The majority of the world's population is to some degree lactose-intolerant. Among people of European descent and in certain parts of Africa, however, the ability to digest the lactose in milk and dairy products is very common. This ability is brought about by a genetic mutation that causes those who carry it to continue producing an enzyme called lactase well into adulthood.
Lactose and Lactase
Both human and cow milk are rich in a sugar called lactose. Lactose is a disaccharide, a molecule made by combining two smaller sugar molecules named glucose and galactose. In water, the lactose sugar tends to break down into glucose and galactose, but this reaction is very slow. The enzyme lactase acts as a catalyst to facilitate the reaction and make it happen very quickly. This enzyme is made up of four separate subunits that come together to form a single functioning enzyme. Each subunit is a long chain of amino acids strung together. Altogether, if you count up the number of amino acids in each chain, there are 4,092 amino acid units in the protein.
Conditions for Enzyme Function
The lactase enzyme only achieves its optimal performance if magnesium is present, and it functions best when the pH is close to 6. When the enzyme is fully saturated -- in other words, when the concentration of lactose is so high that increasing it further doesn't increase the reaction rate -- it can break down 60 molecules of lactose a second. The mechanism by which it facilitates the reaction involves two glutamate amino acids situated in such a way that once the lactose molecule sticks to the enzyme, these amino acids cooperate in splitting it in two.
Genetics of Lactase Persistence
As infants, all humans produce the lactase enzyme in their intestines. Most humans, however, cease producing the enzyme in early childhood. A single mutation close to the gene for this enzyme enables you to continue producing lactase into adulthood -- and thus digest lactose even as an adult. This trait is called lactase persistence, and people who lack it are said to be lactose-intolerant, although the extent and severity of lactose intolerance varies widely among individuals.
Origins of Lactase Persistence
Humans only began dairy farming some 10,000 years ago. There is a strong correlation between the popularity of dairy farming in a given region and the frequency of the lactase persistence mutation. The two regions where lactase persistence is most common are Europe and some African countries, both regions where dairy farming has been practiced for millennia. This implies that lactase persistence is a recent evolutionary innovation and that there has been strong natural selection favoring this mutation, meaning that in regions where dairy farming was practiced, people who could digest dairy products were much more likely to survive and have children. Why the ability to eat dairy was so beneficial remains unclear.