Reading Strange Parallels, Southeast Asia in a Global Context, I have begun to think about the differences between the eruption of Inner Asian nomads in the early modern period, and in prehistory. The author points out that the arrival of Mughals, and even to a greater extent the Manchu, to the ancient and dense civilizations of South and East Asia did not change the cultural substrate in the main. Yes, Turco-Persian Islamic (“Islamicate”) culture became both prestigious and relatively popular in South Asia. But it was, and still is, a minority tradition set against the indigenous religious system, bracketed under the term Hindu today. In Ching China the Manchu had an even less obvious effect. Arguably they assimilated to the Neo-Confucian mores of the Han elite far more than the Mughals did in India in relation to indigenous South Asian gentry.
The dynamics in this context always need to take into account the numbers of the conquerors in relation to the conquered. The Manchus were less than 1 percent of the population of their domain. The foreign Muslim elites (Turk, Afghan, and Persian, with some Arabs in South India) and their scions were never more than a few percent, at most, of the population of South Asia. These alien elites rested atop an extractive system which predated them (in some cases by thousands of years). It was an institutional arrangement that was useful in terms of subsidizing their lifestyles. China and India were attractive to the nomadic populations beyond the frontier because they were rich with people, and therefore resources which could be deployed in consumption as well as marshaled for war (the Mughals milked India to finance wars in Afghanistan).
There are other cases which are similar in terms of numbers. Both the Magyar and Bulgar incursions into Europe seem to have resulted in an Inner Asian elite acclimating itself on top of a broad mass of peasants, from which it extracted rents. Though the Magyars imparted their name and language upon the populace of Roman Pannonia, genetically their impact has been fairly marginal, if detectable. The Bulgars, who exist only as they contributed to the appellation Bulgaria and Bulgarians, lost their language, and to my knowledge their genetic impact was even fainter.
But there are other cases. Both the Turks and Arabs seem to have more substantial genetic contributions, even if on the peripheries it was very marginal. Vast eras of Central Asia once inhabited by Persian and related Indo-European peoples has become a hybrid zone of sorts between West and East Eurasian peoples thanks to the Turkic migrations. Differences between Muslim and non-Muslim populations in the Fertile Crescent are evident.
Which brings me back to the Indo-Europeans. Even if they were not nomads of a classical sort which emerged later on in history, they seem to have been agro-pastoralists. There is now circumstantial evidence for their impact all across Europe, especially the north and east. There is also likely evidence for substantial Indo-European admixture in India. Herodotus reported 2,500 years ago that India was the most inhabited land on the face of the earth. But was it so 4,000 years ago, during the later stages of the Indus Valley civilization?
I will admit I was not primed to accept the idea of mass replacement of indigenous populations in what would later become the Ecumene by populations form the steppe because of the later record of conquest, which was more a matter of elite replacement, than social turnover. But if the genetic data is correct we need to update our models. If the first farmers of Europe were marginalized by invading Indo-Europeans, could not the same have happened to some extent to the agriculturalists of South Asia, who descended from the people of Mehrgarh? The tension between the interior and littoral in Eurasia is an old one, but it seems to have evolved over time, from one of inter-group competition and meta-population dynamics (read: extinction), to exploitation by Inner Asian steppes of the human resources of the littoral. Social complexity and institutional robustness were the best long term investments for farmer populations against nomads, who always outmatched them in individual skill, and often in terms of the tanks of the ancient world, horses.