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Vedas are written in stages by Vedic people who were mostly pastorals settled in region of Punjab. Linguistic study coupled with geological studies of Saraswati-ghaggar basin, archaeological findings of IVC and SSC, all these things suggest that Vedic people composed Vedic literature over the span of about 1000 years. The fact that vedas are written in stages can also be proven by means of archaeoastronomy. Vedas, at different stages mention beginning of monsoon on different stars ((nakshatras). So, using that data, the probable time and interval between those two can be deduced by calculating back the data of modern planetary motion.
Roughly from 2200 BC to 1200 BC. Mandal 2 to Mandal 6 of Rigveda are considered as Family Mandals and are composed earlier than rest of the Vedic literature. Mandal 10 is the latest composition as the sanskrit in Mandal 10 is very similar to post Kapila Sanskrit and not very much similar to the one from mandal 2-6. Thus, we see a gradient of composition and changes in language.
Such philosophical inquiry, without the prejudice of GODDIDIT, shows an existence of free thinking society which in turn shows an existence of a settled life. Such questions are not asked by people who have to struggle everyday to get food. Only a society whose material needs are taken care of finds enough free time to contemplate on such subjects which otherwise are worthless. With stability and prosperity, man tends to think more rationally about supernatural and divine. Not in times of adversity.
10th Mandala of Rigveda is the most recent of all the mandalas. Hence 10th mandala is considered contemporary to time of around 1200-1300 BC. The 6 darshans were composed later, starting with Samkhya in 1000 BC and last is Advaita, dvaita and vishisitadvaita in 700's AD, 1100's and 1200's AD. Buddhism propounded in 500's BC along with Jain thought. The date of Paranjali and Yoga is not known, but given its similarity with Buddhism and long association with Samkhya, it was around 700 BC (this is my personal opinion though). Krishna yajurveda (taittiriya samhita) is considered to be composed around 1300-1100 BC. dates of Sam and atharvadeva irrelevant in this topic.
So, in that frame of reference, what they had and what we know for sure they had was first 9 mandalas of rigveda. This is the literature they had in hand and that literature does speak of gods and demi gods but does not inquire about creation. Nasadiya shows this first OR rather second Inquiry towards the concept of creation of Universe. And in this first inquiry, they put forth their agnostic views.
Interesting thing is, Vedas show the first instance of the process of peer-review. As I said, vedas were composed over time of 1000 years. So anything that was being incorporated in Vedas must have been extensively peer-reviewed by other contemporary sages. Only after a stringent peer-review, a particular sukta must have been given a place in Vedas, hence such a high reverence towards Vedas is seen. It is the large database of peer-reviewed opinions in literature of ancient India.
Today, this database has increased with lot many different and logical and proven explanations of things than Vedas. Hence, according to the Nyaya approach SHOULD include modern scientific literature as Shruti. There are opinions in shruts which are contradictory to each other, and same is in science. Both are essential because one never knows which opinion will trigger a new discovery of an idea or a philosophy in future.
Furthermore, Shrutis talk about Asambhuti and Sambhuti and Vidya and Avidya. and recommends that both should be learnt. Learning just one of them leads to a terrible downfall says Ishopanishat.So, scientific literature dealing with jagat(material world) which is associated with brahman, can be considered as shruti as well. Perhaps more stringent peer-review to be applied to avidya than to vidya, if we apply the algorithm provided to us by Nyaya system of logic.
Anthropologically, people were updating Vedas for thousand years. Afterwards they stopped their growth and the practice of updating the database and conjured up the idea of Apaurusheyatva (Divine origin, which is ludicrous, BTW). This led to people gradually getting overly ritualistic and arrogant in next 400 years and Buddha came and cleaned the mess. The world was 500 years ahead of the point where Vedas had stopped updating themselves.
Of course, the oral traditions have their weaknesses. A point comes when either you have to stop somewhere as there is limit to memorizing power of humans. Or delete the old files and make room for new ones. Thankfully, they did not delete anything. So they had to stop. Perhaps this may be one of the reasons. Of course, every civilization has to fall down, as there always is a generation OR two which spoil everything achieved by the ancestors. This is a general trend of history. It can also happen due to some adversary, natural or man-made. But there are no evidences for any natural or man-made adversaries from 1000 BC to 500BC, which were strong enough to bring about such a radical change. So I assume decadence was one of the major factors.
Vedas stopped expanding around 1000 BC. Saamkhya philosophy of Kapila originated around 1000 BC. However, by that time, society was already growing rigid and ritualistic. The mahajanapadas had sprung up and decadence was seen widely amongst Vedic people. Nothing creative is known to have happened from 800 BC to 500 BC when Buddha arrived. The arrival of Buddha marked the end of classical Vedic age and beginning of a evolution of a new school of philosophical schools which today make up the conglomerate of Hinduism.