A large proportion of East Asian people carry unique versions of the lactase gene that enables humans to digest the sugars in milk, new research hints.
These genetic variants were likely inherited from humans’ extinct cousins, the Neanderthals, tens of thousands of years ago, according to a study published Mar. 10 in the journal PNAS. And it may be that the variants offered an evolutionary advantage by helping early hunter-gatherers thwart infections, new data suggest.
In some people of European or African descent, the lactase gene helps carriers digest a sugar called lactose in milk after they’re weaned off of breast milk; this phenomenon is known as “lactase persistence.” Many people who carry alternative versions of the gene can’t continue breaking down lactose after infancy, so they become lactose intolerant.
The versions of the lactase gene behind lactase persistence are believed to have emerged in populations in Europe and Africa between 5,000 and 10,000 years ago, around the same time humans on these continents began rearing animals and consuming their milk.
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This has led scientists to theorize that the lactose-tolerance variants were selected for within these populations, meaning they became more common over the course of evolution because they offered an advantage. In this case, they may have increased survival rates by allowing adults to absorb nutrients from milk.
However, until now, little was known about the genetic mechanisms behind lactase persistence and lactose intolerance in East Asians. About 65% of adults worldwide are thought to be lactose intolerant, but this figure rises to between 70% and 100% in East Asian populations.
In the new study, scientists compared thousands of genomes sampled from modern humans, including those with East Asian, European or African ancestry. The data came from individuals in a range of countries, such as China, Japan, Spain, Italy and Nigeria.
The analysis revealed that around 25% of the people with East Asian ancestry carried versions of the lactase gene that are not found within the European or African populations. That’s around the same proportion of people who have lactase persistence in East Asian populations. These genetic variants trigger an increase in the activity of the lactase gene in the body, the researchers found.
At first glance, this suggests that these genetic variants may have also been selected for during evolution for dietary reasons, the researchers said. However, further analyses in the same study revealed that this may not have been the case.
Indeed, the researchers went on to compare samples of East Asian genomes with the genome of a Neanderthal who lived in the Altai Mountains in Siberia around 120,000 years ago. This led them to discover that the East Asian variants of the lactase gene were likely inherited by hunter-gatherers from Neanderthals as a result of interbreeding events between the two groups.
Neanderthals and Homo sapiens are known to have interbred on multiple occasions over the millennia. This happened after both species came into contact with one another following the migration of the latter group out of Africa into Eurasia, possibly up to 250,000 years ago.
The new results suggest that the lactose-intolerance variants started being selected for between 25,000 and 28,000 years ago — more than 10,000 years before dairy culture emerged in the Altai Mountain region.
In other words, the variants emerged before people in the region started regularly consuming milk. This suggests that the selective pressure on these variants was not related to improving people’s ability to digest lactose.
To figure out what other advantages these variants might offer, the team scoured a database chronicling gene activity in different cells. They discovered that the lactase gene variants seen in East Asian people altered the activity of three genes in immune cells, causing them to expand in number.
Based on this result, the study authors think it’s possible that these variants were selected for because they somehow enhanced the ability of East Asian hunter-gatherers to combat infections.
“Neanderthals — having inhabited Eurasia for approximately 400,000 years — likely carried alleles [genetic variants] adapted to local pathogens and environmental challenges,” study co-author Shuhua Xu, a professor of human population genetics at Fudan University in China, told Live Science in an email.
Taken together, the findings imply that variants of the lactase gene may have been selected for different reasons across the world, the researchers concluded. These findings may prompt scientists to question why selection of the lactase gene occurred in European and African populations, they added; perhaps the answer isn’t solely about drinking milk.
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