David Reich

And it's tempting to think that both the Neanderthal mitochondrial DNA and Y chromosome come from that event.

But the probability of that happening by chance is only 5% squared, which is a very, very small number.

And people have evoked epicycles, for example, natural selection for the mitochondrial DNA coming from modern humans

natural selection coming from the Y chromosome coming from modern humans, somehow being more advantageous and pushed up in frequency.

But that would have to really happen on both these parts of the genome to produce this pattern.

And it just seems surprising.

So what's been put together is a complicated model and

epicycles, ideas like natural selection, to kind of make it work.

It's not impossible.

It may be the case.

But one wonders whether profoundly different models might actually explain the data.

And so that's something that we and others have been thinking about.

Can there be other models?

An example of another model that might be able to explain the data that we've been playing with is one where there's much more DNA in Neanderthals from modern humans than the three or 5% that's been estimated.

And we can get such models to fit, but here it's 30% or 50% or 70%.

So in that view, Neanderthals and Denisovans are not sisters.

In fact, modern humans and Neanderthals are just as qualified to be sisters as Neanderthals and Denisovans.

And in that case, maybe it's not clear what's modern and what's archaic.

Are modern humans archaic?

Are modern humans modern?