Frederick J. Simoons (1969, 1970) has advanced the thesis that lactase persistence is closely linked to dairying. His culture-evolution hypothesis is that groups that kept cattle and other milk animals would gain a selective advantage if adults retained the ability to use milk and milk products as food. A mutation like LAC*P would be nutritionally beneficial, and the growing number of milk-using adults would then be encouraged to devote more effort toward livestock raising. In general, the distribution of adult lactase persistence and dairying shows a positive relationship. In areas with no dairying tradition, such as China, Oceania, PreColumbian America, or tropical Africa, few adults can digest lactose.
Northern Europe presents the opposite case. More data around the periphery of the two postulated centers would be highly desirable, and we know little about most of the stock-raising societies of central Asia. Still, although the correspondence is not perfect, and gene flow through population mixing complicates the picture, the association seems strong. Given the origins of cattle keeping about 4000 to 3500 B. C. in northern Europe, and even earlier in the Middle East, there probably has been enough time for modest selective pressures to have produced observed LAC*P rates (Sahi 1994).
Other selective forces may also have been at work. Flatz (1987) has suggested that calcium absorption was such a factor in northern Europe. Lactose is known to facilitate calcium absorption in the intestine. The cold, cloudy climate frequently discouraged skin exposure to sunlight, thereby reducing the body’s production of vitamin D. Relatively little dietary vitamin D was available, and so in its absence, calcium was poorly absorbed. Northern populations were thus vulnerable to rickets and osteomalacia. Pelvic deformities made births more difficult. The gradual extinction of the Greenland Viking colony is an example; skeletal evidence shows that such bone diseases were common among this moribund population. A mutant LAC*P allele would not only allow adults to use an excellent source of calcium, but the lactose would also facilitate its absorption. While not proven, this hypothesis has attracted much attention. It would complement the theory that the pale skin of northern Europeans is a genetic trait maximizing the utility of sunlight in vitamin D production and, hence, calcium absorption.
Similarly, other selective pressures facilitating the survival of mutant LAC*P alleles have been postulated for the Sahara-Arabian Peninsula desert region. There is a high degree of dependency on milk among many groups of desert pastoralists, and so a positive link between lactase persistence and milking seems very plausible. In addition, it has been argued (Cook 1978) that the simple fact that milk is a liquid would give adults who could consume it in large quantities a powerful selective advantage. The theory, while unproven, certainly seems plausible. G. C. Cook’s suggestion that lactase persistence conveyed some resistance to gastrointestinal diseases has attracted much less support. At least for cholera, his claim must be rejected, based on what we know of the historical geography of the disease. Cholera seems to have been restricted to the Indian subcontinent until very recent times.
Finally, it seems most likely that the European and Arabia-Sahara centers of LAC*P prevalence, and the Uganda-Rwanda center (if it in fact exists), arose independently. Population movement and gene flow can be very extensive and, no doubt, have played a substantial role around the centers. Despite the efforts of some authors to find a common origin in the ancient Middle East, it is simpler to suggest independent origins than to postulate gene flow from the Middle East to Scandinavia and to the interior of East Africa. The problem might be resolved in the future if gene sequencing could show that the LAC*P alleles in Sweden and Saudi Arabia are, in fact, the same or are distinct forms of the gene with a similar function.
World History
