January 23, 2014
Study Re-examines The Evolutionary Origins Of Lactose Tolerance
Ranjini Raghunath for redOrbit.com - Your Universe Online
Life in current times sure beats living in the Stone Ages. But there is one trait that many modern humans often take for granted - a trait that their primitive ancestors didn’t have: lactose persistence, or the ability to drink milk and not get cramps, flatulence or diarrhea.How and why humans evolved this ability to tolerate lactose has long fascinated evolutionary biologists, as milk has now become a dietary staple throughout much of the world. Most modern humans of European descent are lactose persistent, which means they can produce the enzyme lactase which breaks down the milk sugar lactose into glucose during digestion.
Our Stone Age ancestors, however, didn’t have this luxury. Lactose persistence has evolved only over the last ten thousand years. The secret lies in the genes: over time, European descendents have developed a unique mutation in their genes that allows them to produce the enzyme lactase.
Archaeologists have shown that the ability first evolved in farming populations. A popular theory fielded by biologists is that natural selection favored its evolution so farming populations would not succumb to calcium deficiency. This was especially likely among early European farmers who didn’t get much sunlight which helps the body naturally synthesize vitamin D. In turn, vitamin D is needed for the body to absorb calcium. Milk fortunately has plenty of both calcium and vitamins.
Now, a new study by researchers at Uppsala University and Stockholm University, Sweden shows that the calcium-and-sunlight theory might not give the entire picture.
One question that the theory raises is: what about people who used to live in sunny regions such as Spain? They had all the vitamin D they ever needed. Why did they also need to develop this ability to digest milk if there was no evolutionary pressure to do so?
“If natural selection is driving lactase persistence evolution in a place where people have no problems making vitamin D in their skin, then clearly the vitamin D and calcium explanation (known as the calcium assimilation hypothesis) isn't cutting it,” said Oddný Sverrisdóttir, lead author of the paper said. “So while the calcium assimilation hypothesis may have some relevance in Northern Europe it's clearly not the whole story.”
To test the theory, the researchers carefully recovered DNA samples from the bones of ancient Spanish farmers and examined them. To their surprise, they found that the lactose mutation was absent in their genes, although most Spanish descendants today do have lactose persistence.
“The evolution of lactase persistence is one of the best known and most dramatic examples of recent human evolution. One of the ironies of working in this area is that we know it happened but we still don't fully know why,” Sverrisdóttir stated.
He and his colleagues have proposed an alternate explanation. Famine could have been the trigger, they suggest in their paper published in the journal Molecular Biology and Evolution.
Before early European farmers developed this ability, when they couldn’t drink milk without showing the related symptoms, they could still opt for fermented milk products such as yogurt and cheese. This is because, during fermentation, most of the lactose gets converted into fats.
When crops failed and famine hit, however, they most likely ran out of their fermented products supply and had no choice but to go for milk and other high-lactose products. Given a choice between dying from hunger and diarrhea from milk, they most likely chose the latter. Over time, they probably began to develop a stronger stomach and a tolerance to the effects of milk, the researchers believe.