The evolution of lactose tolerance, and it’s distribution.

 

Stone Age Adults Couldn’t Stomach Milk, Gene Study Shows.

  
James Owen
for National Geographic News

February 26, 2007
Milk wasn’t on the Stone Age menu, says a new study which suggests the vast majority of adult Europeans were lactose intolerant as recently as 7,000 years ago. While cow’s milk is a mainstay in the diet of modern-day Europeans, their ancestors weren’t able to digest the nutritious dairy product after childhood, according to DNA analysis of human skeletons from the Neolithic period. The findings supports the idea that milk drinkers became widespread in Europe only after dairy farming had become established there—not the other way around.

Most mammals lose their ability to digest milk after being weaned, but some humans can continue to benefit from the calcium-rich, high-energy liquid. This is because they carry a mutation that lets them continue producing lactase, the gut enzyme needed to break down the milk sugar lactose, in adulthood. Lactose tolerance is most common in people of European origin, especially those from the northern and central areas of the continent. It is relatively rare in Asian and Native American populations.

Lactose Tolerance

High levels of lactose tolerance among these European groups are thought to reflect an evolutionary advantage. Early farming communities that could digest milk could consume the liquid during otherwise poor harvests, for instance. Some scientists argue this adaptation was previously very rare in humans, spreading only after the introduction of farming to Europe. Others say prehistoric populations were already split between those who could and couldn’t drink milk as adults. This split, the researchers say, determined which groups became dairy farmers.

Burger’s team analysed the DNA of well-preserved Stone Age skeletons from locations in northern and central Europe
Bones dated to between 5800 and 5200 B.C. were tested for a genetic marker associated with the production of lactase. The team says it found no trace of the lactase gene, indicating that people from the period weren’t yet able to drink milk.
Natural Selection

The study suggests that the lactase gene spread rapidly in the human population only after dairy livestock were introduced to Europe about 8,000 years ago, Burger says. (Related: “Goats Key to Spread of Farming, Gene Study Suggests” [October 10, 2006].) “I think it’s a very old mutation that was completely useless before farming started,” he said. But then the gene suddenly became useful, and its presence in the population quickly grew through natural selection, Burger said. “People who had cows, goats, or sheep and were lactose resistant had more children, and those children survived infant mortality and years of poor harvests,” he said. The legacy of this evolutionary process is very apparent in the DNA of northern and central Europeans today, Burger notes. In parts of Sweden, he says, 100 percent of people carry the lactase gene, whereas the average figure for the whole country is about 90 percent.

In Scandinavia, Holland, Britain, and Ireland, he added, “you can say most of the people are the descendents of dairy farmers.” (See a map of Europe.) Milk tolerance also exists in southern and eastern European populations, while certain prehistoric farming communities in North Africa and the Middle East also developed the trait, scientists say.
But in other populations the lactase gene is largely absent. “All over the world most people can’t drink milk when they’re adults,” Burger said. “It’s only some populations in northern Africa and Europeans that can.”

http://www.ntanet.net/milk-consumption.pdf

Rates of lactose intolerance.

Southeast Asians/98%
Asian Americans/90%
Alaskan Eskimo/80%
African Americans Adults/79%
Mexicans from rural communities/73.8%
North American Jews/68.8%
Creek Cypriots/66%
Cretans/56%
Mexican American Males/55%
Indian Adults/50%
African American Children/45%
Indian Children/20%
Caucasians of N. European and Scandinavian decent/5%

http://www.cambridge.org/us/books/kiple/lactose.htm

Adult lactase capability appears to have evolved in two, and possibly three, geographic areas. The case is clearest and best documented for northern Europe, where there are very high percentages around the Baltic and North Seas. High levels of lactase persistence seem closely linked to Germanic and Finnic groups. Scandinavia, northern Germany, and Britain have high levels, as do the Finns and Estonians, the Finnic Izhorians west of St. Petersburg, the Mari of the middle Volga basin, and, to a lesser extent, their more distant relations, the Hungarians.

There is a general north—south gradient in Europe, which is evident within Germany, France, Italy, and perhaps Greece. As noted, more information is needed for Spain, Portugal, and eastern Europe, but there may be something of a west—east gradient in the Slavic lands. Varying frequencies of the LAC*P allele among Lapp groups may be related to differing lengths of historical use of reindeer and cow’s milk and to admixture with other Scandinavians (Sahi 1994).

The second center of adult lactase persistence lies in the arid lands of Arabia, the Sahara, and eastern Sudan. There, lactase persistence characterizes only nomadic populations heavily dependent on camels and cattle, such as the Bedouin Arabs, the Tuareg of the Sahara, the Fulani of the West African Sahel, and the Beja and Kabbabish of Sudan. Lower rates among Nigerian Fulani may indicate a higher degree of genetic mixing with other peoples than among the Fulani of Senegal. In contrast, surrounding urban and agricultural populations, whether Arab, Turkish, Iranian, or African, have very low rates. It is interesting to note that the Somali sample also had a low frequency of the LAC*P allele. Possibly, pastoral Somali have higher prevalences than their urban compatriots.

A third center of adult lactase persistence has been suggested among the Tutsi population of the Uganda-Rwanda area of the East African interior. The Tutsi are an aristocratic cattle-herding caste of Nilotic descent who have traditionally ruled over agricultural Bantu-speakers. Table IV.E.6.1 shows that only 7 percent of a sample of 65 Tutsi adults were lactase deficient, but the data are old, there certainly has been some mixture with Bantu-speakers, and the study should be replicated. The Nilotic peoples of the southern Sudan, whence the Tutsi originated a few centuries ago, do not display this trait. Unless the Tutsi result can be confirmed, and the Maasai and other East African Nilotic groups can be tested, this third center of the LAC*P allele must be considered doubtful. If it does exist, it probably arose as a fairly recent mutation, as there are no obvious historical mechanisms to account for gene flow between the Tutsi and desert dwellers farther north.

lactose-tolerance.jpg

Figure 1. Geographic coincidence between milk gene diversity in cattle, lactose tolerance in humans and locations of Neolithic cattle farming sites in NCE.
(a) Geographic distribution of the 70 cattle breeds (blue dots) sampled across Europe and Turkey. (b) Synthetic map showing the first principal component resulting from the allele frequencies at the cattle genes. The dark orange color shows that the greatest milk gene uniqueness and allelic diversity occurs in cattle from NCE. (c) Geographic distribution of the lactase persistence allele in contemporary Europeans. The darker the orange color, the higher is the frequency of the lactase persistence allele. The dashed black line indicates the limits of the geographic distribution of early Neolithic cattle pastoralist (Funnel Beaker Culture) inferred from archaeological data15.

http://www.nature.com/ng/journal/v35/n4/full/ng1263.html

This article states that the gene for lactase persistance is older than dairy farming in Europeans. I can’t help wandering if maybe Cro Magnons practised a Sami-like herd management that gave them access to milk.

About these ads

2 responses to “The evolution of lactose tolerance, and it’s distribution.

  1. While this is obviously far from the central point of the article you here apparently cite in its entirety, I still want to point out that the brief mention of the Rwanda Tutsi and their origins has been largely discredited. Worse, this passage, “[T]he Tutsi are an aristocratic cattle-herding caste of Nilotic descent who have traditionally ruled over agricultural Bantu-speakers,” and the divisive idea that the Tutsi are of some comparatively recent “non-Rwandan” origin, i.e. Nilotic, from “southern Sudan,” was used as a prime basis for organizing the 1994 genocide against the Tutsi. The Hutu (the “agricultural Bantu-speakers” mentioned here) were rallied to the extremist cause of genocide by propaganda blaring out a message that the Tutsi were the great “other,” usurping visitors from the North, who should be driven out and exterminated. In fact Tutsi, to the extent that they can be identified, largely self-identify through linguistic cues and attitudes, not through visual or genetic traits. This article unquestioningly propagates the “Hamitic myth” of Rwandan division exploited by Belgian colonialists, with devastating results in 1994.

  2. Fair enough, I’m just scrapbooking articles on this page. I’m not well informed on the Rwandan situation, or the origin of the Tutsi. North Africa and the middle East is more my thing.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s