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.