The Ancient Indian Literature consists of the four Vedas and the Upanishads. The four Vedas provides information regarding the development of early and the later Aryan civilization.
The four Vedas are Rig Veda, Sama Veda, Yajur Veda and Atharva Veda. It took a very long time to compose the four divisions of the Vedas, viz. the Samhita, the Aranyaka, the Brahmanas and the Upanishads.
The difference in the language, the philosophy and the region that was occupied by the Aryans in the Rig Vedic time and of the days of the Atharva Veda must have taken a few hundred years, if not a thousand years.
The compilation of the four Vedas and their various divisions took a very long time and with the passage of time, there were so many other changes in the concept of the people, the regions they inhabited, the importance they attached to various forces of nature and the economic and administrative growth which made all the difference for the civilization.
Historians and Sanskrit scholars agree that Rig-Veda and especially its Samhita is altogether different from the later three Vedas and their different divisions. The Sama Veda, the Yajur Veda and the Atharva Veda had a very late development and perhaps not much difference of time in their compilation.
During this period of the Rig Vedic Samhita and the later three Vedas, the centre of importance had shifted from North-West to South-East.
The Aryan civilization had enveloped during this later period the whole of the region from the Himalayas to the Vindhayas and from the North-West frontier to Bihar. One can very well imagine that during this period of expansion, newer things must have come in the political, social and religious life of the people due to the factors of time, place and the people with whom they had to put up for centuries.