From about 2000 B.C. there was a movement of Mongoloid populations from the north to India through Assam and these people along with others who migrated from northern Burma formed from the remote past the bulk of the population of Assam. In ancient Sanskrit texts, such as the two great epics, the Mahabharata and the Ramayana and also in the Puranas, the inhabitants of Assam are referred to as Mlecchas, Kiratas and Cinas (Chinese), in other words, as non-Aryan barbarians.
There were apparently wave after wave of these migrations and the invaders belonged to the Indo-Chinese linguistic family, of which the two most important sub-families are the Mon-Khmer and the Tibeto-Burman. The third, Siamese-Chinese, includes Shan, which was spoken by the Ahoms, the last of these invaders. The Mon-Khmer speakers appear to have come earlier than the others. They were apparently driven by subsequent Tibeto-Burman hordes into the Khasi Hills, which is the only part of Assam in which the sub-family now exists. Of the Tibeto-Burman sub-family, there were three groups viz. Naga, Kuki-Chin and Bodo. The Naga and Kuki speakers were driven to the hills and Bodo became the dominant language. It includes all the surviving non-Aryan languages
of the plains, the Garo Hills and the North Cachar Hills. Kachari, Mech, Garo, Dimasa, Tipra, Lalung, Rabha and Chutiya are derivations of Bodo.
About the time the Mongolian hordes were entering Assam through the north-east, the Aryans started spreading over the whole of northern India across the Gangetic plain, driving the indigenous inhabitants to the south. Eventually, some of the Aryans reached Assam in their eastward movement through Bihar and north Bengal. Judging, however, by physiognomy, the strain of Aryan blood in Assam seems very thin. Assamese of pure Aryan stock can perhaps be found only amongst the descendants of Brahmin scholars who were invited by the non-Aryan kings to their courts for their learning and advanced culture. Some others of high and low castes will have also accompanied these Brahmins. For instance the Kalitas (Kayasthas) of Assam are of distinctly Aryan appearance and are supposed by Sir Edward Gait to be "the descendants of the first Aryan immigrants by women of the country".
A very significant fact about Assam is the fusion which took place here of Aryan and Mongolian cultures. We find here an example of how a dominant culture, although supported numerically by few, can absorb and impose itself on weaker cultures. Thus we find in Assam, a people by and large Mongolian, speaking at the present day, Assamese, which is an Aryan Sanskritic language. This happened not merely because of the superiority of an Aryan language over the rude tribal dialects but also because of the absorbing power and influence of Hinduism. The Brahmins from Northern India succeeded in absorbing into Hinduism all the tribal people of the plains including the powerful Ahoms who ruled over Assam for over six hundred years and who in course of time not only adopted the Hindu religion, discarding their own, but also forgot their own Ahom language and adopted Assamese. Ahom is now a dead Language known only perhaps to about half-a-dozen deodhais (Ahom priests). The same thing happened to all the languages of the Bodo group. The complete disappearance of such tongues like Rabha and Kachari, which are still spoken in the interior by old men and the womenfolk, is only a matter of time.
From these facts, it would appear that the bulk of the population of Assam is tribal or at least of tribal origin. Most of these tribal communities through assimilation have lost their tribal characteristics, and similar assimilation of such communities as are called plains tribals is continuing. People who can still be regarded distinctly as tribals can be found now only in the hills.
The hill-tribals of Assam live in the districts of Garo Hills, United Khasi and Jaintia Hills, United Mikir and North Cachar Hills and the Mizo Hills. The North Cachar Hills are inhabited by several tribes such as the Dimasas, the Kukis and the Zemi Nagas.