Did Agriculture in Europe Spread by Cultural Transmission or by Population Replacement?
There's a long-standing debate in the anthropology community over how agriculture spread throughout Europe. One camp proposes that agriculture spread by a cultural route, and that European hunter-gatherers simply settled down and began planting grains. The other camp suggests that European hunter-gatherers were replaced (totally or partially) by waves of agriculturalist immigrants from the Middle East that were culturally and genetically better adapted to the agricultural diet and lifestyle. These are two extreme positions, and I think almost everyone would agree at this point that the truth lies somewhere in between: modern Europeans are a mix of genetic lineages, some of which originate from the earliest Middle Eastern agriculturalists who expanded into Europe, and some of which originate from indigenous hunter-gatherer groups including a small contribution from neanderthals. We know that modern-day Europeans are not simply Paleolithic mammoth eaters who reluctantly settled down and began farming.
OK, so Europeans are a mix of early agriculturalist and local hunter-gatherer genes, including neanderthals and perhaps other non-human hominids. What fraction of the collective European genome derives from each? This is where the evidence gets contentious. Early studies indicated overall that European ancestry derives primarily from local hunter-gatherers that had been in Europe for thousands of years before the domestication of plants (1, 2, 3). However, recent studies with more sophisticated methods, and larger sample sizes of modern and ancient genomes seem to be painting a different picture. Here are a couple of quotes from a recent paper on mitochondrial DNA (4):
The observed changes over time suggest that the spread of agriculture in Europe involved the expansion of farming populations into Europe followed by the eventual assimilation of resident hunter-gatherers.This paper has been criticized by people more knowledgeable about genetic anthropology than myself (5). However, different methods, including approaches based on skull morphology rather than genetics, have yielded similar results (6). Here's a paper from 2010 on the Y chromosome (7):
The [mitochondrial DNA] data thus suggest that the pre-Neolithic populations in Europe were largely replaced by in-coming Neolithic farming groups, with a maximum [mitochondrial DNA] contribution of around 20% from pre-Neolithic hunter-gatherers.
Taken with evidence on the origins of other haplogroups, this indicates that most European Y chromosomes originate in the Neolithic expansion. This reinterpretation makes Europe a prime example of how technological and cultural change is linked with the expansion of a Y-chromosomal lineage ...The genetic contribution from Middle Eastern agriculturalists may decrease as one moves Northwest throughout Europe, and thus further from the Middle East. A brand new paper attempted to estimate the proportion of the genome that derives from incoming Neolithic farmers in different European populations (8). In the following image from the paper, the proportion of the genome derived from Neolithic farmers in different modern European populations is represented in red:
This is the authors' best guess, based on a very limited number of ancient Paleolithic and Neolithic DNA samples from archaeological sites. The picture will certainly change as more data come in. However, I think it illustrates the overall points clearly that a) modern Europeans are a genetic mix of indigenous Paleolithic and incoming Neolithic farmer populations, and b) the proportion of Neolithic genes generally decreases with increasing distance from the Middle East.
The story of human evolution is a story of population expansions that displace and assimilate surrounding populations. For example, humans expanded and replaced the non-human hominids neanderthals and denisovans in Europe and Asia (although some human populations also assimilated a portion of their genome into our own, so they aren't genetically extinct). Europeans expanded into North America and Australia, and today represent the majority of the human genetic material on those continents. Han Chinese expanded and assimilated surrounding populations in China, and continue to do so today. African Bantu expanded and assimilated surrounding cultures in a large swath of Africa. There is some evidence for similar events occurring in Native American history before the arrival of Europeans. This is due in large part to cultural and genetic adaptations that favor the expansion of certain populations. Like it or not, this is the story of human evolution, and this expansion/assimilation scenario is a plausible explanation for what happened in ancient Europe when agriculturalists arrived.
Do the Proportions Even Matter for Our Purposes?
The issue of how much modern European DNA comes from local hunter-gatherers, and how much comes from Middle Eastern agriculturalists, is still hotly debated, and I won't pretend to be an authority on the matter. It will certainly vary by population. However, I'm going to argue that for our purposes, it doesn't even matter, because the majority of modern Europeans probably carry the most important genetic adaptations to agriculture regardless of the proportion of our genome that has Middle Eastern agriculturalist ancestry. Why? Natural selection. If there has been a significant amount of early agriculturalist genetic material in the European gene pool for thousands of years, which we know at a minimum to be the case, even if that amount is relatively small, natural selection would have favored the propagation of the specific genes that increase reproductive success in an agricultural environment.
Take the example of lactase persistence, a genetic mutation that allows adults to digest the milk sugar lactose. The mutation that's most common in Europeans arose in a single individual about 7,500 years ago, shortly after the introduction of dairying, and today is present in 590 million Europeans (80 percent).
Here's a hypothetical example to illustrate the point. You have a group of 90 hunter-gatherers eating large game in Europe 5,000 years ago. 10 Middle Eastern agriculturalists who have been farming for the last 5,000 years come along, teach the hunter-gatherers how to farm, and have children with them. This newly agricultural population is now 90 percent hunter-gatherer, and 10 percent agriculturalist, genetically speaking. We know that early adopters of agriculture had serious health problems that must have exerted major selective pressures on them, favoring genetic adaptations over time. The offspring from this hybrid hunter-gatherer-agriculturalist population would be subject to the same damaging effects of the agricultural diet and lifestyle. Over time, if the agriculturalists carried any significant genetic adaptations to an agricultural diet and lifestyle, these would be favored by natural selection and increase in frequency, just like lactase persistence. Fast forward 5,000 years, and you could end up with a hypothetical population that's overall 88 percent descended from European hunter-gatherers, 12 percent descended from Middle Eastern agriculturalists, but nevertheless carries all the key genetic adaptations to an agricultural diet and lifestyle that the early agriculturalists brought along with them when they immigrated to Europe, not to mention any new ones they acquired in the meantime.
In the next post, I'll explain that this process of rapid genetic adaptation is not only plausible, it has already been convincingly demonstrated in humans.