Abrupt drop in Steppe in Brahmins, by the time you enter Bengal and Assam/Dooars: Geographical reasons?
This isn't about anyone's superiority or anything nor about if Steppe is good or AASI battle. Neither is good, neither is bad. It's about Geography, only.
Post the Delhi Ridge and West UP where the former pastoral lands have high steppe groups like Jats and Rors, in the native communities (Khatris are non native to Delhi), and the Brahmins of Punjab and Northwest to Bihar, the ones that came out of the Kuru kingdom, and even through MP, oscillate at 25-30% Steppe, throughout.
But a strange ridge exists beyond Bihar, as you enter Bengal, it drops to 17-20%, aka South Indian levels.
In the South of Vindhyas, the reason is the existing established communities, there, into whom the Brahmins merged. What's the reason for the drop into Bengal? Similar one, I guess? The Native Austroasiatic, Dravidian etc tribes and traders, were well established, there, I guess, when the Brahmins started moving in, or the fact that the region couldn't support a large Brahmin migration, unlike Bihar and UP, because it was a marsh? Because if not, it's UC would likely look similar to Rajasthan and MPs..
And what about Assam? Why is it's profile too, with abrupt drop? The fact that it was heavily forested?
My reason is however likely this: the Propulsion effect. Punjab, West UP and Haryana are drylands with pastoral and floodplain landscape before the rise of later irrigation and cultivation. East of Yamuna and Ganga, the land starts becoming wetter and forested and clearing them, meant too much abundance. The propulsion weakened by the time they settled Bihar, not needing to go to Assam or Greater Bengal. There were attempts by Kannauji kings, Guptas, Janapadas, etc to settle Bengal including Bangladesh, Assam, Northeast, etc, but most fizzled out, after the initial Ashwa Medha Yaagnya, because nobody were willing to move and build, especially the Aryan classes, who were happy with the feudal and trade wealth in the Ganges.