The Loom

My ancestry forms a smear across northern and central Europe, a region of the world where many people have a peculiar gift: they can drink milk as adults. Almost all people can digest milk sugar (lactose) as babies, but in many parts of the world they lose this ability after they stop nursing. The change is due to an enzyme called lactase, which breaks down lactose into digestible fragments. Most people stop making lactase as they grow up. If they drink milk, the lactose builds up in their guts, where it can be devoured by microbes that produce gas and other discomforts. (It's not so unpleasant for such people to eat cheese or yogurt that's low in lactose or which contains bacteria that make their own lactase.) You may know this condition as lactose intolerance. Geneticists, perhaps preferring to look on the bright side, like to talk about lactase persistence.

Lactase persistence has an odd geography. In a few parts of the world, it is relatively common, and in others it is rare or altogether missing. The borders between these regions can be sharp. In northern and eastern Europe, many people have lactase persistence, but in southern Europe and the Near East few do. In Africa, some tribes living close to each other are dramatically different. Most of the Tutsi of Rwanda and Burundi are lactase persistent. Most of the Hutu in the same small part of the world are not.

In recent years, scientists have learned a lot about how milk digestion is encoded in the human genome. The lactase gene, LCT, is controlled by neighboring segments of DNA that act like on-off switches. Certain proteins grab onto certain switches, allowing genes to make proteins only under certain conditions. As people grow older, the signals to LCT's switches change, shutting it down. But in populations where people can digest lactose as adults, mutations to these switches change LCT's response to signals. Instead of switching off in adulthood, the gene stays on.

About forty years ago, scientists first began to notice that different ethnic groups had different levels of lactase persistence. As they surveyed more groups, a striking pattern emerged: the groups with the highest levels of lactase persistence had traditionally herded cows or other milk-producing animals, while the groups with low levels did not. Some scientists proposed that lactase persistence was the result of recent natural selection. Shutting down lactase production is common not just in people but in all mammals. Before the rise of livestock, it was an adaptive strategy, since lactase were useless after nursing. Once pastoralists had a steady supply of milk, however, digesting milk could boost a person's odds of survival. Any mutation that allowed a person to absorb more nutrients from milk might be favored by natural selection. In parts of the world where herding never became a way of life, people with the mutation had no advantage.

Scientists have put this hypothesis to the test in a number of ways. As people passed down lactose-associated DNA to their descendants, they also passed down neighboring regions of their genome. Some of those regions are prone to very high rates of mutation. By comparing the mutations in those regions, one team of scientists estimated that lactase persistence first emerged in Europe some time between 7450 and 12,300 years ago. Other researchers have come up with similar estimates by analyzing other DNA segments. These estimates put the origin of lactase persistence back at a time when archaeologists find some of the earliest clues of herding in Europe.

Still, there are other plausible ways to interpret the evidence. Perhaps, for example, human populations were already split up into lactse persistent and non-persistent before the rise of herding. Their different genetic profiles might have even determined which populations ended up herding animals, and which didn't.

Scientists now have a new way to test hypotheses like this one. Instead of looking at the DNA of living populations and inferring their history, scientists can look at the DNA of people who've been dead for thousands of years. A team of scientists based at Mainz University in Germany recently looked for the lactase persistence mutation in ancient bones. They took samples from eight individuals dating back almost 8,000 years. The bones came from sites in Gemrany, Poland, Hungary, and Lithuania. Looking for such a precise bit of DNA is a tricky undertaking, because scientists may end up looking at genes from the archaeologist who found the bones, or even from themselves. So the scientists took pains to confirm that the DNA they extracted really was ancient, such as sequencing their own genes to know what they would look like.

This week the scientists are publishing their results in the Proceedings of the National Academy of Sciences. They found the segment of DNA where the lactase persistence mutation can be found, but in none of the samples did they find the mutation itself. These ancient Europeans, in other words, were lactose intolerant.

A survey of eight people is hardly enough for the scientists to make categorical statements about the genetic makeup of all of Europe. But statistically speaking, it's striking that the scientists failed to find what is now a common mutation in Europe in people who were separated by many hundreds of miles. The scientists calculate that no more than a third of Europeans could have had the lactase persistence gene at the time, and the true figure could even be zero.

It will be fascinating to see this research grow. As scientists extract more DNA from this period, will they continue to find no mutation? Perhaps they will only find it in one place in Europe--the birthplace, perhaps, of lactase persistence on the continent. And this case, at least, scientists will come closer to seeing human evolution in real time, rather than in retrospect. And if I ever want to pay homage to the ancestors who let me enjoy a fine slice of Gruyere, I'll know where to go.

"Absence of the Lactase-Persistence associated allele in early Neolithic Europeans" by J. Burger, M. Kirchner, B. Bramanti, W. Haak, and M. G. Thomas. http://www.pnas.org/cgi/doi/10.1073/pnas.0607187104