How Big Was The Yamna Demographic Contribution?

This isn't really anything new, but Razib Khan neatly sums up the magnitude of the Yamna demographic contribution to the ancestry of modern populations (which was the demic component of the expansion of Indo-European languages) in one post.

The best estimates seem to be that ancestry from the steppe is somewhat more than half the total in northern and eastern Europe, and somewhat less than half in southern and western Europe (i.e., northeast to southwest cline). 

More recently, it also seems that a substantial, though a smaller, proportion of the ancestry in southern Asia also derives from the steppe peoples. Within India itself, the range seems to be from 25-30% among some groups, such as North Indian Brahmins and Jatts, to a more typical range between 5 and 15% (peasant castes in South India are closer to the former, peasant castes in the Gangetic plain are closer to the latter). 

Using the proportions in various ethnic groups in the Indian subcontinent, as well as across European nations, I have come to conclude that around ~10% of the ancestry in the world derives from people who were members of the “Yamna Horizon” ~3000 BC. I don’t know the archaeology well enough to be highly informed, but I’m willing to bet that closer to 1% of the world’s population lived in and around the Yamna Horizon, so over the last 5,000 years, you’ve seen a 10-fold increase in representation of this ancestral component. More concretely, I think that the vast majority of the increase occurred between 2500 BC (when expansions into Britain and Southern Europe seem to have occurred) and 1000 BC (when the core area of the Indian subcontinent was Aryanized).

Generally, I'm in full agreement, but I think that he estimate that "1% of the world’s population lived in and around the Yamna Horizon" ca. 3000 BCE is far too high.

Why Did This Culture Explode In Demographic Impact?

The Yamna expansion in Europe was driven by the domestication of the horse, the practical utilization of the wheel, wide use of copper and bronze tools, and climate events that lead existing civilizations to collapse in the early the Bronze Age to which the steppe people were better adapted than Europe's first farmers. Excessive depletion of soils by the first farmers due to non-sustainable farming practices may have also played a part in the collapse of Neolithic farming in Europe that left the door open for Indo-European expansions. Lactase persistence in Northwestern European Indo-Europeans may have also played a part, although that role is unclear.

Similar, although not quite identical factors, fueled Indo-European expansion into first Central Asia, and then South Asia. 

On balance, South Asia saw more cultural fusion with Indo-European culture, while Europe saw more cultural replacement (particularly in Corded Ware culture regions which have the most Yamna ancestry). The differing percentages of Indo-European admixture between South Asian and Europe are consistent with these observations.

Who Else Contributed To Modern Ancestry?  

The Balance Of European Ancestry And Distinct Migration Waves

We also know that both the Anatolian first farmer Neolithic revolution wave in Europe, which accounts for much of the balance of European ancestry, and the subsequent wave of European steppe ancestry, each expanded in two main waves: the LBK (linear pottery culture) and Corded Ware cultures respectively in the east and north, and the CP (Cardial Pottery) culture and Bell Beaker culture in the south and west. 

Mesolithic European hunter-gathers with some geographic differentiation, and Neanderthal admixture (from all contributing waves dating to the Upper Paleolithic era), make up the balance of European ancestry. Uralic ancestry is also present in Eastern Europe. There are minor components of ancestry from elsewhere (e.g. Jews and Gypsies) in historic times, apart from modern post-colonial era immigration to Europe from all over the world.

I had once hypothesized that the two Yamna waves had separate language families, Corded Ware that was Indo-European, and Bell Beaker, that was closer to Basque (a.k.a. Vasconic). The available evidence still can't rule out this possibility and it shouldn't be thoughtlessly assumed.

But, I now think that Basque and other extinct non-Indo-European languages in Europe (that are not Uralic languages, whose origins in Europe are well understood) are more likely to be Neolithic relict languages that were adopted by Bell Beaker migrants in some places due to the particular circumstances of their arrival, rather than being one of two European steppe language families.

The Balance of South Asian Ancestry

In South Asia, the main non-Yamna ancestry components are (1) the Dravidian component (a South Indian Neolithic component arising ca. 2500 BCE that was derived in turn, almost entirely, from autochthonous South Asian hunter-gatherers and differentiated from that hunter-gatherer source only modestly prior to Indo-Aryan expansion), (2) a Harappan component derived from the Neolithic Indus River Valley civilization (ca. 6,000 BCE) and its pre-Indo-Aryan successors, (3) the Munda component derived from immigrating Southeast Asian rice farmers dating from only a few centuries before Indo-Aryan expansion (ca. 2000 BCE), and (4) the comparatively late arriving Tibeto-Burman component (derive from the highlands of Tibet, Burma and western China). Some isolated populations (mostly the Zoroastrian Parsis population) are migrants in historical times from Persia. All of these populations, moreover, have Neanderthal admixture to a greater or less degree, dating to the Upper Paleolithic era.

I still think that the likelihood of cultural and linguistic influences from the Sahel of Africa, where many of the core crops of the South Indian Neolithic were domesticated, is likely, despite the lack of a strong population genetic signal of that impact (with the possible exception of Y-DNA T in mostly Dravidian parts of East Central India that I lack the data to investigate more carefully). 

Also, I continue to think that it extremely likely that the Dravidian languages do not have any common linguistic origins with Harappan, although there may have been some loan words or other areal influences. I also think that it is very likely that the apparent lack of time depth in the Dravidian languages probably arises from a post-Indo-Aryan expansion contraction that eliminated most of its dialects and then was mostly reversed in a reconquest from the remaining Dravidian core that held out from Indo-Aryan expansion.