During the Neolithic (~10000 BCE), human history witnessed a turning point: a transition from a nomadic lifestyle involving hunting and gathering to settled agriculture. The region where this happened – stretching from Egypt in the east to Iraq in the west – is known as the “Fertile Crescent”. This shift paved the way for humans to colonize all of earth’s habitable continents. Slowly, these farming communities would cross geographic corridors to Europe and Asia, and in two to three thousand years almost all areas of the Old World would have been introduced to agriculture.
But who were these first farmers? A study published in Cell this week, led by a team of geneticists from Switzerland and Germany, attempts to answer this question by tapping into ancient genomes extracted from archaeological remains discovered in Neolithic Europe and Southwest Asia. In particular, the archaeological remains consisted of 15 Neolithic individuals (13 farmers, two hunter-gatherers) from as far away as Luxembourg in the west and Iran in the east. This was complemented by ten previously published genomes (six farmers, four hunter-gatherers).
Previous studies of ancient DNA have largely argued that early European farmers and European hunter-gatherers were genetically distinct, at least in the early stages of agriculture, and did not intermix until later. According to these studies, the farmers who inhabited continental Europe ~9 kya (thousand years ago) came from the Aegean basin (mainly Greece and northern Turkey). The DNA of these early Aegean farmers bore significant similarity to those of central Anatolia (present-day Turkey) and the southern Levant (essentially present-day Jordan, Israel, and Lebanon).
Previous studies have also noted genetic similarity between the Epipalaeolithic (a transitional period between Paleolithic and Mesolithic, ~20-10 kya) and Neolithic in Turkey, suggesting little or no gene flow in terms of migrations, etc
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Speaking about the link between farmers in the Aegean and those in central Anatolia, Daniel Wegmann, one of the study’s corresponding authors, told indianexpress.com, “the theory of a common origin seems to be the most likely. But of course, other theories could also be plausible. We currently lack a high-quality genome from the southern Levant and cannot test these hypotheses to the extent that we would like.
The work of Marchi et al. (2022) reached four main conclusions. To facilitate interpretation, he identifies three metapopulations: western, central and eastern. The Western metapopulation gave rise to the European hunter-gatherer group, which are genetically different from modern Europeans. The central metapopulation gave rise to the group of early Western farmers (from Europe and present-day Turkey). The eastern metapopulation gave rise to the group consisting of the early farmers of Iran and the hunter-gatherers of the Caucasus.
The study found that European hunter-gatherers had much lower genetic diversity due to a demographic bottleneck imposed at the time of the Last Glacial Maximum (LGM). LGM was a period between 26 and 20 kya when the ice sheets were at their greatest extent. After the genetic bottleneck imposed by the last glacial maxima, European hunter-gatherers split into two subgroups around 23 kya. This departs from previous studies which maintained that low genetic diversity among European hunter-gatherers was a consequence of small population size.
Instead, Marchi et al. (2022) find that the effective population size (i.e., roughly, the number of individuals in a population that contribute to the genetic makeup of the next generation through reproduction) was actually higher among European hunter-gatherers than among the first contemporary farmers.
The group’s other major finding was that the western and eastern metapopulations had diverged from each other around 25.6 kya, long before the Neolithic. The central and eastern metapopulations had diverged around 13.6 kya. However, this date is still much later than what we had arrived at earlier. Previous studies had argued that the ancestors of European hunter-gatherers and those of early Iranian farmers at 46-77 kya. This, Marchi et al. (2022), is due to the possibility that demographic bottlenecks have been overlooked.
Later populations in northern Turkey and northern Greece diverged by about 9.1–9.3 kya, around the same time the Aegean Peninsula was settled by early Neolithic farmers. Turkic and Aegean populations show different levels of recent gene flow from western metapopulations, suggesting that their interactions with contemporary hunter-gatherers were not entirely uniform.
While at LGM the eastern and western metapopulations diverged because they were stuck in pockets not covered by ice caps, they were able to spread beyond these refugia after these retreated ice caps. The period from around 14 to 12 kya is known as the “interstadial”, when temperatures were relatively warmer compared to the periods before and after. Around 14.2 kya, central hunter-gatherer metapopulations came into contact with populations ancestral to both Caucasian hunter-gatherers and early western European farmers. Based on the information we have about the glacial extent, the study argues that these “mixings” likely took place in southeastern Turkey and the northern Levant.
It is, however, difficult to identify when and where exactly the first populations of Turkish and Central Aegean farmers differentiated, because not only did the ancestors of the early Western farmers spread further west, but there were also multiple ‘mixing events’ during the interstadium. They may have been part of the same “wave of expansion”, for early farmers in the Aegean Peninsula and central Turkey. share similar genetic signatures. It could also be that Turkic and Aegean populations had already mixed before the transition to agriculture, or that hunter-gatherer populations from the Fertile Crescent had settled in the region.
The study, as a whole, challenges the current idea that all early farmers in Europe owe their cultural and biological origins to early farmers in the Fertile Crescent. The picture is, in fact, much more complex, with early Western farmers experiencing multiple ‘mixing’ events with European hunter-gatherers and Southwest Asian farming populations. In a press release published by the University of Bern, Laurent Excoffer, one of the corresponding authors, specifies that “spatial and temporal gaps remain, and this does not imply the end of studies on the evolution of man in this field”.
The author is a researcher at the Indian Institute of Science (IISc) in Bengaluru and a freelance science communicator. He tweets at @critvik