About five months ago I read Peter Bellwood's First Farmers: The Origins of Agricultural Societies. Bellwood's thesis is simple: that the first adopters of farming entered into a period of rapid demographic expansion and by and large replaced non-farming groups. The populations which dominate the world today in this model are then the descendants of the very small set of cultures which ~10,000 years ago triggered the Neolithic Revolution. When Bellwood presented his thesis in the mid-2000s many would have dismissed it out of hand. Today I believe we have to take this model seriously. There are two primary reasons from my perspective why I am now thinking about Bellwood's thesis a great deal. First, the archaeogenetic inferences based on distributions of modern allele frequencies which suggested that the Neolithic Revolution in Europe was a matter of cultural diffusion seem far shakier. With such genetic models no longer taken for granted the recent historical, semi-historical, and ethnographic evidence, on farming transitions must be given much more weight. The case of the Bantu expansion in Africa seems to be semi-historical. The Bantu farmers themselves were not literate but their wave of advance was in historical time. Tellingly, the Bantu speaking populations of Southern Africa are genetically more similar to the Fang of Cameroon then they are to the Khoisan to their west! More well documented has been the attempts by Europeans to settle various lands overseas in their colonial adventures. They have been able to marginalize populations which did not habitually practice intensive agriculture relatively easily (note that the locus of Afrikaner settlement was initially around the Cape, where Bantu influence was minimal and Khoikhoi pastoralists were dominant). In contrast, in regions like Mesoamerica where obligate intensive agricultural civilization had deep roots there was no biological replacement, but hybridization. An argument can be made that the initial farmers did not have so many advantages over their hunter-gatherer neighbors. So the power and force of the mass agricultural way of life which bore down upon the indigenes of Australia was qualitatively different because the Europeans who arrived were the outer wave of an ancient and ruthlessly efficient civilization of farmers, honed to brutal perfection through the cauldron of inter-group competition over thousands of years. I think the best counter argument against this is the evidence of rapid sweeps of cultural forms in European in prehistory, as well as rapidity of the success of the Bantu agricultural toolkit. The new genome blogger Diogenes expresses the thesis of agricultural replacement to near maximal levels in the model which he is attempting to test with ADMIXTURE runs. Here are his propositions (formatting reedited for clarity):
1. The Neolithic Revolution was about a significant change in Humans. I happened due to selection over thousands of years in very few very special and uniquely rich and unstable environments around the world. This change influenced and was influenced mutually and gradually by technology and culture of the originally Forager people subjected to it. [agree, Razib] 2. Agricultural lifestyles allow for at least 10x higher population differences. Imagine for once this theoretical unreal situation: A land divided as a chess board into 10 squares; 5 inhabited by Neolithic people at 10ppl per square and 5 by Forager people at 1 person per square exclusively. Now imagine they mix. Total resulting population 10x5+5x1=55. Total resulting Forager contribution to gene pool: <10%. Once Neolithics dominate some regions, Foragers are a minority in their land even without wars and genocides (even though these last likely occurred). [agree, Razib] 3. Foragers don't invent or adapt to agricultural lifestyles because they do not possess such changes and can't develop them fast enough in most circumstances. They're predisposed to fight or flee instead. Their much lower densities make them prone to high-density disease they didn't evolve immunity to (but Neolithics did). [lean toward agreement, Razib] 4. Foragers thus get swamped by agriculturalists, and populations become dominated by the Neolithic Core Area population. Since mostly men migrate, in ever larger numbers, almost all Y-DNA is Neolithic; much of the mt-DNA is forager (since early women were taken from forager populations, and later women descend from these); but autossomes are overwhelmingly Neolithic too. [very slight lean toward agreement, Razib] 5. Established Neolithic populations live at the Malthusian limit. Extra food means extra surviving children eating it. [agree, Razib] 6. The only major changes in such a setting involve invasion by populations with food producing advantages. Like better seeds, tools (techs); better organization for irrigation works (culture); major genetic advantages like better digestion of food products (lactose tolerance). Advantageous alleles still diffuse, but neutral genes remain overwhelmingly local. [lean toward agreement, Razib] 7. "Invasions" and "Conquests" in such a setting are about militarized elites subjecting the peasants. Travelling is difficult and they are few compared to peasants. They live off rents collected from them, rather than join in the miserly peasant life. Since they despise peasants as serfs, they refer to themselves and their land by their minority identities. Thus we get "Roman" Tunisia; "Gothic" Ukraine; "Celtic" Anatolia, "British" Jamaica. This has no meaning as far as actual genetic constitution of the majority peasant population, but it's all contemporary authors talk about, as well as most contemporary luxury works. [lean toward agreement, Razib] 8. If subjected populations live for long enough under the alien elite, they mix with it, appropriate their prestige tags, assimilate their prestige language with their substrate one, and much of material culture too. Thus French "Latins" with Celtic substrate, Bulgarian "Slavs", Anatolian "Turks", Egyptian "Arabs", etc. Elites do make a contribution on Y-DNA, since their societies transmit prestige patriarchally. But almost no mt-DNA. And little autosomal DNA, since elite Y-DNA bearers persist but successive wives are mostly local. [lean toward agreement, Razib] 9. Slaves: slaves in settled agricultural societies do not have the impact they have in mostly unsettled Forager inhabited frontiers. Demographic success of agricultural slaves in the Americas is the exception not the rule for slave owning societies. Just like Agricultural minority success at Forager inhabited frontiers is the exception, and elite assimilation into settled populations the rule. African genes in the Americas expanded because they were able to join the early "Neolithic" gene pool there. They had high density disease immunity, agricultural knowledge, social and genetic adaptations, and the right crops versus Forager Amerindians in some regions. Slaves functioned as new rural "peasants" in untilled land from whom Barons extracted rents. [agree, Razib] In Old World regions, where large peasant populations lived at the Malthusian limit, there was no advantage bringing slaves to till the land and replace the peasants. If a Baron could move the peasants from productive land so could he make them work as hard as slaves. Slaves were useful for house work, prestige, for city and mine labour. They had very severe social disadvantages. They generally died at far higher rates and reproduced a lot less than local peasants and so had to be continuously imported. They could not make large contributions to gene pools of very dense agricultural peoples, except if they carried food producing advantages, such as crops specially adapted to local circumstances. Slaves as a rule contribute a little mt-DNA, almost none Y-DNA and residual autossome DNA in such societies. [lean toward agreement, Razib] 10. Over time, once a functioning advanced agricultural community is established, no matter how many elite invasions and conquests, or how many slaves brought; there might be small but significant changes to Y-DNA and mt-DNA respectively. But autossomes are likely to remain the same, with only residual contribution (except if food producing improvements are brought by migrants as said). At least until such times when machines till the soil, people don't live at the Malthusian limit, and they don't continue to reproduce even though food available. [very slight lean toward agreement, Razib]
All this brings me to my final point. The closest analog to the "Ancestral South Indians" (ASI), who contribute ~45% of the ancestry of modern South Asians, are the native peoples of the Andaman Islanders (the period of divergence may be on the order of 20-30,000 years B.P.). The Andaman Islanders were obligate hunter-gatherers, and have a clear difficultly adapting to agricultural life. Were the ASI then hunter-gatherers assimilated by the "Ancestral North Indian" farmers? Diogenes says no. Rather, the survival of a substantial ASI element in the South Asian population is due, in his mind, to the fact that the ASI were themselves indigenous farmers. The analogy here then may be best made with the New World where agricultural indigenous populations in Mesoamerica and the highlands of South America were able to hold their own, and amalgamate with the Old World settlers, European and African. Greg Cochran recently said something similar as well, and the issue has been gnawing at me since First Farmers. I think I agree. My confidence is the proposition is very modest...but I have a difficult time understanding why ASI hunter-gatherers managed to contribute so much to the South Asian genome when related indigenous groups in Southeast Asia were so quickly marginalized by East Eurasian groups moving from the north.