Evolution of Adult Lactose Tolerance in HumansMost humans and other mammals only produce the enzyme lactase in their intestines when they are young. (Lactase is needed to digest milk, and most mammals only subsist on milk when they are infants.) Later, lactase stops being produced. But some populations of humans have evolved to be able to produce lactase and to digest lactose as adults.
Evolution depends on two things:
- Genetic variation (often arising from mutations in existing genes, or from new combinations of genes), and
- Selection—some genes or combinations of genes must contribute to greater reproductive success for their carriers.
Humans are mammals
As babies we are nourished by milk. The principal carbohydrate in milk is lactose. Lactose isn't found in many sources other than milk.
Babies have a digestive enzyme that can break down lactose to provide the energy they need. That enzyme is called "lactase". (Its formal name is β-galactosidase. The human digestive enzyme is called lactase-phlorizin hydrolase. Learn more about it here.)
After we are weaned most of us don't need that enzyme any more, and the gene that codes for it is turned off. Among our hunter-gatherer ancestors, only babies needed to be able to digest lactose, since only babies were fed milk. All humans were hunter-gatherers until about 10,000 years ago.
Adult lactase persistence is not generally an advantageous trait. If it was everybody would have it. The adult-persistence form of the gene may be present at low frequency in most human populations. But it only becomes widespread in a population if strong selection pressure is applied, by making milk and lactose-containing milk products an important part of the diet of older children and adults of reproductive age.
Milk could only have become an important part of the adult human diet when animals like reindeer, cows, water buffalo, horses, camels, goats and sheep were domesticated and developed as significant parts of a herding/pastoral economy. This apparently began to happen about 10,000 years ago. That is only about 400 or 500 generations. Yet adult lactose persistence has become established in a number of populations around the world. (See supporting table showing percentage of adults lacking lactase in various populations.)
Western medicine sees the inability to digest lactose by adults as an illness. It is called Hypolactasia, Lactose Maldigestion and Lactose Intolerance. This is a strange way of looking at it. It is based on the European idea that consumption of unprocessed milk products by adults is "normal", so if it causes you digestive upset you must be "sick". The condition is discussed at this site and this one.
The increase of the adult-lactase-persistence form of regulation of the lactase gene is an example of coevolution. Lactase was of no use to adults until they had supplies of milk for food. They couldn't comfortably consume milk after weaning until they developed adult-lactase-persistence. The chicken and the egg.
The adult-lactase-persistence form of the gene seems to be an old mutation. (I wouldn't be surprised if it were eventually found even in other mammals.) But it was just lurking in the background until selective pressure was applied.
As milk-producing animals began to be domesticated, and milk became a more available food, people who could comfortably eat milk had a new source of nutrition. Those with only the "standard" form of the gene could not efficiently use this food source. More nutrition meant greater reproductive success for the possessors of the mutant form of the gene. Since the gene is dominant, if you get it from either of your parents you will be able to digest milk as an adult. The gene could rapidly increase its frequency in the population.
[Update 13 June 2015: Recent research into the genes of very old human remains suggests that even 4,000 years ago lactase persistence genes were very rare in Europeans. This BBC article discusses research published in Nature. It may be that the selection pressure for lactase persistence is more recent than domestication of milk-producing livestock. There is some discussion of the evolution of adult lactase persistence. (Full text of Nature article here.)]
Control of Gene Expression
Obviously the expression of genes must be controlled. Lactase is needed to break down lactose in certain tissues at certain times, but not in all tissues nor at all times. Almost every cell has the gene for lactase production. How is it managed?
The lactase gene is transcribed and translated to make a glycosylated enzyme precursor, which is then transported to the cell membrane and cleaved to produce the active enzyme. The enzyme dangles into the small intestine, bound to the membranes of cells of the intestinal wall.
The presence or absence of lactase is regulated at the level of gene transcription. That is, if lactase is not needed the transcription of the gene into messenger RNA is not initiated. Of course, most genes are "turned off" most of the time. How is the lactase gene turned on when lactase is needed? It's all rather complicated, but here is a simplified version.
- Genes, like the gene that codes for lactase in humans, have "promoter regions" of nearby DNA that control whether or not they are transcribed into RNA to make proteins.
- These promoters permit the transcription of the associated gene when they bind to specific "transcription factors".
- The promoters of intestinal genes, like lactase, are activated when they bind to specific transcription agents that are expressed in the cells of the intestinal epithelium.
- Presumably, when it is time for the production of lactase to be turned off at the time of weaning, the necessary transcription factors are no longer produced. You will ask, what then controls the expression of the transcription factors? Mutations have been identified in DNA associated with lactase regulation, found on the same DNA strand as the lactase gene itself, upstream in an intron in another gene.
- What, then, controls the expression of the intestinal transcription factors that control lactase expression in time and space? I have no idea. They're still working on that.
David Wheat's Science In Action site has articles about science and math in the real world, weird science, science news, unexpected connections, and other cool science stuff. There is an index of the articles by topic here.
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