IRA FLATOW: This is “Science Friday.” I’m Ira Flatow. This week one of the rituals of science, featuring the iconic early morning phone call.

SANTE PAABO: I first thought this is probably an elaborate prank you’ve done by people in my group. But then it sounded a little bit too serious to me. So I sort of accepted the fact.

IRA FLATOW: That’s Dr. Svante Paabo He’s the Director of the Max Planck Institute for Evolutionary Anthropology in Leipzig and winner of this year’s Nobel Prize for Physiology or Medicine. The Prize Committee awarded the prize, quote, “for his discoveries concerning the genomes of extinct hominins and human evolution.” What that means, he found that genes from ancient Neanderthals reside in people today.

SANTE PAABO: So what we do is to look for the genetic material, for DNA, from people who have lived here long before us and try to see how they are related to us and how they are related to other forms of humans that were also here, such as Neanderthals.

IRA FLATOW: He’s been a guest on this program several times over the years. Back in 2017, I talked with Dr. Paabo about his work on trying to pin down the answer to the question, how much of our DNA comes from Neanderthal ancestors. He spoke to me by phone.

How many people are descendants of Neanderthals then?

SANTE PAABO: Well so, everybody whose genetic roots are outside Africa are partly descended from Neanderthals. So there are billions of people, in the order of six, seven billion people who actually carry parts of the genome of Neanderthals today.

IRA FLATOW: And how did how did we come to that 2% number and then raising it?

SANTE PAABO: Back in 2014, we published high quality Neanderthal genome. But that genome came from Southern Siberia, so very far east in the distribution of Neanderthals. So we were, of course, aware that genome was not really close geographically to where Neanderthals probably met early modern humans when they came out of Africa. So what we have now done is to sequence the genome from Southern Europe, from Croatia, that is also closer in time to when that interbreeding might have happened. It’s between 50,000 to 65,000 years old.

And indeed we can show that this individual from which it comes was quite substantially closer related to the Neanderthals that interbred with modern humans than the Siberian ones. And that results in that we can identify more pieces, more fragments of DNA in people today that come from Neanderthals, in the order of 10% to 15% more than we had earlier. That’s in the order of 4 million more base pairs per individual that we identify.

IRA FLATOW: Are we more interested than in the Neanderthals because we want to know about them or because what they can tell us about who we are?

SANTE PAABO: Well, I would say that I would be interested in both things. Of course, it is quite interesting to know what aspects of our physiology today derive from Neanderthals. In what ways do they live on in us, if you like. But these variants may also allow us at least in the future to tell us more about what they were like.

IRA FLATOW: So how is it that just two genetic sequences, we’re talking about two Neanderthals now, tell us so much about a whole species of hominid.

SANTE PAABO: Well, of course that is because when you have a whole genome from an individual, you have of course two versions of that genome, the version that individual inherited from the mother and from the father. So, when we are sequenced to high quality now, so we see both chromosomes in an individual, we have four genomes in reality. And we can then get a fairly good idea, particularly when they come from different parts of the distribution, geographic distribution like this, of the variation in the species.

IRA FLATOW: How come you were able to get such a good, high quality sequence?

SANTE PAABO: We have looked through a lot of bones from this site in Croatia, this cave. We have analyzed 19 different bone fragments to identify the ones that have the most Neanderthal DNA, but at the same time the lowest proportion of bacterial DNA in the bone from soil bacteria that lived in the bone when it was in the ground. So this particular bone, there were parts of it that up to 10% of the DNA actually were of Neanderthal origin.

And then we use techniques that we have developed in our lab over 20 years now to extract as efficiently as possible the DNA, process it in a way that we can feed it into sequencing machines, and then map it to the human genome, see what these short fragments we get will fit in the genome.

IRA FLATOW: Dr. Paabo, what are you working on now? Do you have anything in the pipeline that you’re sequencing?

SANTE PAABO: So one direction is to try to go back further in time. So, the oldest sort of hominin remains we’ve been able to get tiny amounts of DNA from is over 400,000 years old, that’s some early Neanderthal ancestor from Spain. So we’re trying to get more DNA there. And the other direction is to try to understand what’s special about modern human genomes, what are those variants that made it possible for us and not the Neanderthals to develop technology and culture that allowed us to expand and colonize the whole world and compete so successfully and detrimentally in from the point of view of this other hominids that became extinct?

IRA FLATOW: On Monday, Dr. Paabo compared his work to an archeological dig.

SANTE PAABO: What really drives our work is really curiosity, I would say. It is just as if you do an archeological excavation to find out about the past, we sort of make excavations in the human genome. What my own group and my biggest interest is actually to study the genetic differences between present day people and our closest relatives in Neanderthals, particularly genetic changes that exist in everybody today or almost everybody. And that may be important for why modern humans became so numerous, formed big societies, and so on.

IRA FLATOW: Dr. Svante Paabo, winner of this year’s Nobel Prize for Physiology or Medicine. Congratulations to all of this year’s winners. We hope to talk with more of them in the weeks ahead. I’m Ira Flatow and this is “Science Friday” from WNYC Studios.

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