It is well known that some people are able to digest milk while others aren't, and this ability seems to be concentrated in some populations and lacking in others. The usual explanation for this is that in cultures where milk is used regularly, the population has evolved the ability to process it. But, why do some cultures use milk in the first place? A new paper shows that extreme climate conditions are associated with lactose malabsorption (LM), since it may be impossible to maintain cattle at such conditions. More importantly, LM seems to be associated with the presence of several dangerous cattle diseases. Therefore, in those parts of the world where it was not possible to raise cattle safely and efficiently, humans avoided doing so, and hence they did not develop the necessary mechanism for absorbing lactose.
Evolution and Human Behavior
Volume 26, Issue 4 , July 2005, Pages 301-312
Dairying barriers affect the distribution of lactose malabsorption
Gabrielle Bloom and Paul W. Sherman
Most mammals stop drinking milk at weaning, which is also the time when they cease producing lactase, the digestive enzyme that hydrolyzes lactose. Cessation of lactase production and milk drinking also characterize most human populations, especially those of African and Asian descent. However, a genetic mutation that maintains the functionality of lactase production into adulthood occurs commonly among populations from northern Europe, where dairying is practiced routinely. Indeed, the ability to absorb lactose is nutritionally beneficial for adults only if milk consistently is available. What determines the distribution of dairying? We hypothesized that specific environmental circumstances affect where milk-producing ungulates can be raised safely and economically, thus influencing the geographical occurrence of dairying and lactase persistence. To evaluate this hypothesis, we compiled data on adult lactose absorption (LA) and malabsorption (LM) frequencies in 270 indigenous African and Eurasian populations (Appendix A). Partial correlation analyses revealed that, as predicted, adult LM is associated with extreme climates (at high and low latitudes) and, more significantly, with the historical (pre-1900) geographical occurrence of nine deadly, communicable diseases of cattle. These results suggest that areas where adult LM predominates are those where it is impossible or dangerous to maintain dairy herds.