Extreme founder events among ancestral Europeans

 

 

Expansion of steppe pastoralists (Narasimhan et al. 2019)

 

How much of the present-day European gene pool comes from the indigenous hunter-gatherers? How much from Neolithic farmers of Anatolian origin? And how much from steppe pastoralists of the Don-Volga area? There is no easy answer.

 

 

A “founder event” results when a few people split off from their original population and found a new one. The fewer they are, the likelier they will differ genetically, on average, from the original population. The founder event is a “bottleneck” through which only a small fraction of the original genetic diversity can pass into the new population.

 

A founder event is one of three reasons why adjacent populations may differ from each other genetically. The other two are:

 

Natural selection – The boundary between two adjacent populations often corresponds to a change in the natural environment (vegetation, climate, etc.) or the cultural environment (diet, way of life, rules and prohibitions, sexual division of labor, etc.). The two populations are thus subjected to different regimes of natural selection.

 

Population admixture or replacement – One of the two adjacent populations has admixed with or been replaced by a population that has moved into the area.

 

In practice, a founder event overlaps with differences in natural selection. When pioneers move into a new area, they tend to be better suited to the local conditions than the people they left behind. The less suited are less likely to go, and if they do go they are more likely to go back … or die. So, from the outset, there already is some selection.

 

A new study of ancient DNA has shown that founder effects have been more frequent and more “extreme” than previously thought: “In humans, we find that over half of the analyzed populations have evidence for recent founder events, associated with geographic isolation, modes of sustenance, or cultural practices such as endogamy” (Tournebize et al. 2022, p. 1)

 

Contrary to popular belief, Ashkenazi Jews are not the outcome of a particularly extreme founder event:

 

Across worldwide populations, we identified 53 groups that have experienced more extreme founder events (with significantly higher founder intensity) than AJs [Ashkenazi Jews], who have high rates of recessive diseases due to their history of founder events (Tournebize et al. 2022, p. 7)

 

Perhaps those recessive diseases are not due to a founder event. Perhaps they are a side-effect of selection for an adaptive trait. If a founder effect had been the cause, those diseases would be distributed randomly over different metabolic pathways. Actually, they are all associated with excessive storage of sphingolipids, a key component of neural tissue (Cochran et al. 2006; Diamond 1994).

 

Founder events seem to have been frequent among ancestral Europeans, regardless of whether they were hunter-gatherers, farmers, or pastoralists. But those events seem to have been more “intense” among European hunter-gatherers. In other words, founder groups were smaller and spent more time passing through the population bottleneck.

 

Recent analysis has shown that present-day Europeans are a mixture of three major ancestry groups related to ancient European hunter-gatherers, Anatolian farmers, and Eurasian Steppe pastoralists. […] Across the three groups, we found that the frequency of founder events was similar, ranging between 90–100%. However, the average founder intensity was significantly higher in European hunter-gatherers […] compared to the Near Eastern farmers […] or the Steppe pastoralists. (Tournebize et al. 2022, p. 11)

 

Hunter-gatherers had more intense founder events because they had a lower population density. Founder groups were thus smaller and less representative of the original population from which they came.

 

… we found local hunter-gatherer groups had more extreme founder events than the Neolithic farmers or Bronze Age individuals. This suggests that population sizes in Europe have increased over time, coupled with changes in ancestry and transitions in lifestyle. Our results are consistent with a recent study that measured short runs of homozygosity in ancient Europeans and found a similar increase in population size during the Neolithic period. Our results are also in agreement with archeological evidence for increased population size during the Neolithic transition. (Tournebize et al. 2022, p. 14)

 

This takes us back to the previous quote: Europeans are a mix of indigenous hunter-gatherers, Anatolian farmers, and steppe pastoralists. These three groups, and their roles in European prehistory, can be summarized as follows:

 

·         Farmers began to move into Europe from present-day Turkey about 10,000 years ago. Initially, they advanced rapidly through territory inhabited by small nomadic bands.

·         About 7,000 years ago, the wave of advance stalled along a line running from the Low Countries to the Black Sea. To the north, along the North Sea and the Baltic, were large semi-sedentary communities of hunter-fisher-gatherers who could less easily be replaced because they were so numerous.

·         About a thousand years later, farming resumed its northward advance, although the advance was now much more a matter of people adopting farming rather than being replaced by farmers.

·         Meanwhile, around 5,400 years ago, some hunter-gatherers in the Don-Volga area adopted pastoralism and began to expand westward into Europe and southeastward into the Middle East, Central Asia, and South Asia. They may have been ancestral Indo-Europeans.

 

How much did each of the three groups contribute to the European gene pool? Which group contributed the most and which the least? The question is hard to answer, for three reasons:

 

Double counting

 

The hunter-gatherers of Mesolithic Europe contributed to the present European gene pool both directly and indirectly. The steppe pastoralists were themselves indigenous hunter-gatherers who had adopted pastoralism, plus an admixture of up to 18% from Anatolian farmers. As the Anatolian farmers pushed into Europe, they became gradually “Europeanized” through intermixture with local hunter-gatherers. It is also possible that the Anatolian farmers were themselves the product of an earlier expansion of European hunter-gatherers into the Middle East (Frost 2014).

 

Founder events

 

We measure population replacement by measuring the degree of genetic difference between the original group and the one that replaced it. Is that a valid method? Let’s take the replacement of hunter-gatherers by farmers, and let’s assume that all of the farmers were descended from hunter-gatherers who had adopted farming. The two groups would still be genetically different. The farmers would have been the product of a founder event—a small and genetically unrepresentative group of hunter-gatherers who had decided to take up farming.

 

Differences in natural selection

 

Population replacement is hard to measure for another reason: hunter-gatherers and farmers lived under different regimes of natural selection. They were selected for their ability to adapt to different diets, types of shelter, and means of subsistence. To go from one way of life to the other required not only cultural change but also genetic change.

 

For instance, the population frequency of haplogroup U shows a sharp break at the time boundary between late hunter-gatherers and early farmers (Bramanti et al. 2009). That break strongly suggests that the original Europeans were largely replaced by farmers spreading into Europe from the Middle East. Yet haplogroup U would persist in Denmark at high frequencies long after the transition to farming (Melchior et al. 2010). In Latvia and Ukraine it would persist into Neolithic times (Jones et al. 2017). Haplogroup U probably disappeared from the European gene pool because it ceased to be adaptive. It has been shown to shift the energy balance away from ATP synthesis and toward production of body heat, a useful adaptation if you sleep in makeshift shelters and pursue game in all kinds of weather (Balloux et al. 2009; Montiel-Sosa et al. 2006). It’s less useful if you sleep in a warmer environment and can plan your outdoor activities.

 

Conclusion

 

Whenever I make this argument, the counter-argument is that founder events and natural selection could not possibly explain all of the genetic difference we see between late hunter-gatherers and early farmers in Europe. I agree. I’m just saying that the magnitude of the demographic replacement has been overestimated.

 

References

 

Balloux F., L.J. Handley, T. Jombart, H. Liu, and A. Manica. (2009). Climate shaped the worldwide distribution of human mitochondrial DNA sequence variation. Proceedings of the Royal Society B. Biological Sciences 276: 3447-3455.

https://doi.org/10.1098/rspb.2009.0752

 

Bramanti, B., M.G. Thomas, W. Haak, M. Unterlaender, P. Jores, K. Tambets, I. Antanaitis-Jacobs, M.N. Haidle, R. Jankauskas, C.J. Kind, et al. (2009). Genetic discontinuity between local hunter-gatherers and Central Europe's first farmers. Science 326: 137-140. https://doi.org/10.1126/science.1176869

 

Cochran, G., J. Hardy, and H. Harpending. (2006). Natural history of Ashkenazi intelligence. Journal of Biosocial Science 38(5): 659-693. https://doi.org/10.1017/S0021932005027069

 

Diamond, J.M. (1994). Jewish Lysosomes. Nature 368: 291-292. https://doi.org/10.1038/368291a0

 

Frost, P. (2014). The new European phenotype: expansion into the Middle East. Evo and Proud, January 25. https://evoandproud.blogspot.com/2014/01/the-new-european-phenotype-expansion.html

 

Jones, E.R., G. Zarina, V. Moiseyev, E. Lightfoot, P.R. Nigst, A. Manica, et al. (2017). The Neolithic transition in the Baltic was not driven by admixture with early European farmers, Current Biology 27(4): 576-582.

https://doi.org/10.1016/j.cub.2016.12.060

 

Melchior, L., N. Lynnerup, H.R. Siegismund, T. Kivisild, and J. Dissing. (2010). Genetic diversity among ancient Nordic populations. PLoS One 5(7): e11898

https://doi.org/10.1371/journal.pone.0011898

 

Montiel-Sosa, F., E. Ruiz-Pesini, J.A. Enriquez, A. Marcuello, C. Diez-Sanchez, J. Montoya, D.J. Wallace, and M.J. López-Pérez, (2006). Differences of sperm motility in mitochondrial DNA haplogroup U sublineages. Gene 368: 21-27.

https://doi.org/10.1016/j.gene.2005.09.015

 

Narasimhan, V.M., N. Patterson, P. Moorjani, N. Rohland, R. Bernardos, S. Mallick, I. Lazaridis, et al. (2019). The formation of human populations in South and Central Asia. Science 6: 365(6457): eaat7487. https://doi.org/10.1126/science.aat7487  

 

Tournebize, R., G. Chu, and P. Moorjani. (2022). Reconstructing the history of founder events using genome-wide patterns of allele sharing across individuals. PLoS Genet 18(6): e1010243. https://doi.org/10.1371/journal.pgen.1010243