Milk and the Modern Man
The rise of adult milk drinking
Like a lot of people, I love ice cream. And I am one of the few lucky adults who can enjoy it.
But wait a minute. Can't most adults drink milk and enjoy ice cream? No.
Here in the U.S. 90% of adults can drink milk. But around 75% of the world's adults can't. They are lactose intolerant.
Of course, since this is a majority, lactose intolerance isn't a very good name. This is the normal condition. The 25% of adults who can drink milk have lactase persistence.
Note that the "o" in lactose changed to an "a" in lactase. Lactose is a sugar in milk. Lactase is the protein we make that breaks that sugar down.
Almost all of us start out being able to drink milk. But over time, the majority of people lose the ability.
Why is this? How do people lose the ability to drink milk? And why can Northern Europeans and a few other groups drink milk when almost no one in East Asia can?
The answer has to do with genes of course. Almost all of us have the gene for making lactase. The gene just stops working in most adults.
See, genes are really just recipes for proteins. The lactase gene is just a recipe for the lactase protein.
Just like a real recipe, the lactase gene needs to be read and its directions followed to have any lactase made. A spaghetti recipe in a book is not the same thing as a spaghetti dinner. And a lactase gene is not the same thing as lactase.
So we all start out with our cells being able to read and follow the recipe of the lactase gene. Over time, most of us lose that ability. And so can't enjoy ice cream anymore.
The first new research tries to figure out what DNA changes are responsible for the lactase gene staying on in some adults. And the second bit of research tries to figure out when this useful trait became common in some populations.
Why some adults can drink milk
As I said, most of our cells stop being able to read the lactase gene as an adult. For a few of us, our cells keep on reading. Why is this?
A few years back scientists found the DNA change responsible for keeping the lactase gene on in many Northern Europeans. It was in a part of the DNA a good distance from the recipe itself.
There are other populations that can drink milk as adults too. Some researchers looked to see what DNA changes were responsible for the ability of many East Africans to drink milk.
What they found were DNA changes in a very similar location to the ones found in Europeans. These changes were different but near one another.
This tells us that this is a very important area of the DNA that is involved in somehow shutting off the lactase gene as humans grow older. And that the changes that kept the gene on evolved independently in the various populations.
Why do these DNA changes cause the lactase gene to stay on? No one knows for sure but one idea is that it keeps a repressor from sticking to the DNA.
Repressors are special proteins made by a cell. They stick to the DNA and hide the presence of a gene.
These proteins help to make sure that the right genes are on in the right cells. Imagine that as we get older, a repressor gets made that shuts off the lactase gene. How does this repressor know where to stick normally? And why can't it stick in people with lactase persistence?
A repressor knows where to land on DNA because it likes certain DNA sequences. Something like GAATTC for example. If this set of letters is near a gene, the repressor binds and hides the gene from the cell.
These proteins are incredibly fussy. They won't stick particularly well to GGATTC for example. So a small DNA change like this might make the repressor no longer bind. And so the cell can keep reading the gene.
Perhaps this is what these clustered DNA changes are doing in people who can drink milk as adults. The repressor still gets made in these people but can no longer bind to the altered DNA. So the lactase gene stays on and these folks can keep enjoying their Ben and Jerry's!
Lactase persistence may not have been common in Europe 5000 years ago
So when did some of our ancestors gain the ability to drink milk as an adult? Certainly it isn't the norm. Which means that at some point in our past, the ability to drink milk as an adult must have arisen.
One idea is that the trait has always been in some populations. And so these populations developed dairy industries.
Another idea is that there were always a few folks who kept making lactase. But when people began drinking milk, these people had a huge advantage and outbred their lactose intolerant neighbors. Soon most people in these populations were lactase persistent.
Some genetic work suggested that the selection model was the right one. But this evidence is indirect. Scientists looked at the DNA around the change and from that estimated how quickly lactase persistence spread through a population.
But some scientists wanted a more direct test. So researchers decided to do just that. How? By looking at the DNA of Europeans who died thousands of years ago.
The researchers got good DNA reads from 10 different skeletons. One skeleton was from about 500 years ago, eight were from about 5000-6000 years ago. And one was from about 2300 years ago.
Of these, only the 500 year old skeleton showed the DNA change. All of the other skeletons had DNA consistent with lactose intolerance.
These data suggest that lactose persistence wasn't yet common in Europe 5000 years ago. Now of course this is a very small sample. And although they did their best to rule out contamination, you never know with historical DNA like this.
But it is a start. And an important one. Once they look at many more skeletons, scientists may be able to have a better handle on when lactase persistence became common.
They may also be able to figure out when the paleness of Northern Europeans became more common. Or their red hair or blue eyes. Or sickle cell anemia in African populations. Or
As more genes become identified with traits, we will be able to learn even more about when they became common. It is so exciting to be able to look at our ancestors' DNA and learn about our history.