Poets such as Śamyu Bārhapatasya 6.45.1, some early Kaṇvas (in book 8) [belong to the] Early Ṛgvedic level. - Michael Witzel

"The structure of the text has been more extensively studied, already by Bergaigne (1878-83) and Oldenberg in the 19th century. From the latter's Prolegomena (Oldenberg 1888), it appears that the Ṛgveda was composed and assembled in the following stages, beginning 'at the centre' with books 2-7" (WITZEL 1995b:309).

The so-called ‘invasion’ of IA speakers is not (yet) visible in the archaeology.

North-West India was a large "colonial" area, where the Indo-Iranian or early Vedic immigrant clans and tribes (including their poets) were struggling with each other and with more numerous local populations of non-Aryan descent which belonged to the post-Indus civilizations (c. 1900 B.C. and later).... The immigrating group(s) may have been relatively small one(s), such as Normans who came to England in 1066 and who nearly turned England into French speaking country- while they originally had been Scandinavians, speaking N. Germanic. This may supply a model for the Indo-Aryan immigration as well...…..However, the introduction of the horse and especially of the horse-drawn chariot was a powerful weapon in the hands of the Indo-Aryans. It must have helped to secure military and political dominance even if some of the local elite were indeed quick to introduce the new cattle-based economy and the weapon, the horse drawn chariot, - just as the Near Eastern peoples did on a much larger and planned scale. If they had resided and intermarried with the local population of the northern borderlands of Iran (the so called Bactro-Margiana Archaeological complex) for some centuries, the immigrating Indo-Aryan clans and tribes may originally have looked like Bactrians, Afghanis or Kashmiris, and must have been racially submerged quickly in the population of the Punjab, just like later immigrants whose staging area was in Bactria as well: the Saka, Kusana, Huns, etc……