David Reich

Well, I think what's very interesting is that what we have data from now are modern humans, the sequences of people living today.

And we also have data from Neanderthals who are archaic humans who lived in Western Eurasia for the last couple of hundred thousand years.

And we have now sequences from many Neanderthals.

And we also have DNA from Denisovans.

Denisovans are archaic humans who were discovered from the DNA.

from a finger bone that was found in a cave in Siberia, not anticipated to be a new group of humans, but were sequenced.

So we have DNA from these different sources plus bits of DNA from these sources mixed into modern populations.

And based on this, in the last 10 years or 14 years, we collectively have been piecing together an understanding of how

modern humans are related to our closest relatives who are now no longer with us in unmixed form, the Neanderthals, Denisovans, and maybe others who are no longer, not yet sampled.

And the model that we have is really a model based on accretion.

So we start with the modern humans and then we add the Neanderthals once we obtain that sequence and we add the Denisovans.

And then the model doesn't quite fit and we add...

other mixture events to make the model fit.

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.