Only a small number of human beings live like this today.
But from the time our species evolved some 200,000 years ago until the not too distant past, all of us lived as hunter gatherers.
Then, around 10,000 years ago, people started domesticating animals for food, living in settlements and cultivating crops. These cultural changes have profound biological impacts on our species. And you’re about to encounter one surprising example. It has to do with the familiar food.
I’m talking about milk, the main ingredients, some of our favorite things. Almost all of us can digest it as babies. But the story of how many adults can use it as a food is a fascinating case study. A study of the coevolution of human culture and biology.
All infant mammals can digest milk. In fact, producing milk for babies is a key trait that distinguishes mammals from all other types of animals.
The main sugar in milk, lactose can’t easily pass through the intestinal wall, so cells here make an enzyme called lactase, which breaks lactose into glucose and galactose.
These two simpler sugars can then enter the bloodstream where they can be used for energy.
Around the time young mammals stop drinking milk. Almost all of them stop making lactase so they lose their ability to digest milk. They become lactose intolerant. What typically happens when an adult mammal drinks milk? It’s not pretty. The lactose goes undigested straight through the small intestine to the large intestine. Here, bacteria eat the sugar and can cause cramps, gas, and diarrhea.
It’s a bad idea to offer a bowl of milk to an adult cat.
We only know of one mammal species in which some adults can drink milk without getting sick. Yes, it’s us. Not all of us, but worldwide, about a third of adults can digest lactose. This minority is called lactase persistent because their ability to produce the enzyme that breaks down lactose persists beyond childhood and in fact, throughout their lives. How did lactase persistence come about? Why does it occur only in some people?
I’ve come to University College London to start my quest to find out.
Geneticist Dallas Swallow will show me how to figure out whether someone can digest the sugar in milk. You’re going to do a, uh, lactose tolerance test. I am. The idea is to look to see what the level of glucose is in the blood of the volunteer before the lactose load has been taken of the blood of. After measuring my baseline glucose level. Okay. I now have to chug a liter of milk. You’re allowed to breathe in between. It’s all right. If my body is still making lactase, my blood glucose will shoot up. After I drank the milk. Here’s what happened.
No doubt about it. My lactase enzyme is still working. Where do your family come from? Britain. On my father’s side. Um, Denmark. Holland on my mother’s side, but kind of. Northern Europe. Northern Europe. Okay. You can see, first of all, that most people in Europe are lactase persistent. My family background makes sense.
In only a few regions is a large majority of people lactase persistent?
In other parts of the world, few adults easily digest lactose.
What exactly is different about people who are lactase persistent?
To get a clue. Researchers looked at DNA. They first compared the part of the lactase gene that encodes the enzyme across persistent and non persistent people. They didn’t find a change in the DNA to distinguish the two traits. So what could explain the difference? We know that genes, including lactase, are regulated, turned on or off, dialled up or down by other pieces of DNA that act like switches. In search of a possible mutation in a lactase switch. A research team identified Finnish families that had members who were lactase persistent, as well as those who weren’t.
Statistical geneticist Joe Terwilliger was part of the team. We then looked to see if they shared DNA around the region where the gene was that we knew was affecting the metabolism of lactose. On chromosome two. In and around the lactase gene. A number of shared markers in the DNA allowed Terwilliger and his colleagues to hone in on a segment of DNA likely to contain the lactase persistence mutation.
By comparing this segment based by base across lactase persistent and non persistent individuals, they discovered the critical one base difference a T instead of a C at one non-coding position. The researchers had made an important discovery. They found a mutation that causes lactase persistence in Finns and other Europeans. Do all lactase persistent people carry this mutation? I thought that would be one mutation and that would be it. So we went off, uh, to study, uh, samples from Africa. And to our surprise, we found that the mutation barely existed. Was a different mutation at work on this continent. Then a young professor, geneticist Sarah Skov, traveled to a number of African countries to find out. We’ve now looked at Tanzania, Kenya and Sudan and Ethiopia. And so we’ve really looked at a broad range of