In the 1970s A. J. Ammerman and L. L. Cavalli-Sforza argued for the validity of a model of Neolithic expansion of farmers into Europe predicated on a “demic diffusion” dynamic. This is in contrast to the idea that farming spread through the diffusion of ideas, not people. The formal theory is inspired by the Fisher wave model, but empirically just imagine two populations with very different carrying capacities due to their mode of production, farmers, and hunter-gatherers. In a Malthusian framework, the farmer carrying capacity in a given area of land might be ~10× greater than that of hunter-gatherers. Starting at the same initial population, the farmers will simply breed the hunter-gatherers out of existence.
As the farmers reaching their local carrying capacity, migration outward will occur in a continuous and diffusive process. For all practical purposes, the farmers will perceive the landscape occupied by hunter-gatherers as “empty.” This is due to the fact that hunter-gatherers often engage in extensive, not intensive, exploitation of resources. In contrast, even slash and burn agriculturalists leave a much bigger ecological footprint. They swarm over the land.
The beauty of the demic diffusion process is that that it’s analytically elegant and tractable. Families or villages engaged in primary production to “fill up” a landscape through simple cultural practices which manifest on the individual scale that allow for aggregate endogenous growth. And this model underlies much of the work by Peter Bellwood in First Farmers and Colin Renfrew’s theories about the spread of Indo-European langauges. You can call it the Walder Frey theory of history.
I didn’t really think deeply about this theory because I didn’t have much empirical knowledge until I read Lawrence Keeley’s War Before Civilization. In this book, Keeley observes that the archaeological record suggests that there was violent conflict between the first farmers and hunter-gatherers in northwestern Europe, near the North Sea. He reports that there seems to have been a broad front of conflict, presumably a prehistoric “no man’s land.” Not only that, but Keeley claims that the spread of agriculture stopped for a period. The barrier between hunter-gatherer occupation and farmer territory was not permeable. Not diffusion.
As a stylized fact, the demic diffusion framework treats all farmers as interchangeable and all hunter-gatherers as interchangeable. On the face of it, we know that this is wrong. But the assumption is that to a first approximation this axiom will allow us to capture the main features of the dynamics in question. This may be a false assumption. The fact is we know that some hunting and gathering populations can engage in intensive resource extraction and remain sedentary.
The Pacific Northwest Indian tribes of the United States of America are the best-known examples of such hunting and gathering peoples. Because of the concentrated runs of salmon, these people could remain hunter-gatherers while maintaining relatively sedentary and dense societies characterized by social stratification (e.g., they practiced slavery). As it happens, it seems that it is on the maritime fringes of Northern Europe than the hunter-gatherers flourished the longest. Agriculture took ~1,000 years to transplant itself from northern Germany to southern Scandinavia, and even then hunter-gatherer lifestyles persisted in many locales for several thousand years until the Nordic Bronze Age (and in Finland even longer).
The flip side of the variation in intensity and density of hunter-gatherers is that the early farmers were probably less efficient and intensive than later agriculturalists. And, as the Anatolian farmers pushed into Northern Europe their cultural toolkit would be less and less effective. Even assuming local dynamics of reproductive increase as the primary driver for farmer expansion, the growth parameter of the agriculturalists in comparison to the hunter-gatherers may not have been that different in many contexts.
But the second major issue is that the assumption of continuous and diffusive expansion over wide areas is probably wrong. The early Neolithic farmers may have been stateless in a modern sense, but they were almost certainly not primitive anarchies. They were pre-state polities of some sort no doubt and exhibited coordination and cultural uniformity over large distances. An illustration of what might happen to small groups of farmers is what happened to white American homesteaders who occupied territory too close to the Comanche lands. Future archaeologists may see an empirical pattern of demic diffusion of white Americans from the east to the west, but that expansion occurred only within the scaffold of a political-military superstructure.
On a fundamental level demic diffusion, and the higher reproductive value over time of farmer peoples than hunter-gatherer peoples, are essential pieces of the puzzle of the peopling of Europe during the Holocene. But they need to be framed in the context of the discontinuous expansion of cultural zones of activity and freedom for farming communities, under the umbrella of some supra-village social and political order. This step by step expansion in a piecewise fashion probably explains the “hunter-gatherer resurgence” that David Reich’s lab has found in the temporal transects within a given region. Even if socially and politically dominant within a particular region, the farming communities likely targeted the richest and most suitable lands as predicted by classical economics. The hunter-gatherer populations likely persisted in more marginal areas and only assimilated with the dominant farmers over time. The invasion dynamics locally would exhibit patchiness in the early phases, allowing for hunter-gatherer persistence.
The fundamental lower-level dynamics are those of panmictic local populations expanding over time in a continuous fashion. These can be modeled by a few parameters. The problem is that the older idea that this could be generalized over time and space is surely wrong. Rather, inter-group dynamics probably govern a lot of the coarse-scale patterns we see. Over time farmer populations always won, but “on any given day” the outcome was always in doubt.
And so it was with agriculturalist conflicts as well. This is on my mind partly because I recently reread Genetic origins of the Minoans and Mycenaeans and The Beaker Phenomenon and the Genomic Transformation of Northwest Europe. There are lots of details within these papers that are easy to miss on first or second or even third read. For example, I noticed a sample dated to between 2200 and 1900 BC (so probably 2050 BC?) from Parma in northern Italy from a Bell Beaker cultural context which has a lot of steppe ancestry. Contemporaneous samples from Iberia seem spottier in their steppe ancestry, but that’s around when it shows up in that peninsula. Similarly, steppe ancestry arrived in Greece at some time after the Neolithic but before the Bronze Age collapse.
We know that the Beaker people arrived in Britain and Ireland rather suddenly ~2500 BC, even though the earliest evidence of the canonical beakers diagnostic for this culture are found in western Iberia in ~2900 BC. The Reich group concluded, rightly I suspect, that the cultural phenomenon of the Beaker people transcended a particular socio-cultural group bounded by kinship and genetic affinity. In other words, the Beaker culture was a set of peoples, in the plural.
And yet outside of Iberia and some Mediterranean locales, The Beaker Phenomenon and the Genomic Transformation of Northwest Europe makes it clear that a genetic disruption of the local demographics occurred when the society adopted the beaker. Whereas in Central and Eastern Europe Indo-European languages probably arrived with the Corded Ware people ~2900 BC, the Beaker come to our attention somewhat later, and in fact, pushed eastward into Corded Ware territory. Though the Beaker people seem to have been the vectors for steppe ancestry in many areas of Western Europe, they generally have less of it than the Corded Ware.
The Corded Ware frontier with non-Indo-European peoples to their west, south, and north, can be thought of as a cultural innovation zone. This is historically the trend, with frontier areas producing a vigorous and cohesive, yet often innovative, identity group that can mobilize resources and engages in expansion and domination. The Zhou and Chin states in China are examples of this, as is the ascendence of Roman Emperors from the trans-Danbunian region after 200 AD. It seems entirely possible then that the explosion of Indo-European Beaker people on the West-Central European frontier occurred through cultural synthesis and transmission from non-Indo-European Western Europe of the 3rd millennium, and once this society became cohesive it expanded outward aggressively.
In sum, while genetic processes are continuous and gradual, cultural processes are often discontinuous and may exhibit a phase of fluctuating change alternating stasis (perhaps modeled by a Poisson distribution of periods of expansion against the typical stationary background state?).
Addendum: The Slavic expansion in Eastern Europe and the Balkans fits with this model. Their success both demographically and culturally was due in large part to an ability to adapt to the regression of social complexity. Slavic societies were antifragile. They degraded well. In contrast, the Latin and Greek peasantry were more reliant for their existence and cultural continuity on the Roman state. With the collapse of the Balkan limes in the last quarter of the 6th century, the East Roman Empire lost total control of its Europeans interior communications, and Constantinople, Thessaloniki and the Peloponnese remained connected through maritime means through the Imperial navy’s total control of the Aegean.
And the Slavs were not an anarchic people. Though organized around small tribes, they existed under the hegemony of the Avars, and in multiple instances seem to have coalesced under the leadership of non-Slavic peoples who provided a leadership caste before these groups were culturally assimilated. Their demic diffusion through the Balkans was only enabled through the scaffold of an expansion pastoralist ascendancy in areas heretofore dominated by the Roman state.