The Following is what I came across while reading Matsya Purana Chapter 144.
Before, There was Only Veda having four feet(parts). That Got changed on and on, due to the short lives of the people, until at last in Dvapara, the one veda was abridged and divided in four Vedas.The sons of the Risis, again due to their respective faulty understandings explained them in various ways.They inserted the Brahmana Portions within the samhita portions of the Rik,Yajuh and Sama vedas. They even changed the Svaras or musical tones in the songs of the Vedas. They did not fully grasp the meanings partly owing to their habits and faulty understandings, and partly owing to many corruptions and interpolations in the Vedas, of the Brahmana portions, of the Kalpa sutras,of the Bhasyas and various other things. Some parts were correctly explained. It is in this Dvaparayuga that persons adopted various customs and rites and began to hold different opinions.
At first the Advharyu's work was one.Afterwords, it was divided into two.Owing to distortions and twistings in the meanings, the Sastras have much transformed.Therefore, the Adhvaryu's works are performed in different ways.The Sama and Atharva vedas also were turned and twisted owing to the want of knowledge of the munis and their want of confidence. Thus the state of things in the Dvapara age was in a chaotic condition. And in the Kaliyuga the vedas became extinct.
Is All this this really true? That Brahmana,Kalpasutra etc. portions have been inserted in the Vedas by later rishis and their Svaras etc. have been changed?