David Reich

said, and that's still used as the explanation for the vast majority of the genealogies, the DNA lineages connecting them.

So maybe except for 5% of the DNA, that's what we think is going on.

Modern humans are one group, and then there's a sister of modern humans, the Denisva Neanderthal group, and they separated 500 to 750,000 years ago.

But what's become very, very clear in a really important series of papers since that time is that, in fact, there are exceptions to this.

And one exception to this is the mitochondrial sequence, what you get from your mother and she gets from her mother and so on going back in time.

And there, the shared ancestor between Neanderthals and modern humans is only maybe three or 400,000 years ago, which is after the split that's very well estimated from the whole genome.

And what we've also learned is that's also true for the Y chromosome.

So that's inherited from your father and his father and so on.

And that too is only maybe three or 400,000 years separated between Neanderthals and modern humans.

And like the mitochondrial DNA, the Denisovans are much more distant maybe.

800,000 years, 700,000 years, a million years.

So the story told by these two parts of the genome is one that's really, really different from the rest of the genome and incompatible with the main story, too recent sharing.

And we know in these papers that maybe a few percent, 5%, 3%, 8% of the DNA of Neanderthals comes from a gene flow event, a migration event into the ancestors of Neanderthals from the modern human lineage a few hundred thousand years ago.

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