As early humans shifted from hunting and gathering to agriculture, their genomes shifted, too. But according to new research, the adaptations that allow us to enjoy dairy products without digestion problems may have arisen much later than expected in some populations.
The new study, published Tuesday in Nature Communications, sequenced the genetic information of 13 individuals who lived on the Great Hungarian Plain during the 5,000 years leading up to the Iron Age. The area represents a good cross-section of human development at the time, the researchers reported, showing a transition from hunter-gatherer cultures to farming.
In studying the genetic information of these individuals, the researchers found that shifts in culture could be pegged to corresponding shifts in genes. In other words, new practices – like farming or heavy-metal working –didn't appear from nowhere.
"The genomes do seem to shift as new technologies come about," co-author and Trinity College Dublin professor Daniel Bradley said. "You can't look at this and think that farming and metallurgy are technologies that come into the culture by osmosis. They come with people. Genomes and technology migrate together."
More surprising were the findings related to lactose tolerance. "We thought we'd look at some genes that had been previously discussed as being important, that we knew human populations had selected for during the course of human pre-history," Bradley said.
In Europe – and particularly in Ireland, Bradley's neck of the woods – lactose tolerance, or the ability to break down the sugar in non-human milk, is incredibly common. But it's not something that humans had in the early days of our evolution. A genetic variation allows humans to digest lactose.
"Ireland is the place in the world with the highest concentration of lactose tolerance," Bradley said, "and undoubtedly that’s to do with a heavy reliance on drinking unprocessed milk in pre-history, and a culture focused on dairying."
Previous research had estimated that this variation must have cropped up about 7,000 years ago or more, when evidence of cheese-making became abundant. But Bradley and his colleagues didn't find the variation in their Hungarian DNA samples until 3,000 years back.
So it could be that the presence of milking wasn't enough to spur genetic change, and that populations had to rely heavily on dairy before individuals adapted to tolerate it in abundance.
Thirteen individuals' worth of DNA might not sound like a lot, but Bradley and his colleagues were able to pull an unprecedented amount of information from their remains. By using the densest bone in the human body (one found in the inner ear) they were able to recover from 40 to 87 percent of each individual's genetic information –compared to the 1 percent that most bone samples yield.
"Our methodology really opens up the possibility of the genomic analysis of human remains," Bradley said. And it's possible that with more data, his team's findings on lactose intolerance will prove to be incomplete. "We'll have to wait and see," he said. "It will be very interesting to see what happens elsewhere."