Not only the language, but also the culture of the newly arrived elite was appropriated, including the 'Vedic Tank' the horse drawn chariot. - Michael Witzel

None of the archaeologically identified post-Harappan cultures so far found, from Cemetery H, Sarai Kala III, the early Gandhara and Gomal Grave Cultures, does make a good fit for the culture of the speakers of Vedic […] At the present moment, we can only state that linguistic and textual studies confirm the presence of an outside, Indo-Aryan speaking element, whose language and spiritual culture has definitely been introduced, along with the horse and the spoked wheel chariot, via the BMAC area into northwestern South Asia. However, much of present-day Archaeology denies that. To put it in the words of Shaffer (1999:245) ‘A diffusion or migration of a culturally complex ‘Indo-Aryan’ people into South Asia is not described by the archaeological record’ […] [But] the importation of their spiritual and material culture must be explained. So far, clear archaeological evidence has just not been found"

Theoretically, since Gartsamada Saunaka [the eponymous Rsi of Mandala II of the Rigveda] is made a Bhargava, he could be later than Book 6.

[the Sintashta-Arkaim culture on the W. Siberian plains east of the Urals,] "dated to c.2200/2100-1700/1600 BC", [where] "the earliest attested traces of Aryan material culture "and even of Aryan belief". ..[there we find] "remnants of horse sacrifices (aśvamedha) and primitive horse drawn chariots (ratha, raθa) with spoked wheels [....] a real tripura [....] adobe bricks (*išt) [....] frame houses (which reminds of Rgvedic kula 'hollow, family‘ [....]) [....] Most tellingly, perhaps, at the site of Potapovka (N. Krasnayarsk Dt., near Kybyshev on the N. Volga steppe), a unique burial has been found. It contains a human skeleton whose head has been replaced by a horse head, a human head lies near his feet, along with a bone pipe, and a cow‘s head is placed near his knees. This looks like an archaeological illustration of the Rgvedic myth of Dadhyanc, whose head was cut off by Indra and replaced by that of a horse. The bone pipe reminds, as the excavator has noted, of the RV sentence referring to the playing of pipes in Yama‘s realm, the world of the ancestors".