We have seen that not all people can survive well at high altitude or under intense ultraviolet radiation. Similarly, there are major differences around the world in how effectively our bodies process particular foods. In addition, some people live well on daily diets that would be at a starvation level for others. Different human populations not only eat different foods, but their digestive systems often use them in somewhat different ways. For instance, the Inuit of Alaska, Northern Canada, and Greenland traditionally ate far more fat than most other populations and their gastrointestinal systems apparently are more capable of breaking fats down for use by their bodies. This is an indication that adapting to local nutritional opportunities has led to the evolution of related genetic differences among the populations of the world.
Many Indians in the Southwestern United States have what have been described as "thrifty genes." Their bodies are unusually efficient at utilizing the calories in their food and, subsequently, need to consume less than other people of their size in order to maintain a stable weight. The Tohono O'Odham Indians of Southern Arizona are an example. Until well into the 20th century, these traditional subsistence farmers had diets that mostly consisted of beans, squash, and corn with little animal protein and fat. Late 19th century photographs usually showed them to have slender to medium body builds. That is no longer true. Now, obesity and associated type 2 diabetes are very common problems for most of them. A dramatic change in diet was responsible for this deterioration in health. Most of them abandoned their traditional diet in favor of the fat and protein rich foods of their Mexican and European American neighbors. Their bodies are responding to what for them is an excessive amount of calories by storing much of it as body fat. This fat is storage for a famine that never comes.
NOTE: "Tohono O'Odham" literally means "Desert People". They were formerly known as the Pima and Papago Indians.
The best documented differences in nutritional adaptation relates to milk sugar, or lactose , which is commonly found in uncooked dairy products. A liter of full fat cow's milk contains 650 calories of which 250 are from lactose. Most human adults have moderate to severe difficulty in digesting lactose. They experience bloating, stomach cramps, belching, flatulence, and even diarrhea when they drink milk. Not surprisingly, this commonly results in a decision to remove dairy products from their diet. This problem is most often due to an inability to produce sufficient amounts of the enzyme lactase , which breaks down lactose into glucose and galactose in the small intestine to aid its absorption into the blood stream. Those who have this problem are said to be lactose intolerant due to their lactase deficiency.
The ability to produce lactase is genetically controlled. The gene that codes for it (LCT) is on chromosome 2. The vast majority of babies throughout the world can digest their mother's milk. However, there is a decline in lactase production as people grow older. This decline usually begins by two years of age, which is shortly after the time when babies are weaned in most societies. For some people, the reduction in lactase production does not start to occur until they are around twenty. More rarely, lactase continues to be produced at sufficient levels to consume milk throughout life.
Lactose intolerance is at its highest frequency in some parts of Africa, East Asia, and among Native Americans (as shown in the table below). Northern Europeans generally have the lowest frequency of this dietary problem.
U.S. European Americans 2-19 % Latinos (Hispanic Americans) 52 % African Americans 70-77 % Native Americans 95 % Asian Americans 95-100 % Mexico 83 % Europe Sweden 4 % Switzerland 12 % Spain 15 % Finland 18 % Estonia 28 % England 32 % Hungary 37 % Greece 88 % Jordan 79 % Africa Southern Sudan (cattle herders) 17 % Ibo and Yoruba (Nigeria) 99 % Asia Japan 90 % Thailand 99 % Australia (Aborigines) 85 %
Source: Robert D. McCracken, "Lactase Deficiency: An Example of Dietary Evolution,"
Current Anthropology 12 (Oct.-Dec. 1971, pp. 479-517) and Norman Kretchner, "Lactose
and Lactase," Scientific American 277 (Oct. 1972, pp. 71-78)
Given this distribution pattern of lactose intolerance, it is not surprising that dairy products are popular among most Europeans but are rarely found in Asian, Native American, and most African cuisines (except among cattle herders in East Africa). In the majority of non-European populations, fresh milk is considered an unpleasant substance to be consumed only as a last resort. It is now clear that lactose tolerant Europeans and East Africans are atypical for humanity and for the entire animal kingdom.
NOTE: In the Indian subcontinent and much of Central and Western Asia, dairy products are consumed frequently but usually only after bacteria (lactobacilli ) have broken down most of the lactose. When this has occurred, milk becomes yoghurt or kumis, both of which are relatively easily digested even by people who produce little lactase.
Evolutionary Significance of Lactose Tolerance
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, which occurred 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 gradually shifted to favor lactose tolerant people, resulting in the progressive evolution of the gene pools of these populations in Europe. 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 common 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 3 different mutations resulting in lactose tolerance in Africa arose between 2,700 and 6,800 years ago. This is a good example of nature coming up with different successful genetic solutions to the same environmental challenge.
Got Lactase? The Co-evolution of Genes and Culture--short film explaining the function of lactase in digesting milk
and the evolution of lactase persistence in different populations. This link takes you to a video at an external website.
To return here, you must click the "back" button on your browser program. (length = 14 min, 52 secs)
Adapting to Alcohol
It is likely that natural selection is largely responsible for population differences in the ability of humans to quickly metabolize alcohol in beverages and subsequently tolerate it. Metabolizing alcohol eliminates its toxicity and removes it from the blood so that it won't cause cell and organ destruction. Frequencies of the alleles in the 2 gene families (ADH and ALDH) coding for the enzymes involved in this protective process vary significantly from population to population. Subsequently, there is a wide range of variation in how rapidly and effectively alcohol metabolism occurs--i.e., whether people become intoxicated easily and how long this state continues.
Beer and wine consumption have been common in Europe and the Near East for 5-6 thousand years. In fact, the first known recorded beer recipe dates to 3,800 B.C. in Mesopotamia (now southern Iraq). Throughout Medieval Europe, beer and wine were far more frequently consumed than water by all classes of society. It is not surprising that many people in these same populations now have the genetic makeup that gives them a relatively high tolerance of alcohol consumed on a regular basis. Presumably nature tended to select against those individuals who lacked the gene variants that made this possible. In contrast, alcohol tolerance is unusual among Native American populations. With few exceptions, they have only had alcohol based beverages since the arrival of Europeans, a few hundred years ago. This is little time for nature to select for gene combinations that would allow them to metabolize alcohol quickly. It is not surprising that alcoholism and alcohol related problems such as cirrhosis of the liver are at comparatively high rates among many Native American groups. They are at a genetic disadvantage when it comes to consuming alcohol.
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