The gene that enables East Asian adults to drink milk without getting side effects like stomach cramps was probably inherited from Neanderthals, thousands of years before humans started consuming dairy, according to a new study. Therefore, researchers think that this genetic variant may have initially spread through hunter-gatherer populations because it enhanced immune system function, with lactose tolerance just a happy side-effect of its activity.
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Lactase persistence (LP), or lactose tolerance, refers to the ability to digest the sugars in milk beyond infancy, and is thought to have evolved once humans started raising cattle for dairy between 5,000 and 10,000 years ago. However, as the authors of a new study point out, it’s odd that the various genes responsible for LP appear to have increased in human populations many millennia before the advent of animal husbandry.
This indicates that the haplotype – or suite of associated genes – responsible for LP may have originally arisen for a different purpose.
To investigate, the researchers compared thousands of genomes from people all over the world, looking for variants in the lactase gene, known as LCT. In doing so, they discovered a previously unknown haplotype among East Asians, a population with high levels of lactose intolerance.
Though different from the European and African haplotypes associated with LP, this variant appears to influence LCT expression in a similar way. Moreover, it occurs in about 25 percent of East Asians, which is roughly the same proportion that are tolerant to lactose.
According to the researchers, it seems likely that this previously unknown haplotype confers LP among East Asians. However, further analysis revealed that this set of genetic variants originated in Neanderthals, and was introgressed into modern humans when our ancient ancestors hooked up with this extinct hominid lineage during the Pleistocene.
The haplotype then underwent positive selection between 25,000 and 28,000 years ago. However, because humans weren’t yet drinking milk at this point, the researchers suspect that this selection was driven by something other than the need to digest lactose.
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“Given the origin of this haplotype from Neanderthals and the old age for the onset of selection, it is highly unlikely that the reason for positive selection involves LP,” write the study authors.
Importantly, the East Asian haplotype doesn’t just influence the LCT gene, but also alters the expression of other key genes such as DARS1, which plays a crucial role in immune cell development. As it turns out, the European variants linked to LP also affect DARS1, all of which suggests that these genes may have become prominent across ancient human populations because of their immunity benefits.
Based on these findings, the researchers say that “the LCT region in Europeans may not be associated with LP phenotype alone but be associated with an ancient adaptation to famine or increased pathogen exposure.”
And while more work is needed to figure out exactly how these variants influence our biology beyond lactose digestion, the authors conclude that “either there has been selection on [these haplotypes] for different reasons in different populations, or the selection in European and African populations is also not associated with LP.”
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The study is published in the Proceedings of the National Academy of Sciences.
Source Link: Neanderthal DNA May Be The Reason Some East Asian People Can Tolerate Lactose
