Rise and fall, prosperity and decline are just natural phenomenon that no one can elude. Any civilization, how much prosperous could it have been, has to see a phase of decline, making space for another race or nation of civilization to rise. Indus Valley Civilization is also no different from this natural phenomenon. The prosperous Indus Valley Civilization started to decline by 1700 BC, might be due to flooding and changing course of the Indus river, drying up of Ghaggar-Hakkar rivers, many portion of the erstwhile Indus Valley turning arid and dry and some other natural calamities. The natives of Indus Valley started moving eastwards in search of greener pastures. Eventually, over the next hundreds of years, they migrated to different parts of India, thus spreading the original inhabitants of the Indus Valley to the entire Indian subcontinent. Where ever they went, they carried with them their language, which was perhaps one of the proto-Dravidian languages, belonging to the same Elamo-Dravidian family of languages spoken still in many parts of India, Pakistan. The language of Indus Valley might well be the missing link between the present day Dravidian languages and the ancient Elamite language. In most parts of northern Indian subcontinent they have eventually absorbed the subsequent Indo-Aryan languages. But they retained their native family of language predominantly in south (Tamil, Telegu, Kannada, Malayalam etc), some parts of central (tribal Gondi, Manda, Koya, Kurukh) and eastern India (Malto) and even some parts in Pakistan (Brahui). It can be said that they are indeed the major constituent of the population of Indian subcontinent even now. Over the next 3500 years several people have come into Indian subcontinent, but no one displaced or conquered the native Indian people totally. All the foreign elements or genes got absorbed into the native Indian genetic pool. But from time to time the Indian culture, religion and language got changed, sometimes drastically and sometimes in a mild ways under the impact of foreign people entering into India. The immediate foreign people in India after the decline of Indus Valley were the Indo-Aryans, who had started moving into the Indian sub-continent since 2000 BC.
The Indo-Iranians were settled in the Iranian plateau and regions north of Hindu Kush mountains since 2800 BC. They had introduced the culture of horse and chariots to the Sumerians around 2500 BC. They were the successors of the proto-Indo-European people. The Iranians and the Indo-Aryans separated from each other sometime around 2000 BC and started moving towards the east and west. The Iranians moved deep into present day Iran. Indo-Aryans subsequently played a very prominent role in the history of India and the Iranians played a key role in the Persian history. It was around 1700 BC that the east bound Indo-Aryans crossed the Hindu Kush mountains and entered into the Indian subcontinent in the Punjab in present day India and Pakistan. The west bound Indo-Aryans reached Assyria in Mesopotamia and ruled over Mittani sometime around 1500 BC. The east bound Indo-Aryans first reached Swat in northern Pakistan around 1600 BC. Remnants of these early Aryans in Indian subcontinent can be found in the numerous graves spread across the Swat valley dating back to 1500 BC. Remains of horse have been found at least in one grave. Unlike all the contemporaries, horses were a trade mark of the Indo-Iranians and Indo-Aryans. This adds credence to the fact that the Indo-Aryans did move into Indian subcontinent before 1500 BC. Eventually the Aryans did enter into some confrontations with the native people of the declining Indus Valley Civilizations. The Rig Veda, the earliest literature of the Aryans, dating back to 1500 BC, speak about several such clashes with the 'dark skinned' natives, who are none other than the people of Indus Valley, who had already started spreading across India.
In the subsequent centuries the Aryan culture, religion and language became so powerful in the northern part of India that even though the Aryans were a minority compared to the natives they overshadowed the latter in all aspects. There are references to many non-Aryan kingdoms and kings in both north and south India since the days of Rig Vedas (1500 BC).
Controversies around Aryan Migration and People of Indus Valley
There has been too much of controversy with regards to the origin and decline of the people of Indus Valley. A common view, endorsed by likes of Jawaharlal Nehru in 'Discovery of India', later acknowledged by Amartya Sen in 'The Argumentative India' and backed by various researches in archaeology and genetic anthropology in recent times, is that they belonged to the relatively dark skinned Dravidian people, the original inhabitants of the Indian subcontinent. The Rig Vedas, the earliest book written by the Aryans around 1500 BC has reference to confrontations with the dark skinned natives, who can be very well thought to be the people of the declining Indus Valley.
By 1700 BC, when the Indus Valley civilization was on decline, might be due to drying up of rivers, floods or some other natural calamity, the light skinned Indo-Aryans had already started migrating into India from Hindu Kush mountains. They started settling in many parts of the erstwhile Indus Valley. Though there is no evidence of any large scale invasion by the Aryans but it’s not unlikely that the incoming Aryans had some confrontations with the native people. The Aryans had a different physique, language and culture. Gradually they might have overshadowed the natives to a great extent. Also those were the times when the erstwhile flourishing Indus Valley Civilization was declining fast. Cultivation and trade and commerce would have also declined. The incoming Aryans might have been stronger than the people of the declining Indus Valley Civilization. It's not unlikely that the incoming Aryans, belonging to the larger Indo-European and Indo-Iranian groups of people, who had already scattered across various parts of West Asia and East Europe, would have got a good hold of the land trade routes across Asia. Thus access to prosperous trade would have made the Aryans quite strong financially. On the contrary the native people of the declining Indus Valley might have become economically quite fragile due to the breakdown of their civilization due to natural calamities. Being vulnerable in front of the stronger Aryans, the natives might have gradually migrated away from the areas occupied by the Aryans and moved towards the South and other parts of India. The natives who stayed back and confronted and finally defeated were not thrown out or enslaved by the Aryans. They were eventually accommodated in the lowest stratum of the society as Shudras, doing agricultural and other household tasks for the upper three classes or Varnas – namely Brahmanas (the teachers and priests), Kshatriayas (the rulers or warriors) and Vaishyas (the traders). But that doesn't mean that all the natives were converted to Shudras. Many non-Aryan clans and rulers eventually became very prominent and played critical roles in North India. For example the people and rulers of Magadha, Anga and Gandhara were referred in early Vedas as non-Aryans and 'despised'.
This whole theory has been much criticized for being racist and also propagating the idea that the sophisticated art, culture, religion, philosophy and literature of the Aryans is of a foreign origin rather than being indigenous. Some historians do accept the theory of Aryan migration but reject the Dravidian connection of Indus Valley Civilization on the ground that there's not much commonality between the subsequent Dravidian civilization in Southern India, which was predominantly rural, with the urbanized Indus Valley.
I don’t think the theory of Aryan migration is racist. In any other place any new and more powerful immigrant has always treated the natives very badly. In most cases they were enslaved or even executed, the most recent example being the handling of the Native Americans by the European settlers. But on the contrary the Aryans neither executed nor enslaved the natives. Off course they were not given the highest social status, but they were indeed accommodated in the Aryan society. I've mentioned earlier, that it's not unlikely that the native people might have become financially quite fragile during the period of decline of the Indus Valley Civilization and it's not irrational for the stronger Aryans to use them as labourers or workers. Similar things happen even now-a-days. The maid servant working at my house in Bangalore is a native where as I've migrated from outside. But still she has been put in a 'class' lower than me just because of the financial conditions. So accommodating the economically weaker natives in a lower class is not something unusual or vindictive. The issues of untouchability, which is a much later phenomenon in Indian society, can't be attributed completely to the Aryan class system, which was purely based on the division of labour. Though there was a gradation in the class system, but still all the four classes had their own importance in the society. There are enough references of people from the 'Shudra' class attaining very important positions in society. Mahabharata mentiones that Veda Vyasa, the person to whom is attributed the authorship of Mahabharata and compilation of the Vedas, is dark skinned and belongs to a 'Shudra' fishermen class. Even the most popular of the personalities from Mahabharata, Krishna, is raised in a family of milkmen, Yadavas, which also belong to a lower class. Even Krishna is described to be dark skinned. So it’s not very correct to connote the theory of Aryan migration as racist.
The second point of criticism is that the theory of Aryan migration (or invasion, whatever you say) tends to preach the foreign origin of the Indian or Hindu Culture. The very greatness of Indian Civilization or culture lies in the fact that it has enriched itself with the cultures from across the world. What’s the harm in accepting that the Aryans brought a very advanced form of culture and art and religion with them and enriched the Indian civilization? If we accept that the Aryans did come from outside and confront with the native Dravidians of the Indus Valley does that belittle our culture or civilization? I don’t think so. On the contrary accepting this makes our culture more tolerant and adaptive – which is perhaps the strongest feature of Indian Culture and civilization.The third point is that if the native people of Indus Valley were indeed the Dravidians, who eventually moved to South India, why didn't they create similar urban civilizations in South? It should be remembered that it took more than 1000 years for the Indus Valley Civilization to reach the rich Harappan phase and after that it survived for less than a millennium. The migration towards south would have started by 1700BC, the time when the Indus Valley Civilization started declining. From that time it took about 1000 years for the Dravidians to establish the prosperous Pandya, Chera and Chola Kingdoms in south India. Rich cities, prosperous kingdoms, flourishing trade and important ports came up in the south by 5th century BC. Those civilizations might not have been exact replicas of the Indus Valley, but then why should we assume that the same lot of people would replicate something of the past? The urbanization of the Indus Valley Civilization might have been the need of the time to remain economically prosperous. Some thousand years down the line, it's not neccessary that a similar establishment would serve the same purpose. Throughout the world the earliest forms of most of the ancient civilizations have been in the form of City States. The earliest form of civilization in Mesopotamia were the Sumerian States. The Greek and the Roman civilizations also started with City States. But with expansion of the civilization it's not feasible to have cities throughout a large kingdom or empire. The same is also true for the Indus Valley Civilization, which gradually evolved into later smaller and then bigger kingdoms both in the North and South.
Basis of theory of Aryan Migration
Recent researches in archaeology and genetic anthropology have added enough credence to the fact that there was indeed an Aryan Migration into India. But it can't be considered as an invasion, as is commonly referred to. The Aryans migrated into the Indian subcontinent and brought with them the Indo-European languages, but the Indian population always remained Indian to a great extent.
The two main Y-chromozones found predominantly in Indians are R1a1 and L1. The frequency of L1 among Indians is between 7-15%, with higher frequency in South India. South Asia is considered to be the place of origin for the L Y-chromozone. R1a is bearer of the proto-Indo-European language and people. Most of the East Europeans, Russians have high frequencies (nearly 50%) of R1a. The frequency is also very high (around 50%) among North Indians and highest among the Kashmiri Pandits (72%). But then R1a1 is also found in many tribes and people of lower castes whose chance of migration from East Europe into India as Aryans is almost zero. So it can't be concluded decisively if the high content of R1a1 among Indians, specially North Indians, can be linked with an influx of large numbers of Into-Europeans in the form of Aryans into India. It's believed that South Asia might be also a probable place of origin of the Ra1 Y-chromozone. Nevertheless, the high frequency of R1a1 among people from Kashmir, Punjab and Sind (north western part of South Asia) do suggest that there was some form of migration of the Indo-Europeans into India from the north western side. But this migration didn't change the demography of India, which remained predominantly Indian throughout. Most of the other predominant chromozone components of East Europe are not available among Indians.
A very interesting study has been done by Rosenberg in 2006. He did the cluster analysis of genetic components of a wide range of people across the globe. With cluster size of 7 he observes a very interesting behavior: there is a distinct cluster for the people of Indian subcontinent from different regions and speaking different languages- but there is not much of the East European genes in Indian cluster.
The chart below shows the 7 clusters of people. The clusters of our interest are Blue - denoting the European or the Indo-Eurpoean genetic cluster and Red - which can be seen as the genetic cluster component of the people of Indian subcontinent.