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Famine and Disease Helped Milk Digestion in Humans: New Study

The researchers propose that lactose tolerance was enhanced during events such as famines and diseases where lactose intolerant people would die compared to people who got the beneficial gene variants.
milk

Representational use only.Image Courtesy: Wikimedia.Org

Even when many of us have lactose intolerance and find it difficult to digest milk, consuming and digesting it appears to be something that does not attract much attention; however, milk digestion has remained an active field of research for scientists. How humans have evolved the capacity to digest milk and the multitudes of factors involved in it have attracted researchers for years.

Dairy farming in Europe started thousands of years prior that people could develop the ability to drink milk without getting ill. Scientists have been intrigued about how this happened.

Now, recent research published in Nature adds different aspects to these questions. The researchers claim that lactose tolerance (the ability to digest milk) was evolutionarily beneficial only during occasional events of famines and disease. They believe this happened for thousands of years before milk consumption became widespread. The researchers analysed thousands of pottery shards and hundreds of ancient genome analyses coupled with modelling studies to decipher how milk consumption became common among Europeans when dairy farming started.

The ability to digest milk is also known as lactase persistence. An enzyme called lactase can break down the milk sugar molecules, and this stops after young children are weaned. Experts believe that the study reveals new aspects of lactase persistence evolution, which seems to be much more complicated than previously thought.

Digesting milk evolved among different populations in the ancient world independently. The researchers mapped the genetic basis which instructs human cells to produce lactase. It is worth mentioning here that proteins are produced in cells according to the codes or instructions embedded in the genes. The enzyme ‘Lactase’ is a protein, and the researchers studied the gene variations that could instruct lactase production in early populations.

Earlier, one theory suggested that the gene variations proved helpful only after people in the ancient period started consuming milk routinely. The other strong theory suggests that the cattle domesticated as early as 10,000-12,000 years back were used mainly for their meat and consumption of their milk developed millennia later.

Richard Evershed, the lead author of the latest Nature study and a biogeochemist of the University of Bristol, the UK and his team, extracted fat milk residues on ancient potsherds that dated back to the advent of domesticating the animals. On the other hand, genomic studies on ancient animal farmers revealed their lactose intolerance. Lactose tolerance (and hence digesting milk) was not common among the Europeans until 5,000 to 4,000 years ago, which is the period of the bronze age.

Evershed and his team collated archaeological and genomic data and then modelled them to understand how lactose tolerance evolved. They analysed more than 1,700 genomes of ancient Eurasian people.

The team found less overlap between leaps in lactose tolerance and increased milk consumption. This was inferred by the milk fat found in about 13,000 potsherds collected from more than 550 archaeological sites.

Mark Thomas, a co-author of the study and an evolutionary geneticist at the University College, London, who collated archaeological and genomic data, commented, “All previous hypotheses for what was the natural-selection advantage of lactase persistence pegged themselves to the extent of milk use, because of the presumed nutritional benefits of being able to consume milk without getting sick.”

The researchers attempted to see how lactose tolerance is related to milk consumption in modern Europeans. After analysing the UK Biobank, which is a repository of genomic and health data of half a million people in the UK, the researchers found that there is little correlation between lactose tolerance and milk consumption, and 92% of the lactose intolerant people were found to prefer fresh milk over other alternatives. Interestingly, lactose tolerance showed no clear benefit to health or fertility that could drive natural selection.

The modelling that the researchers developed in the study suggested lactase persistence as more likely to have developed among ancient people exposed to animal pathogens and famine than those exposed to other factors that they examined. The researchers propose that lactose tolerance was enhanced during such events as famines and diseases where lactose intolerant people would die compared to people who got the beneficial gene variants.

However, experts are not fully accepting what the new research suggests. They say that many ifs and buts and further exploration of the topic are needed before coming to anything conclusive.

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