, (with the exception of few claims made recently).
It is true that the earliest reference of war chariots is known from Samsi-Adad (1800 BC) in north Syria but, for the harappan sites, we are yet to demonstrate the presence of horse conclusively.
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In the Boghaz keui tablets dated to 1380 BC referring to a war treaty of the Hittites one can see the mention of such Rig-Veda gods’ names as Mitra Varuna and Indra. But again this is nearly 400 years later than the date of the end of Indus Civilization.
Under the circumstances as above an “Aryan Invasion” as a direct causative force for the decline would not seem very tenable. This would mean that the arrival of the Indo-European speakers and the cause of the decline of Indus Valley have to be treated as mutually exclusive phenomena separated by time.
2. Foreign Invasion Theory:
Wheeler (1947), Piggott (1950) and Gordon (1958) and have separately supported a foreign invasion theory to explain the rather abrupt decline of Indus culture. The invasion model is sought to be supported by the fact that the defence wall was periodically reinforced at the urban sites and also the ancient texts, particularly (Rig-Veda) mentions of similar invasions and conquering of cities and towns by the Indo-Europeans. Bronze weapons and other kinds of Indo-European associated traditional weapons have been found on the surface of Urban Harappan levels.
The invasion theory seeks support from these archaeological evidences. It was difficult to support this theory primarily because almost all scholars agree that a decline starts much before the accepted date of Rig-Veda.
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Waves of nomads and mauroders must have been coming from central Asia periodically and a defence against them was done by the walls. However, the supporters of invasion theory have their own body of evidences.
Wheeler, for instance, has referred to the discovery of skeletons form the topmost level at Mohenjo-Daro with signs of violent massacre, similar evidence have been found in HR area where a group of 13 skeletons of males, females and a child were found in a state which suggests their killing one after another.
Dales and Raikes argue that not all the skeletons at Mohenjo-Daro belong to the final phase. There are no signs of burning and destruction and there were no skeletons found in the citadel area which was the main center of power. In his later writings, Wheeler became less emphatic about this theory and today it is more or less discarded.
3. Climatic Change:
In early 1950s, both Wheeler and Piggott also supported this theory. As early as 1930s Marshall and Aurel Stein had opined that the climate of this region during the Harappan period was better than today and it was the slowly increasing aridity which caused a failure of the economy and hence the civilization.
They collected several archaeological evidences to support their theory but these were not conclusive because these evidences could be explained with alternative causes as well. Recently, Gurdeep Singh studied the palynological spectrum of the Rajasthan salt lakes and opined that a causal relationship exists between increased rain fall and the development and reduced rainfall and decline in the Harappan civilization. Singh could not get a large support from the archaeologists.
4. Tactonic Phenomena Theory:
This theory is basically derived to explain the water deposited layers encountered at Mohenjo-Daro city area. Amri and Chanhudaro also produced similar evidence. It was argued that the river Indus was flooded suddenly because of tectonic phenomena. But Kalibangan or for that matter the Saurashtra sites show no such evidence.
In 1952, M. R. Sahani, a geologist and paleontologist studied the silty deposits of Indus plain and suggested that flood in this region was not a case of mere overflow of the river but was probably an event more than that.
Thus, the tectonic theory was once again saught to be substantiated. This theory was more elaborated by Raikes (1964, 1975) and Dales (1966). They carried detailed investigation of the river bank. At places Raikes record silty deposits as high as 30 feet above ground.
According to him the deposits are of still water origin and that these conditions were caused by inundation of the Indus water by uplift. Thus, the dam and lake hypothesis of M.R. Sahni was substantiated. Dales, during his investigation observed that the early Harappan sea ports were now as far as 30 miles inland suggesting thereby that the coastline in this part of Pakistan has risen greatly within the last 4000 years.
Lambrick and Possehl (1967) have rejected the theory. According to them still water origin of the silts could not be conclusively demonstrated and also evidence of tectonic up lift of the river in the geomorphology of the Indus basin is not demonstrable.
5. Hydrological Change Theory:
For the abandonment of Mohenjo-Daro Lambrick (1967) suggested that the east ward shift of the river Indus caused the periodic inundation of arable land in and around the city finally leading to loss of agricultural product. This theory could further argue that the focus of Indus civilization was the new channels of Ghaggar-Hakra.
In this regard the recent analysis of this issue by prof. V. N. Misra will appear to be extremely appealing. In the recent years palaeochannel studies have demonstrated that several Himalayan flows used to discourage huge quantity of water into the Ghaggar, which in turn used to flow into the Indus through the Sutlej.
These feeder channels of Ghaggar have been demonstrated to have changed progressively towards the south east in the past. It is true that we have no dates for these events, but we have evidence that the main feeder channel had changed its earlier course and shifted to Chautang.
This did finally meet the Ghaggar but at a far more easterly shifted spot near Suratgarh. The final and the last shift completely disconnected the feeder channel from the Ghaggar system. It is believed to have now joined the Ganga system at its northerly hilly slopes. This resulted into the complete drying up of the Ghaggar and deprived the Sutlej and hence the Indus of a large amount of its water contribution.
Misra argues brilliantly to show how this might have caused gradual silting of the Indus and thus could, in due course of time bring about a crash in the economic surplus which had maintained the management of labour, craft and trade operations. That such an event was not sudden is almost certain.
A gradual migration of the artisans and tradesmen towards Saurashtra and Haryana must have started as early as 1900 BC. Even at the original Indus Valley towns we see the emergence of an altogether changed feature of the Jhukar and Jhangar culture. There is quite a possibility that this phase represents the first shift of the Ghaggar feeder to Chautang.
People with trade connections had always maintained human contact with the Indus Valley and also various principalities further west which in turn may had contact with the Sumerians first and then the Babylonians. The Indo-European speakers finally entered the Indus plain during these 200 years of slow degeneration of the Indus economy.
They might have come in several waves and brought their culture but their being the cause of destroying Indus culture does not seem attested by either the archaeological evidences or by the numerous revised radio-carbon dates for the various stages of the culture now available.