Most people in the world are lactose intolerant - their bodies (after infancy) aren't configured to deal with milk . John Reader explains how we figured this out, in Africa. A Biography of the Continent. The story is interesting because of the interaction of human cultural and physiological evolution, and because of the way the problem originally presented itself:
...the low calcium (and iron) content of the agriculturalist diet restrained any potential for population growth that the community's increased food production otherwise might have fueled. Adequate supplied of milk from domesticated livestock corrected the deficiency. Sheep and goats were the first step….
This is not to say that people began milking because they were aware of a calcium deficiency in their diet; it is simply that the benefits of the improved diet would have produced a population growth spurt among the people partaking of it. Their increased numbers in turn increased the prevalence of the practice....
...But before this could happen and milch pastoralism could become a dominant way of life in the semi-arid regions of the Middle East and Africa a not inconsiderable adjustment to human physiology was required.
The lactose molecule which enables the human body to absorb calcium is itself unable to pass through the walls of the small intestine in one piece. It is too large and complex, and must first be broken down into its two major parts - the sugars glucose and galactose. Deconstruction is undertaken by an enzyme, lactase, which is secreted by the mucous membrane that lines the inner surfaces of the small intestine.
Lactase is an enzyme which serves no function other than to break down milk sugars (lactose). So, from a physiological point of view, lactase production is superfluous once an infant has passed the age by which the majority of its kind are weaned from their mothers' milk. Accordingly, as this stage is reached, the gene determining lactase production is switched off in all mammalian species except one. The exception, of course, is humans. Large numbers of people continue to produce lactase beyond infancy and remain able to digest lactose throughout their adult lives - but not all. Others experience extreme discomfort whenever they drink so much as a single glass of milk. The undigested lactose accumulates in the large intestine, where it ferments. Flatulence, distension, severe intestinal cramps, and diarrhoea follow in rapid order, as the gut fills with water and the lactose is flushed from the body in liquid stools.
The first hint that milk might not be universally "good for you" was noted by western nutritionists during the 1960s, in the aftermath of international foreign-aid programmes which had shipped millions of tons of dried milk around the world and found their generosity greeted with less than total appreciation. While individual Americans and Europeans were drinking litres of milk every day, in the firm belief that it would build sturdy bodies, maintain good health, settle the stomach, calm the nerves, and relieve insomnia - among other things (and among other things, increased consumption also helped to soak up surplus production) - the inhabitants of impoverished countries around the world were complaining that it made them ill. Such ingratitude. In West Africa, a Peace Corps volunteer reported villagers concluding that the powdered milk from the United States contained "evil spirits" and should be avoided. In Columbia, large shipments were used to whitewash houses, after first adding small quantities of clay to give the wash a more desirable off-white shade.
At first the donors attributed reports of the milk causing intestinal discomfort and diarrhoea to poor hygiene, dirty water, and lack of experience with a new type of food. But while shipping mountains of powdered milk abroad, the United States government was also distributing surplus while milk to needy Americans, and by the mid-1960s blacks in the ghettos were also complaining of discomfort and diarrhoea after drinking milk. In 1965, a team of research physicians at the Johns Hopkins School of Medicine began to investigate, and soon found that 70 per cent of American blacks cannot digest lactose, while 85 per cent of whites can. This pioneer study clearly showed that the European capacity to digest lactose is not universal; furthermore, it suggested that lactose tolerance and intolerance could be a measure of difference between ethnic groups.
The latter possibility was confirmed the following year by a study conducted at Makerere University College in Uganda which examined the capacity to digest lactose in two distinct ethnic groups: the cattle-herding Tutsi, and the agriculturist BaGanda. The study found that while 80 per cent of the Tutsi could digest lactose, 80 per cent of the BaGanda could not. The issues of lactose tolerance and intolerance in adults now became a subject of worldwide study. The initial assumption (based on North American and European experience, where most people consume milk products without ill effects) was that tolerance must be the norm, but results soon began to show that this ability to digest lactose in adulthood was distinctly abnormal, and lactose intolerance was the norm. In fact, a review of the subject published in 1981 concluded that "the vast majority of humankind [is] lactose intolerant." Functional levels of adult tolerance to lactose have been recorded only among northern Europeans and while North Americans (approaching 90 per cent), and nomadic pastoralists in Africa (80 per cent tolerant).
Back on the other side of the lactose divide, non-pastoralist Africans showed around 90 per cent intolerance, with the level approaching 100 per cent among the Igbo and Yoruba people of Nigeria. Most North American blacks are descendents of slaves from West Africa.Their reduced levels of intolerance (down to 70 per cent) indicate that either some north European genes have transmitted a degree of lactose tolerance to the black population since slaves were first sold into America 400 years ago (sixteen to twenty generations), or else that tolerance has evolved among the blacks as a result of their absorption into a dairying culture.
The ability to digest lactose depends, of course, upon the secretion of the enzyme, lactase; and the continued secretion of lactase beyond infancy is determined by whether or not the gene responsible for its production is left switched on. The genetic connection was established to the satisfaction of most commentators in the early 1970s by researchers from the medical schools at Lagos and Stanford universities. From their detailed surveys of cattle herding (Fulani) and agriculturist (Yoruba and Igbo) communities in Nigeria the researchers published results showing that both lactose tolerance and intolerance were inherited. But the gene for tolerance was dominant, they found. In other words, if one parent was intolerant and the other tolerant, only the tolerant gene would be copied when they reproduced and all their children would be able to digest lactose as adults.
The implication of a dominant gene for lactose tolerance is that once the mutation had arisen, the number of people in a group sharing its benefits would increase, generation by generation, until all were lactose tolerant. Populations would have grown and dispersed, especially in the early days of agriculture, when livestock were newly domesticated and the capacity to exploit the nutritional benefits of milk would have been very advantageous indeed.
The evolution of lactose tolerance can only have begun with the domestication of livestock around 10,000 years ago. Whether these crucial developments arose solely in the Near East, or in the Sahara as well…, they combined to expand the food resource potential of semi-arid lands throughout both regions. Cows converted dry grass and corn stalks into nourishing milk. As numbers of both cattle and people increased, nomadic pastoralists moved into ecological zones that could not otherwise be occupied in such numbers...
I learned about this book from Tyler Cowen: How is it I missed this book? (Marginal Revolution, July 10, 2007).
The study of the evolution of lactose tolerance in adult humans is an ongoing research area. There are still a lot of questions. Dennis O'Neill describes more recent research (Nutritional Adaptation):
The common ability of people in Europe and some other areas of the world to continue producing lactase as adults is very likely a relatively recent evolutionary development. Prior to the domestication of cattle, sheep, goats, and horses after about 9000 years ago, milk was most likely only consumed by babies and very young children. That milk was human milk. Dairy products such as cow's milk, yoghurt, and cheese did not exist. When nutrient rich nonhuman milk became widely available in pastoralist societies, the rare genetic variations that allowed some adults to easily digest lactose were selected for and this trait became more common. In other words, natural selection shifted to favor lactose tolerant people, resulting in the progressive evolution of the gene pools of these populations. Support for this hypothesis was provided in 2007 by Joachim Burger and his team of researchers at the University of Mainz in Germany. Their analysis of DNA in bones from 10 Central and Eastern European human skeletons dated between 3,800 and 6,000 years ago showed that the allele that allows lactose tolerance in adulthood was not yet present despite the fact that these populations apparently had been raising milk producing farm animals for hundreds or even thousands of years. Sarah Tishkoff from the University of Maryland also reported in 2007 that the mutations among East Africans that keep the lactase gene permanently turned on are different from those of Europeans who share this trait. Her genetic studies among 43 East African ethnic groups also suggests that that 3 different mutations resulting in lactose tolerance in Africa arose 2,700-6,800 years ago.
The Burger et al. study is here: Absence of the lactase-persistence-associated allele in early Neolithic Europeans (Proceedings of the National Academy of Sciences of the United States, published online February 28, 2007). Sarah Tishkoff's work is described in these articles:UMd-Led Team Discovers Gene Mutation for Milk Tolerance in Africans (press release, December 5, 2006); Got lactase? (Understanding Evolution Web site, UC Berkeley, April 2007).
Stephen Wooding: Following the Herd (Nature Genetics, January 2007) explains the significance of Tishkoff and her co-authors' conclusions:
The discovery that different SNPs account for the same phenotype in different populations is extraordinarily interesting from an evolutionary standpoint, as it suggests that these separate populations have been under strong pressure from natural selection. Throughout history, one of the biggest problems humans have faced is getting enough to eat. Early populations solved this problem by being highly efficient hunter-gatherers. The advent of agriculture 10,000 years ago presented an alternative solution: keep food sources close at hand.
It has long been hypothesized that in the case of dairy animals, domestication had the effect of reflecting selective pressures back at the domesticators. This is because dairy animals are useful even if you can’t drink their milk, but they are much more so if you can: milk is a nutritional bonanza of fat, proteins, carbohydrates, vitamins, calcium and even water… but only if you can digest it. Thus, many have argued that whereas non-dairying populations faced little pressure to digest milk into adulthood, dairying populations were under enormous selective pressure to do so. And if they were, we should see evidence of it in their genes.
Evolutionary studies of lactase persistence alleles in European populations have found just such evidence13,14. In Europeans, the C/T-13910 variant is found on a haplotype background demonstrating long-range linkage disequilibrium, consistent with the recent increase in the frequency of the lactasepersistence variant, exactly as expected if natural selection had favored it15. Moreover, this variant is found at highest frequency in populations that have historically practiced dairying.
Tishkoff et al. find nearly identical signatures of selection in Africans, but with a twist: they are associated with a different lactasepersistence allele. Taken together, these findings tell us that divergent human populations have been under similar pressures in the diet—pressures involving milk—and have converged on the same solution of prolonging LPH expression into adulthood. In a striking testimony to the powerful evolutionary effects culture can have on our genes, not only has the domestication of cattle driven allele frequencies in humans, but it has done so at least twice, in different regions of the world.
The Wikipedia article on Lactose intolerance notes that "Many global cat breeds (Asian breeds in particular) share the mammalian lactose sensitivity, unlike many European breeds that have a mutation similar to the European human mutation."