Source: TW
Population
Around the time the oldest versions of the surviving sUkta-s were composed by our ancestors, the world population was probably just around 20 million people, and the core ethnic group of the said ancestors was probably just around 100-250,000
Mandala order
Short story: White indologists swear by Oldenburg. I remain unconvinced by the counts reflecting age of the family books.
- What we do know is that vishvAmitra-s, vasiShTha-s and jamadagni-s refer to each other or share mantra-s suggesting they were contemporaneous. Thus RV3, RV7 & jamadagni mantra-s in RV9, 10 are likely contemporaneous. - MT_TW.
- atri-s, certain kAshyapa-s and certain kANva-s refer to each other they were likely contemporaneous.
- The RV9 is collection of soma sUkta-s from many families so it is obviously of composite age.
- RV1 shares authors with RV8 - so there is partial contemporaneity of those two.
- RV10 has some clearly late material and also likely early material. So it is a composite.
- All in all I think RV is a very composite text with core of family books which were composed in a small time & space window ~200 years which was then expanded by tacking on left over material in RV10 and expanding RV1. Over all the oldest RV material was IMO much earlier than when the final RV as a saMhitA was put together.