Single Mutation Sparks Dietary Revolution
A small event can have large consequences. So it is with a single gene controlling the production of an enzyme called lactase. Lactase enables infant mammals to digest lactose, the principal sugar in mother's milk ensuring the newborn has a reliable source of nutrition until able to digest solid food.
For mammals in general and most humans in particular, the gene controlling the enzyme lactase switches to the off position some time after the infant is weaned. Within a short time thereafter, lactose can no longer be broken down and causes severe diarrhea if animal milk is ingested. About 7500 years ago in what is now Eastern Europe a genetic mutation occurred to the switch gene, creating a condition called lactase persistence, that is the gene did not turn off lactase production. This mutation, an adaptation permitting the drinking of animal milk throughout one’s life, spread quickly in succeeding generations. An article in Nature this week (read “Archaeology: The Milk Revolution”) outlines the evidence and argues it is more likely than not this adaptation was carried by agricultural people (having domesticated animals and grains) moving into the area, replacing hunter gatherer peoples lacking this mutation.
Presently about 35% of the overall human population, bears a mutation in this switching gene and the lactase is not turned off. Approximately one third of all humans consume milk, (from cows, goats and sheep) throughout their lives without ill effect and thereby can take advantage of another source of nutrition. The article argues the ability to digest animal milk after weaning conveyed an evolutionary advantage in times of reduced supply of other food resources. However, the author does not deal with the counterargument: if grains, meat and milk from domesticated animals, become scarce, so does animal feed, reducing milk production.
This article is clear, engrossing and well illustrated (see “Dairy Diaspora” on this page). Visitors to becominghuman.org are urged to read it.