How did lactose tolerance spread among humans?

Nowadays there are many people who suffer from the so-called lactose intolerance. This is the inability to digest this type of sugar, present in non-fermented milk, and which leads to problems such as frequent diarrhea, flatulence and abdominal discomfort.

Biologically, it would be natural for all adult humans to show this intolerance, since the ability to process lactose is only present in young mammals. As they develop, and especially between childhood and adolescence, the enzyme that allows a correct assimilation of lactose compound sugars disappear naturally, since adult mammals are not supposed to need to consume milk to survive.

Why, then, can 90% of adult humans currently digest lactose without a problem? This curious phenomenon, absent in the rest of the mammals, has generated several theories among the scientific community. ¿How and why humans evolved to be able to generate the enzyme lactase (which allows the assimilation of lactose) in their adult stage?

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Lactose tolerance: story of an adaptation

In recent years, various studies have been carried out that aim to shed some light on this issue. Why is the ability to process lactose so common in adult humans, when in no other species is this possible during adulthood?

One of the most recent studies is the one that was carried out in 2022, promoted by an interdisciplinary team from University College London, the University of Burgos and the University of Bristol. The project took advantage of a pioneering technique, developed by Richard Evershed and his team (University of Bristol): the analysis of containers found in archaeological remains that contained traces of fat dairy. The study of more than 7,000 samples showed that milk consumption was widespread in Europe about 9,000 years ago, from the establishment of agriculture, and that, therefore, the appearance of the gene that allows the assimilation of lactose in adults must have appeared about 10,000 years ago.

The most widespread theory until then claimed that this genetic adaptation arose as a consequence of the extension of milk consumption during prehistory. At a time when the necessary nutrients to survive were not always guaranteed, milk presented a perfect alternative, since it is rich in sugars and proteins. And if the consumption of this food spread, inevitably individuals had to biologically adapt to it.

However, the cited study has shown that this adaptation to lactose was more the result of natural selection. We explain it in another section.

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lactase enzyme in adults

First, it seems appropriate to explain what this adaptation consists of. We have already commented that adult human beings, in principle, should not be prepared to digest lactose, since milk is the food of young mammals. However, we have already seen that this is not the case. How did this adaptation come about?

Lactose is a type of sugar compound found in milk.. Being made up of two sugars, glucose and galactose, a special enzyme is needed to separate both components, an element that the digestive system of an adult mammal does not possess. Unable to be digested, lactose is stored in the large intestine. When the lactose load is high, a series of problems occur, such as diarrhea, flatulence, bloating and abdominal discomfort.

In mammalian offspring, a type of enzyme is present, the lactase enzyme, which is produced in the small intestine. and is capable of "splitting" this double sugar into two simple sugars, which are easily absorbed by the mucosa intestinal. Normally, as we have already mentioned, this enzyme is not present in adults, so lactose becomes practically indigestible by the body.

However, we commented in the introduction that, currently, 90% of adult humans do have the lactase enzyme in their digestive system, which means that the digestion of non-fermented milk does not represent a problem. But what happens to the remaining 10%? Why did some humans evolve towards lactose assimilation, while others remained lactose intolerant?

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A natural selection?

The study by University College, the University of Burgos and the University of Bristol raised the possibility that this unusual adaptation was due to natural selection. Given that, according to the analysis of skeletal remains, this gene only began to be present after the birth of agriculture, we can deduce that, when crops were poor and, therefore, famines occurred, humans resorted to milk to provide nutrients necessary.

In healthy individuals, the consumption of milk without the gene necessary for its digestion causes discomfort such as those we have already mentioned, but it does not necessarily lead to death. However, if a person who is nutritionally deficient (and therefore much more vulnerable to diseases) consume lactose without having the lactase enzyme in their digestive system, diarrhea consequent they can cause a state of dehydration such that, in this case, it can have fatal complications.

This is the opinion of José Miguel Carretero, a researcher at the University of Burgos, present at this project and in which, in 2014 and led by Mark Thomas, he analyzed the remains of the Portalón de Cueva Mayor, in Atapuerca. Carretero assures that the adaptation to lactose would therefore be a turbocharged natural selection, according to which the individuals that reached the reproductive age they were the ones who had developed the gene to digest milk, since those who were intolerant died much earlier due to complications derived from their intolerance.

According to this theory, only through this natural selection could this curious genetic mutation, known as lactase persistence, in populations of Europe, Asia, the Middle East and Africa over a period of about 10,000 years.

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A dizzyingly fast adaptation

This study is not the first (and probably will not be the last) to analyze the evolution of adaptation to lactose in adult humans. In 2020, another study led by the Johannes Gutenberg University of Mainz (Germany) analyzed the skeletal remains of more than 130 people, found in the mud of the Tollense river, north of Berlin.

These remains, in addition to providing information about the gene, discovered a very relevant piece of information when studying European prehistory: in the Tollense valley, around the year 1300 a. C., an important battle between clans that was most likely the most enormous warfare in Europe in the Bronze Age. Among the bones found were fragments that still contained arrowheads, as well as bluntly crushed skulls.

But, story aside, these remains provided significant information regarding the persistence of lactase in adults and its evolution over time. It was discovered that only one in eight of the individuals analyzed had this gene; a much lower percentage than we currently have.

In other words, the adaptation to lactose evolved with vertiginous speed, since, in only 120 generations (which are the ones that separate these remains of the Tollense valley of current humans) the percentage of individuals tolerant to this sugar increased to nine out of every ten. A rapid and highly efficient evolution that allowed adult humans to survive times of disease and famine.