It Took Humans 5,000 Years to Evolve the Ability to Digest Dairy

A new DNA sequencing technique has allowed scientists to study ancient genomes with better accuracy.

Oct 21 2014, 4:15pm

Image: Graeme Maclean/Flickr

If you are one of the lucky few people who are able to digest lactose, you have roughly 5,000 years of human suffering to thank: A new genetic analysis has found that our ancestors probably didn't have the alleles necessary to process the stuff for most of recorded human history.

The finding is exciting primarily because of the fact that we discovered it at all: It's one of the first times that ancient human DNA has been sequenced, and it's thanks to a new technique of gathering it by researchers at the University College Dublin and Trinity College Dublin.

In the past, most of the DNA that was sequenced from ancient bones was actually from bacteria that had overtaken the fossil, leaving scientists struggling to sort out what DNA was from what organism. But by drilling into the inner ear's petrous bone, researchers were able to get genetic samples that are roughly 100 times better than what's been previously found.

One of the burial sites where skeletons were discovered. Image: Janos Dani

"That inner ear region is extremely dense" and thus protected from bacteria, Ron Pinhasi, one of the authors of a study published in  Nature Communications said in a video explaining the find. "We were hoping to double or triple the yield. We got more than 100 times better."

The better quality DNA allowed Pinhasi and his colleague, Daniel Bradley, to more closely sequence the genomes of a series of 25 ancient humans who lived in the Great Hungarian Plain between 6,000 BC and 1,000 BC. 

Interestingly, the ability to digest lactose didn't show up until the Bronze Age, at which point it took off because, well, people were eating a lot of cheese around that time and it was a distinct advantage. In fact, people had been eating cheese for thousands and thousands of years (at least in this particular population), all without the ability to break down dairy.

The researchers suggested in the paper that they saw the "strongest signals of selection" in the gene that controls for lactose digestion. That means its development was probably quite important to population survival in the area, an idea that scientists have posited before, because dying of diarrhea and dehydration isn't conducive to reproduction.

"It has been postulated that this allele first underwent selection 5,500 years BC," they wrote. "Here, in our temporal sequence its appearance is delayed until the more recent of our Bronze Age individuals, who lived [around the year 1,000 BC]."

The finding suggests that it took more than 5,000 years to evolve the trait. Because just a single allele controls for the ability to digest lactose, it was a good one to look for in the 25 humans. Bradley says in the future, we'll be able to tell how and when other traits evolved in humans.

"The aim of [paleogenomics] isn't just to sequence just small bits of DNA, but whole genomes," he said. "You get very deep information about the characteristics of an ancient person, which population [today] he or she most resembles. It has opened a new vista of deep information in the past."

Of course, lactose intolerance is still a very prevalent trait: Some studies have suggested that as many as 60 percent of the world's people are still unable to digest lactose. As many as 90 percent of Northern Europeans are able to digest the stuff, which makes sense once you consider that milk drinking is believed to have originated in Central Europe roughly 7,500 years ago.

These Hungarians, and many, many of their ancestors, it turns out, probably suffered many upset stomachs before we got to where we are today.