David Reich

And at this point, there's a number of these mixture events that seem increasingly implausible.

They feel to me a little bit like, I don't know if you know the history of models of how the Earth and the Sun relate to each other in ancient Greek times, but there's these epicycles that were attached by

the Greek Hellenistic astronomer Ptolemy to make it still possible to describe the movements of the planets and the stars, given that a model where the Sun revolved around the Earth.

And we've added all of these epicycles to make things fit.

And one wonders whether there's some pretty fundamental differences that might explain the patterns that are observed.

So just to give you an example of this, the standard model is basically this, that modern humans separated from a group that is ancestral to Denisovans and Neanderthals, these two groups for which we have sequences, somewhere between maybe 500,000 to 750,000 years ago.

That's what the genetic papers beginning in about 2012 and 2014 say.

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.