Thursday, 29 October 2009

Are we all Middle Easterners now?

Dienekes is arguing that Middle Eastern farmers demographically replaced Europe’s original population between 8,000 and 3,000 years ago. This argument seems to be proven by two recent papers that show no genetic continuity between Europe’s late hunter-gatherers and early farmers. The continent’s current gene pool seems to owe very little to the original Upper Paleolithic and Mesolithic inhabitants. So goes his argument.

This argument raises one obvious problem. It implies that the physical characteristics of Europeans, especially northern Europeans, arose recently and over a short time.

How short? As late as 7500 years ago, hunter-fisher-gatherers still inhabited Europe above a line running from the Netherlands to the Black Sea. The line then gradually moved north, reaching northern Germany about 5500 BP and the eastern and northern agricultural areas of Scandinavia around 4300 BP. This leaves very little time for the evolution of the northern European phenotype, i.e., lightening of the skin to pinkish-white and diversification of hair and eye color into a wide range of hues. This phenotype is attested by historical records going back over two thousand years, so we’re left with a time window of less than five thousand years.

Is that enough time for so much phenotypic change? Perhaps, but the selection pressures would have to be very strong.

Let’s turn to the first of the two papers. Bramanti et al. (2009) compared mtDNA sequences from late hunter-gatherers and early farmers who had lived in northern and central Europe (Lithuania, Poland, Russia, Germany). There was no evidence of genetic continuity between the two populations.


But this paper raises several other points:

1. Modern Europeans are almost as distant genetically from the early farmers as they are from the late hunter-gatherers. To be ancestral to modern Europeans, these farmers and their descendents would need a very low female population size (less than 3,000 individuals). As the authors admit, this figure is well below current archaeological estimates (124,000 individuals).

2. The sample sizes are very small for the early farmers (25 individuals) and the late hunter-gatherers (20 individuals).

3. The sample of late hunter-gatherers covers a much longer time frame (15,400 – 4300 BP) than does the sample of early farmers (7650 - 7400 BP).

In sum, the authors have tried to describe the gene pool of late European hunter-gatherers with data from 20 individuals spread over four countries and over some 11,000 years.

Can such a sample be representative? Doubtful. Besides the smallness of the sample, the late hunter-gatherers were not a homogeneous population. By their time, Europe had completely changed ecologically. Open tundra had given way to forest and it was no longer possible to hunt wandering herds of reindeer. Hunter-gatherers now lived in smaller and more localized groups. Each group would have had its own genetic profile as a result of genetic drift and founder effects.

Even if these 20 individuals fairly represented late hunter-gatherers, the genetic continuity hypothesis is not disproved by genetic differences between them and early farmers. Undoubtedly, some hunter-gatherers adopted farming earlier than others and thus contributed more to the early farmer gene pool. Others never adopted farming and thus contributed nothing. Founder effects would have been considerable.

There are thus two serious problems with Bramanti et al. (2009):

1. The sample of late hunter-gatherers is too small and too scattered over space and time to be representative of the late hunter-gatherer gene pool;

2. The genetic continuity hypothesis does not assume that the early farmer gene pool was a representative cross-section of the late hunter-gatherer gene pool.

Let’s turn to the other paper. Malmström et al. (2009) retrieved mtDNA from 19 late hunter-gatherers and 3 early farmers who lived in southern Scandinavia. The late hunter-gatherers show no genetic continuity with the early farmers or with modern Scandinavians but they do show genetic continuity with modern Baltic populations (i.e., Latvians). This seems consistent with archaeological evidence that the eastern Baltic was a refugium for Europe’s last hunter-gatherers. Indeed, the inland boundaries of Latvia, Lithuania, and Old Prussia may hark back to a time when these people fished and sealed from coastal stations part of the year and then moved some distance inland to hunt game the rest of the year.

This study has the merit of being more narrowly focused in time and space. Like the other study, however, it suffers from very small sample sizes and the likelihood of founder effects. In fact, the early farmer sample is so small that genetic continuity with modern Scandinavians is unsure.

What now?

The challenge now will be to enlarge this sample of late hunter-gatherers. By ‘enlarge’, I don’t simply mean a larger sample. I also mean a larger number of geographic locations to be sampled. Late hunter-gatherers were a heterogeneous bunch. Some contributed a lot to the future gene pool. Others went extinct.

The ‘losers’ were small inland hunting bands with low population densities. They were less able to integrate agriculture into their nomadic way of life and also more likely to retreat in the face of much larger farming communities.

The ‘winners’ were semi-sedentary coastal groups with relatively high population densities. Because such groups depended more on fishing and sealing than on hunting and gathering, they could more readily integrate farming into their lifestyle, if only as a secondary subsistence activity. They were also more numerous and likelier to withstand encroachment by farming communities.

References

Bramanti, B., M.G. Thomas, W. Haak, M. Unterlaender, P. Jores, K. Tambets, I. Antanaitis-Jacobs, M.N. Haidle, R. Jankauskas, C.-J. Kind, F. Lueth, T. Terberger, J. Hiller, S. Matsumura, P. Forster, & J. Burger. (2009). Genetic discontinuity between local hunter-gatherers and Central Europe’s first farmers, Science, 326, 137-140

Malmström, H., M.T.P. Gilbert, M.G. Thomas, M. Brandström, J. Storå, P. Molnar, P.K. Andersen, C. Bendixen, G. Holmlund, A. Götherström, & E. Willerslev (2009). Ancient DNA Reveals Lack of Continuity between Neolithic Hunter-Gatherers and Contemporary Scandinavians, Current Biology, doi:10.1016/j.cub.2009.09.017

5 comments:

Tod said...

Hunter gatherers in Europe would have been adapted to eating a diet very low in carbohydrates

The switch to agriculture may have significantly disadvantaged most of the European 'low-carb' hunter gatherer mtDNA lines to the extent of them disappearing over thousands of years. European mtDNA that is around today may well have been inherited from far less than 5% of the ancestral hunter gatherer women. It is not terribly surprising that a sample of 20 failed to find a match with later agriculturists.

"The ‘winners’ were semi-sedentary coastal groups with relatively high population densities. Because such groups depended more on fishing and sealing than on hunting and gathering, they could more readily integrate farming into their lifestyle, if only as a secondary subsistence activity. They were also more numerous and likelier to withstand encroachment by farming communities"

Populations which were not so adapted to eating meat, ie those on on the southern periphery of the steppe-tundra hunting area, may have been the source of the mtDNA that later became predominant.

Alternatively, Dienkes' Middle Eastern farmers could have supplied the necessary agriculture adapted mtDNA. As the Middle Eastener's non-mitochondrial DNA was not being selected for in the same way it would never have achieved the same displacement of the hunter gatherer's non mitochondrial DNA.

Mitochondrial DNA haplogroups and type 2 diabetes: a study of 897 cases and 1010 controls "[...]it is shown that European mtDNA haplogroups are unlikely to play a major role in the risk of developing the disorder."

Mitochondrial haplogroup N9a confers resistance against type 2 diabetes in Asians

Women with mitochondrial haplogroup N9a are protected against metabolic syndrome

Mitochondrial DNA adaptations in living human populations (John Hawkes).

"The occurrence of potentially adaptive mtDNA mutations appears to have been quite a common event throughout human prehistory, because today's haplogroups appear to be separated by many mutations that are adaptive in different contexts."

Peter Frost said...

Tod,

The Inuit have made the transition from a low to high carbohydrate diet in two or three generations. There have been negative effects, and I don't want to minimize them, but the most serious ones seem to be due to overeating.

There seems to have been a period of a thousand years or so when the northward movement of hunter-gatherers stalled. There may have been trade between the two groups and this may have allowed some hunter-gatherers to become habituated to cereal foods.

Tod said...

I'm wasn't suggesting a mass die off over a couple of generations that left a few suvivors. Say certain mtDNA lines could produce 10% higher fitness in the new situation of high carb diet diet and epidemics of novel infectious diseases brought by incomers. Over many generations the adaptive mtDNA would supplant the others, gradually becoming the norm.

Would the Kurgan hypothesis be a viable alternative explaination?

Tod said...

Ancient DNA mutations permitted humans to adapt to colder climates.
End of the Ice Age would have led to a relatively swift transition to selection against cold adapted mitochdria which may lower fertility in men..
Otzi the Iceman’s mitochondrial DNA.
Close examination of his mitochondrial DNA showed mutations associated with low sperm mobility, so it’s possible he was infertile. The more interesting thing about his mt DNA is that it is a previously unseen variant of K1, and it’s quite possibly extinct in the modern European population.

Ice Age mtDNA may have died out over thousands of years.

Robert8 said...

What did look like these farmers ?