By By His Divine Grace A. C. Bhaktivedanta Swami Prabhupada
Similarly, previously the Vedic knowledge was coming by hearing. There was no need of books. But when this age, Kali-yuga, began five thousand years ago, the Vedic scriptures were recorded. First there was only one Veda, known as Atharva Veda. Then Vyasadeva, just to make it clear, divided it into four and entrusted his various disciples to each take charge of one school of Veda. Then he wrote the Mahabharata and the Puranas to make the Vedic knowledge understandable by the common man in different ways, but the principle is the same.