David Reich

I'm a geneticist, and I work on human history and how ancient people relate to each other and people living today.

Well, the dream was that when this field started, this ancient DNA field started more than 16 or 17 years ago, that we were going to learn a lot about biology, learn about how people's biology changed over time by getting DNA out of ancient human remains and tracking changes over time.

And that dream has really not been realized since the beginning of this field.

So while the field's been a big success with regard to learning about human history, it's resulted in surprising findings about human migrations, people not being descended from

from the people who lived in the same place hundreds or thousands or tens of thousands of years before, and mixture being common in human history, sex bias processes being common in human history, and things that were not expected from archaeology.

And so the field's been a big success from that perspective.

But what's not been successful is learning about

biology and biological change.

And one big reason for that has been that the sample sizes have been too small.

So when you have a single person's DNA, it provides a tremendous amount of information about history.

And that's because when you look at one person's DNA, it's not a single person.

It's many people.

It's your two parents.

It's your four grandparents.

It's your eight great-grandparents and 16 great-great-grandparents and so on.

And going back in time, thousands, tens of thousands, even hundreds of thousands of ancestors going back in time contributed to people today.

So when you look at the DNA of a single person's genome or a Neanderthal genome, you have effectively tens of thousands of ancestors all represented in your data.

And you can position that individual exquisitely with respect to other people from whom you have data.

But when you are interested in how a particular genetic variant that affects something like your skin pigmentation or affects your ability to digest cow's milk into adulthood or affects a behavioral trait, when you want to see how that changes over time, a single person gives you only one sample or maybe two samples, the one that is in their mother and the one that's in their father.

And so to get a high-resolution picture of how the frequency changes over time, you need to have very big sample sizes of truly very large numbers of people.