Most people from western societies are lactose tolerant and able to drink milk as adults, although intolerance still affects a significant minority.
However, for most races, lactose intolerance is not the minority but rather the norm. It is estimated that globally, only about a third of today’s population can consume milk into adulthood. So, it is lactose tolerance that needs to be explained.
Lactose tolerance seems to have developed in a two-stage process, the second of which was a spontaneous genetic mutation that only occurred in a few places in the world.
Until then, except for children, milk was an unpleasant (but not life-threatening) toxin. The body must produce the lactase enzyme to break down lactose in milk, and adults are not able to do that. Young children of all races produce lactase, and can digest their mother’s milk, but as they grow older the gene producing lactase is switched off (presumably to induce weaning by making milk unpalatable).
But homo sapiens are a resourceful bunch. There is evidence going back at least 9,000 years that humans were fermenting milk to produce yoghurt and cheese, which they could eat.
Microbes that can ferment milk to produce yoghurt are naturally occurring and rather common, so it is understandable that chance could have produced the first yoghurt. The microbes break down lactose (a milk sugar) into an acid (lactic acid). This confers an advantage to these microbes because the other microbes usually found in milk, or that can infect milk, cannot survive in an acidic environment. So they are squeezing out their competition.
This has at least four consequences for us. First, the lactic acid produces a sour taste, which we find refreshing. Second, the acid kills or inhibits microbes that are harmful to us, thus preserving the food. Third, the acidic environment denatures milk proteins (e.g. casein), causing them to unravel and entangle with one another. This is the same process as forming a gel, so the result is a thickened product.
But the final consequence is the most important: By breaking down the lactose for us, the microbes created a milk product that our ancestors could eat even though they were lactose intolerant themselves. A similar but more involved process occurs in the making of cheese (hard cheeses such as Parmesan have virtually no lactose).
This was a very old discovery by our ancestors. Pottery shards have been uncovered riddled with small holes for draining whey, and scientific analysis has dated them, and revealed the expected milk residues. It meant that a food source was available all year round that was not subject to crop failures and famine.
Then around 7,500 years ago the second-stage happened, a single small mutation not far from the lactase gene resulted in preservation of lactase expression into adulthood. The origin of this one mutation has been traced to a region now known as Hungary.
By already farming and producing fermented milk products, the circumstances were in place to take advantage of the new lactose tolerance and consume the milk directly.
The mutation proved to be a powerful force. The ability to drink milk, and derive reliable sources of proteins and nutrients from it, gave people with this mutation a major selective advantage (for example, they may have produced up to 20% more offspring). The lactose tolerant individuals spread across Europe, displacing the native lactose intolerant hunter-gatherers and rapidly increasing in population.
It turned out to be one of the strongest selective pressures in our history.
Two other enzymes that are thought to have arisen from spontaneous mutations are amylase (which enables starch in grains to be broken down) and alcohol dehydrogenase (which enables the body to break down alcohol). Both of these have had a significant impact on the evolution of our civilization, and still do.
Principal source: Curry A, The milk revolution. Nature (1 August 2013) 500:20-22.