Why did Europeans evolve into lactose intolerance? | Chemistry

Just 5,000 years ago, even if it was part of their diet, in fact adult humans were unable to digest milk properly. But in the blink of an evolutionary theory, Nordic people began to inherit the genetic mutations that allowed them to do so.This trait became common in just a few thousand years and is today Up to 95 percent of the population.. By stitching together pieces of Neolithic pottery with the ancient human genome, scientists may have solved the mystery of how European lactose intolerance evolved.

and Study published today of Nature, Researchers have compared 9,000 years of archaeological evidence of European milk use with genetics and discovered an unusually rapid evolution of lactose tolerance long after Europeans first began consuming beverages. The authors suggest that something more extreme than normal milk consumption caused genetic alterations. Exceptional stressers such as famine and pathogens exacerbate the typical mild gastrointestinal effects of milk on lactose intolerance, making milk’s ability to digest more valuable, while diarrhea and It can cause fatal attacks of dehydration.

“We’re rewriting the textbook on why drinking milk was an advantage,” says lead author Richard Evershed, director. Biogeochemical Research Center At the University of Bristol. “In order for a gene mutation to evolve very quickly, something has to kill people who don’t have it.”

An extensive study led by colleagues at Evershed and the University of Bristol and University College London included contributions from experts from 20 other countries.

Almost every baby in the world is born with the ability to digest lactose. After all, lactose is found in breast milk. However, about two-thirds of adults are unable to digest natural lactose because the production of a milk-digesting enzyme called lactase ceases after weaning. As a result, the majority of the world’s adult population is lactase non-persistent and is also known as lactose intolerance.

The remaining one-third of the world’s adult population has evolved lactose intolerance. That is, they continue to produce lactase. This is especially true for groups like Scandinavian groups.

Shevan Wilkin, a bio-anthropologist at the Max Planck Institute for the Science of Human History, says that perhaps five years ago, the story of lactose intolerance seemed simple. When a group of humans grazed animals and started drinking milk, the health benefits of milk favored those who could digest it, but digestive disorders countered the success of intolerance. As a result, the genetic mutations that helped humans digest milk eventually spread to those populations.

“Then we noticed some crazy trends,” says Wilkin, who wasn’t involved in the study. “Looking at the ancient genome, no one has been lactose intolerant until the last few thousand years.” In order for a genetic trait to spread so rapidly, those who have it survive and There must be a very important reason for breeding and dying others.

“We also noticed that the huge population of the entire grassland, the people of modern Kazakhstan, Russia, Mongolia, and those who drink large amounts of milk have no lactase persistence.” Drinking large amounts of milk. If the simple advantage of was to produce and propagate lactase persistence mutations, grassland dwellers would certainly have evolved their traits in the same way as Europeans.

In addition, studies of ancient human DNA have shown that the gene mutations that allowed the persistence of lactase in Europe do not appear to provide a slight nutritional benefit. In the European genome The single trait most preferred by positive natural selection Over the last 10,000 years.

The author used several different research lines to delve into the dark past of European milk.

Richard Evershed and colleagues have mapped breast milk use over the last 9,000 years and created a vast database of 6,899 animal fat residues derived from 13,181 pottery fragments from 554 archaeological sites across Europe. Over the last three decades, scientists who have pushed experts like Evershed to the fore have developed ways to analyze ancient pottery and reveal evidence of what it contains.

Luck, or just as science has it, milk fat is absorbed into ancient pottery and preserved at amazing levels. A study of the carbon isotope composition of the two major fatty acids present and persisting in the decomposed animal fats of the pot reveals that milk is made differently from ruminant carcasses and thus has unique characteristics. You can see that it remains.

Evershed has found a wealth of evidence that humans have been drinking milk extensively throughout Europe for about 9,000 years.

Co-author Mark Thomas, an evolutionary geneticist at University College London, has launched his own mapping project. This shows when and where the genetic variation that allows lactase persistence in Europeans emerged. He examined the DNA sequences of more than 1,700 prehistoric humans and found that their first appearance was not about 5,000 years ago, or about 4,000 years after normal milk consumption began. did. Mutations have become commonplace in the short term since then, but their late appearance means that humans have been drinking milk for thousands of years before they could digest it.

Thomas et al. Compared Evershed’s dataset on historical milk use in Europe with genetic evidence for elevated lactose intolerance. They found that there was no relationship between changes in milk use over time and increased human ability to tolerate lactose.

For humans who do not digest lactose, it is puzzling because sugary milk components can cause intestinal problems ranging from flatulence to diarrhea. For this reason, people with lactose intolerance do not drink much milk. At least that’s what many people mistakenly assumed.

In fact, a study by co-author George Davy Smith shows that they drink milk, according to his study of UK Biobank data, which includes more than 500,000 living individuals. His analysis found no substantial difference in milk consumption between persistent and non-persistent adults. He also found that most non-persistent milk drinkers had no long-term health effects and did not shorten their lifespan or impair fertility. did. “So why were people dairy when lactase wasn’t lasting?” Evershed asks. “Because they are willing to consume milk and then benefit from their health.”

The discovery of George Davy Smith raised another question for researchers. If people with lactose intolerance could drink milk without major adverse effects, what caused dramatic genetic changes and how many Europeans developed lactose intolerance rapidly?

Several factors must have accelerated the evolution of lactose intolerance, perhaps by making it very important and making it a matter of life and death.

“So I started imagining a scenario where this was the case,” explains Evershed.

Mark Thomas theorizes that famine may have played a major role. He states that most intolerant adults usually do not work very well after drinking milk. “Flat flatulence, diarrhea, it’s not good, it can be unpleasant and embarrassing, but no one has died of lactose intolerance.”

“But if you have diarrhea during severe malnutrition, you have real problems,” he continues. “It is still the leading cause of death in the world today.” If foods like grains run out during famine, non-persistent humans consume more dairy products when they shouldn’t. You may rely on it, which can have the greatest negative impact on their health.

Davey Smith, director of the MRC Integrated Epidemiology Unit at the University of Bristol, has another idea with a similar concept. He theorizes that pathogens played a major role. His research shows that drinking milk does not harm the health of today’s intolerant adults, but it is a potentially major problem among people suffering from bowel disorders, dehydration and other illnesses. is. In an era when humans lived nearby in livestock and lacked proper hygiene, the disease was widespread and severely weakened many who found lactose-induced diarrhea and dehydration to be fatal. It may have been transformed. On the other hand, those who could drink and digest milk had resources to help them pass through.

The team tested these ideas using a model. This suggests that lactase persistence gene mutants increased in populations when affected by famine and pathogens.

The environmental stressors that caused lactose intolerance may have worked together and could have been very different in each of the five different periods known to have evolved in Europe, the Middle East and Africa. ..

“In Europe, it can be about settlement and famine, but in Africa, for example, it can be about drought and a higher illness burden,” says Thomas.

Group methods may also be adopted to find out what happened where humans never developed the ability to digest milk, when common sense suggests they may have. Hmm.

” [Eurasian] People in the grasslands who do not have lactase persistence drink large amounts of milk, “says Shevan Wilkin. “When it evolved in Europe, what was happening where it didn’t evolve?”

Wilkin explains the mysteries of lactose digestion, such as how slow and rapidly evolving lactose intolerance and why heavy lactose consumers, such as grassland dwellers, remain lactose intolerant. He adds that people are emerging with various ideas. Currently, she says, there is a framework that can further investigate these questions.

“It’s a very impressive business, and through it they came up with some ideas that made a lot of sense.”