History of the Vedas
The Bhāgavatam clearly states (1.4.14) that the Vedas as we know them are not ahistorical. It says that Vyāsa’s efforts to organize the Veda into its current form began in the final third of Age Three, just as Age Two was dawning.
Translating this into years is complicated because there are many types of “ages.” Ṛg Veda’s Vedāṁga Jyotiṣa, for example, defines a five-year age. Manu Smṛti and some sections of the Purāṇas define four ages as multiples of 1,000 years. While other places in the Purāṇas, and the Surya Siddhānta, define four ages as multiples of 360,000 years. It seems that the duration of an “age” is relative to the context. The five-year age is used in calendric contexts. 1,000 year ages are used in historical context. 360,000 year ages are used in astronomical context. It appears that the correct definition to use in this case is the historical age.
Ages are numbered in reference to their multiple, which is the reverse of their numeric order, and can therefore be confusing: “Age One” is the “Fourth Age” in succession.
Each age has three parts. The main part of an age lasts for its ordinal (1-4) multiplied by 1,000 years, or 360,000 if the context is astronomical. The two other parts are the “dawn” and “dusk” transitions, each of which lasts 10% as long as the main part.
Scholars and scientists know with significant confidence that Age One began very near 3,100 BCE (and it seems that the historical and astrological ages were synchronous at this point). The age before it, “Age Two,” lasts for 2,400 years. Vyāsa’s efforts began in the age before that, “Age Three,” but in the “third part” of that age, the “dusk,” which for Age Three is 300 years long. Bhāgavatam, therefore, states that Vyāsa began his work roughly 2,700 years prior his conceiving the Bhāgavatam – which we reasonably believe to coincide with the beginning of Age One at about 3,100 BCE. This means that the history of the Vedas as we know them begins about 7,700 years ago, somewhere between 5,800 – 5,500 BCE.
From that date, over a period spanning many generations (ŚB 1.4.23), Vyāsa oversaw the evolution of the vast Vedic library. Towards the end of this process he decided to write Mahābhārata. After this, still unsatisfied after 2,700 years of work, Vyāsa conceived of the seed of inspiration to write the Beautiful Tales of the All-Attractive (Śrīmad Bhāgavatam). This was at the very end of the Age Two, just on the dawn of Age One.
The modern rational mind raises several questions, among which are, “How can Vyāsa have lived for 2,700 years?” and “How could there have been well developed human culture 7,700 years ago.” The second question is perhaps not as hard to reconcile, considering that the currently accepted archeological model permits human culture to have begun about 10,000 years ago, and there are several plausible challenges to this model that permit sophisticated civilization much before even that. As for the first question, however, I can reply in two ways: (1), the “religious” way: Vyāsa was an incarnation of God, and had a superhuman lifespan. (2), the “scholastic” way: Vyāsa was the founder of a school, and successors and students took his name as a title and attributed their works to him.
Our current copy of the oldest Veda, Ṛg, contains astronomical information that dates it in the vicinity of 5,000 years ago. This is about two and a half thousand years short of when it was first conceived by Vyāsa. The astronomical information found in our current version of Bhāgavatam dates it in the vicinity of 300 AD, about three thousand years short of when it was conceived by Vyāsa and first given form by Śuka. We can therefore accept that (a) there can be a great deal of time between the original concept and the final version; and (b) there are revisions made over time.