“in contrast to its close relatives in Iran (Avestan, Old Persian), Vedic Sanskrit is already an Indian language”. (p 108) - Michael Witzel

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

By contrast, the Pürus, who along with the Bharatas appeared on the scene later, began to use the designation “Five Peoples” immediately: as dis­cussed above, they probably regarded themselves as being located at the centre. In the later books the tribes mentioned include both the older “Five Peoples” as well as the newcomers, namely the Pürus and the Bharatas.

Nevertheless, in order to bolster his claim for the antiquity of the "Vedic horse (as) a native Indian breed", he connects this dead horse with the Rigvedic one, which is described as having 34 ribs (Rigveda 1.162.18). But, while horses (Equus caballus) generally have 18 ribs on each side, this can individually vary with 17 on just one or on both sides. This is not a genetically inherited trait. ... As for the number 34, numeral symbolism may play a role in this Rigveda passage dealing with a horse sacrificed for the gods. The number of gods in the Rigveda is 33 or 33+1, which obviously corresponds to the 34 ribs of the horse, that in turn is speculatively brought into connection with all the gods, many of whom are mentioned by name (Rigveda 1.162-3). But this is mere philology, not worthy of "scientific" study...