Nowadays there are many people who suffer from so-called lactose intolerance. It is the inability to digest this type of sugar, present in unfermented milk, and which leads to problems such as frequent diarrhea, flatulence and abdominal discomfort.
Biologically, it would be natural for all adult human beings to have this intolerance, since the ability to process lactose is only present in the offspring of mammals. As they develop, and especially between childhood and adolescence, the enzyme that allows correct assimilation of the sugars composed of lactose disappears naturally, since it is assumed that adult mammals do not need to consume milk to survive.
Why, then, can 90% of adult humans currently digest lactose without problem? This curious phenomenon, absent in the rest of mammals, has generated several theories among the scientific community. ¿How and why humans evolved to generate the lactase enzyme (which allows the assimilation of lactose) in its adult stage?
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 the adult stage?
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 dairy fats. 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 spread of milk consumption during prehistory. In a time where the nutrients necessary 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, individuals necessarily had to adapt biologically to it.
However, the study cited has shown that this adaptation to lactose was rather the result of natural selection. We explain it in another section.
The 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 have. Since it cannot 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 intestinal mucosa. The normal thing, as we have already mentioned, is that in adults this enzyme is not present, so lactose becomes something practically indigestible by the body.
However, we mentioned in the introduction that, Currently, 90% of adult humans do have the enzyme lactase in their digestive system , which makes the digestion of unfermented milk not a problem. But what happens to the remaining 10%? Why did some humans evolve to assimilate lactose, while others remained lactose intolerant?
A natural selection?
The study by University College, the University of Burgos and the University of Bristol presented the possibility that this unusual adaptation was due to natural selection. Since, according to the analysis of bone remains, this gene only began to be present from the birth of agriculture, we can deduce that, when crops were scarce and, therefore, famines occurred, humans turned to milk to provide the necessary nutrients.
In healthy individuals, the consumption of milk without the gene necessary for its digestion produces discomfort such as those we have already mentioned, but does not necessarily lead to death. However, if a person with nutritional deficiency (and, therefore, much more vulnerable to diseases) consumes lactose without having the lactase enzyme in their digestive system, the consequent diarrhea 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 in this project and in which, in 2014 and led by Mark Thomas, analyzed the remains of the Portalón de Cueva Mayor, in Atapuerca. Carretero assures that adaptation to lactose would, therefore, be a turbocharged natural selection, according to which the individuals who reached reproductive age were those who had developed the gene to digest milk, since those who were intolerant died much earlier due to the complications derived from their intolerance.
According to this theory, only through this natural selection could this curious genetic mutation, known as lactase persistence in the populations of Europe, Asia, the Middle East and Africa over a period of about 10,000 years.
This study is not the first (nor will it probably 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 fact when studying European prehistory: in the Tollense valley, around the year 1300 BC, an important battle between clans was fought that was most likely the meeting most enormous war in Bronze Age Europe. Among the bones that were found were fragments that still retained arrowheads, as well as bluntly crushed skulls.
But, history 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 what we have today.
That is to say, the adaptation to lactose evolved with dizzying speed, since, in only 120 generations (which are the ones that separate these remains from the Tollense valley from current humans) the percentage of individuals tolerant to this sugar increased to nine of every ten. A rapid and highly efficient evolution that allowed adult humans to survive times of disease and famine.
