David Reich

So the average time to the common ancestor of any two human genes is one or two million years ago.

So if you look at a bit of your DNA that you get from your mother and the same bit of your DNA on the same chromosome, the copy of chromosome three you get from your mother and the copy of chromosome three you get from your father, typical time they share a common ancestor is one or two million years ago.

That's before the split from Neanderthals and Denisovans.

So there's many places in your DNA where you're more closely related to a Neanderthal on your mother's side than you are to your father.

It's the same reason that if you have a sister, you're in some places in your DNA more closely related to her than you are to me because you share a parent.

But in other places, you're more closely related to me than you are to your sister because you happen not to share the same DNA from your parents.

It's just that the DNA that we get from our common ancestral population was already quite variable 500,000 years ago, 700,000 years ago, a million years ago.

And some of us descend from some of those ancestors and others of us descend from other of those ancestors.

And Neanderthals split from our lineage really close in time on human evolutionary timescale, such that in some places in our DNA, we're more closely related to Neanderthals than to each other.

I think that's the main thing that I'm thinking about a lot these days.

You know, I think that I'm really continue to be very obsessed with questions about the spread of human populations around the world and trying to reconstruct that with ancient DNA.

The thing I'm thinking about a lot recently is the possibility that maybe we're not thinking in the right way about the relationship between archaic and modern humans.

So the standard model is one like this, where Denisovans, these archaic humans that were found from ancient DNA, and Neanderthals,

descend from a common ancestral populations five or 600,000 years ago.

And that these two separate earlier, maybe 700 to 800,000 years ago from the ancestors of modern humans, people like us.

So that's the big results of a lot of studies since 2010.

But there's also evidence of interbreeding events that happened maybe 200 to 300,000 years ago.

And that actually resulted in modern humans

contributing DNA to the ancestors of Neanderthals.

So this is maybe 5% of the DNA of Neanderthals comes from this interbreeding event, and a lot of studies have shown this.