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Wednesday, September 02, 2009

The Origins of Lactase Persistence in Europe

Most of the world's population is lactose intolerant, with the latest estimates that a minimum of 60% are. Being lactose intolerant means that you are born with a gene that turns off the production of the lactase enzyme that digests the milk sugar lactose at some point in your life.

Most Northern Europeans and their descendants, which means most Americans, are not lactose intolerant. That makes them a unique population, given the size and numbers and uniformity of the ability to drink milk without symptoms as an adult.

How could this possibly come about? The origins of drinking milk had to come after milkable animals were domesticated in large numbers. That appears to have happened for the first time in the Middle East around 10,000 years ago. That's a very short time in human history. We do know that the mutation that never sends out the signal to stop producing lactose is one that keeps cropping up - in fact, scientists have found 43 separate versions of the mutation - so it must be fairly common. Even so, getting a chance mutation, even a dominant mutation as this one is, to spread completely throughout a population is extremely unusual. No other mutation like lactose tolerance is known today.

Exactly when and where the mutation took hold in a population that would spread it across Europe has been fiercely debated for years. In Milk Mutation "Strongest Signal of Selection", for instance, I reported that:

Tishkoff's team determined the date range when the mutation likely occurred: 3,000 to 7,000 years ago, which matches up well with the archaeological record that places pastoralization coming to East Africa about 5,000 years ago. The European trait dates back about 9,000 years.

A new computer model developed by a group of UK scientists make the time and place much more specific:
We infer that lactase persistence/dairying coevolution began around 7,500 years ago between the central Balkans and central Europe, probably among people of the Linearbandkeramik culture.

Oh, c'mon, everybody knows the Linearbandkeramik culture. They were the first farmers in Europe.
The Linearbandkeramik Culture (also called Bandkeramik or Linear Pottery Ceramic Culture or simply abbreviated LBK) is what German archaeologist F. Klopfleisch called the first true farming communities in central Europe, dated between about 5400 and 4900 BC. Thus, LBK is considered the first Neolithic culture in the European continent.

The word Linearbandkeramik refers to the distinctive banded decoration found on pottery vessels on sites spread throughout central Europe, from south-western Ukraine and Moldova in the east to the Paris Basin in the west. The LBK people are considered the importers of agricultural products and methods, moving the first domesticated animals and plants from the Near East and Central Asia into Europe.

If lactose tolerance is associated with milking and milking is associated with farmers, then the first farmers in Europe are a logical place to look for a beginning. It all seems to come together.

When I was doing the research for Milk Is Not for Every Body, I had to struggle through dozens of articles from medical and archaeological journals, but for sheer total unreadability nothing compared to articles on the math of population genetics. So I was pleasantly surprised to see that the introduction to this particular article from PLoS Computational Biology is written in more or less accessible English. Here goes.

Itan Y, Powell A, Beaumont MA, Burger J, Thomas MG, 2009 The Origins of Lactase Persistence in Europe. PLoS Comput Biol 5(8): e1000491. doi:10.1371/journal.pcbi.1000491

Lactase persistence (LP) is common among people of European ancestry, but with the exception of some African, Middle Eastern and southern Asian groups, is rare or absent elsewhere in the world. Lactase gene haplotype conservation around a polymorphism strongly associated with LP in Europeans (1−13,910 C/T) indicates that the derived allele is recent in origin and has been subject to strong positive selection. Furthermore, ancient DNA work has shown that the −13,910*T (derived) allele was very rare or absent in early Neolithic central Europeans. It is unlikely that LP would provide a selective advantage without a supply of fresh milk, and this has lead to a gene-culture coevolutionary model where lactase persistence is only favoured in cultures practicing dairying, and dairying is more favoured in lactase persistent populations. We have developed a flexible demic computer simulation model to explore the spread of lactase persistence, dairying, other subsistence practices and unlinked genetic markers in Europe and western Asia's geographic space. Using data on −13,910*T allele frequency and farming arrival dates across Europe, and approximate Bayesian computation to estimate parameters of interest, we infer that the −13,910*T allele first underwent selection among dairying farmers around 7,500 years ago in a region between the central Balkans and central Europe, possibly in association with the dissemination of the Neolithic Linearbandkeramik culture over Central Europe. Furthermore, our results suggest that natural selection favouring a lactase persistence allele was not higher in northern latitudes through an increased requirement for dietary vitamin D. Our results provide a coherent and spatially explicit picture of the coevolution of lactase persistence and dairying in Europe.

Author Summary

Most adults worldwide do not produce the enzyme lactase and so are unable to digest the milk sugar lactose. However, most people in Europe and many from other populations continue to produce lactase throughout their life (lactase persistence). In Europe, a single genetic variant, −13,910*T, is strongly associated with lactase persistence and appears to have been favoured by natural selection in the last 10,000 years. Since adult consumption of fresh milk was only possible after the domestication of animals, it is likely that lactase persistence coevolved with the cultural practice of dairying, although it is not known when lactase persistence first arose in Europe or what factors drove its rapid spread. To address these questions, we have developed a simulation model of the spread of lactase persistence, dairying, and farmers in Europe, and have integrated genetic and archaeological data using newly developed statistical approaches. We infer that lactase persistence/dairying coevolution began around 7,500 years ago between the central Balkans and central Europe, probably among people of the Linearbandkeramik culture. We also find that lactase persistence was not more favoured in northern latitudes through an increased requirement for dietary vitamin D. Our results illustrate the possibility of integrating genetic and archaeological data to address important questions on human evolution.

One final technical point. The evolution of "white" skin has usually been explained by the need for northerners to get more vitamin D from the sun in the relatively limited time available. "Black" skin reduces the uv absorbed and vitamin D made, but this isn't a problem in the tropics. Calcium sources were also more limited in the north, making milk a dietary necessity and vitamin D helps calcium absorption. So why isn't there the expected correlation between the people who were drinking more milk and therefore getting more calcium which therefore required more vitamin D? Two reasons. One is that the model found that the coevolution of dairying explained enough so that an additional explanation wasn't needed. The second was that this particular effect wasn't included in the model in the first place.

That's the problem with computer models. They are excellent at checking whether the data meets expectations. Putting every possible variation and explanation into a model exceeds what our computers can do at this point.

The whole article is a fascinating read and a good summary of much of what is known or suspected today on the spread of dairying and lactose tolerance. It's not as difficult a read as most, at least until the equations start taking over.

The history of lactose tolerance is the history of western civilization in many ways. Northern Europeans couldn't have survived the north without milk and dairying. Their aggressive societies took over the rest of the world, taking their milk-based culture with them. Most Americans grew up thinking that dairy was the norm and have been surprising to find themselves a minority in the world. This is history well worth the study and the time and difficulty in tackling it. It's the story of us, tolerant and intolerant alike.

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1 comment:

AccessDNA said...

Here is some additional information about the "genetics" of this condition that was written by our Genetic Counselor and other genetic professionals: I hope it helps. Thanks, AccessDNA