Nobel Prize lecture Svante Pääbo Nobel Prize in Physiology or Medicine 2022

Nobel laureates, distinguished guests, ladies and gentlemen, it's a great pleasure and privilege for me to welcome you all to the 2022 Nobel lecture in physiology or medicine. My name is Ole Petter Otterson, I'm the president of Karolinska Institutet. During the Nobel week we celebrate science and we rejoice at the unique contributions of some exceptional individuals whose discoveries in the words of Alfred Nobel, and I quote, have conferred the greatest benefit to humankind, unquote. Within our field, physiology and medicine, the ultimate goal is to achieve scientific breakthroughs that significantly deepen our understanding of health and disease. And today, I might add, deepen our understanding of our origin as human beings. The discoveries we celebrate this year are stellar examples of such breakthroughs. Today, it's an honor for me to extend my warmest congratulations on behalf of Karolinska Institutet to Professor Svante Pebo, awarded the 2022 Nobel prize in physiology or medicine, and I quote, for his discoveries concerning the genomes of extinct hominins and human evolution, unquote. It will be very exciting and timely to hear more about your important findings today. And without further ado, I now give the floor to Gunilla Karlsson-Hedestam, professor of vaccine immunology and a member of the Nobel assembly. She will introduce this year's laureate. Esteemed Nobel laureates, dear colleagues and friends, dear students, warm welcome to this Nobel lecture 2022. It is a great pleasure to introduce this year's laureate, Professor Svante Pebo, who is awarded the Nobel prize in physiology or medicine for his discoveries concerning the genomes of extinct hominins and human evolution. For this year's Nobel lecture, we have the laureate present in person for the first time in three years. We also have the honor of having several of the laureates of the previous two years in the audience. It is really wonderful to finally have you here present in person. So today it is really with great excitement that we look forward to listening to Dr. Pebo's work and his account of his past and present work in an area of research that I think is of unusually broad interest, namely human evolution. Svante Pebo was born in Stockholm in 1955. Following his high school graduation, a year of military service waited. Serving a year in the Swedish defense forces was compulsory for all 18-year-old men in Sweden at that time, and it was something that was often met with mixed feelings. Svante Pebo was among the few new recruits that was selected to the School of Interpreters. There he learned the basics of the Russian language, something that turned out to be surprisingly useful later on in his scientific career. Following his military service, he applied to Uppsala University. He was interested in a broad range of subjects in the humanities, and he enrolled in programs to study the history of sciences, Egyptology, and Russia, Russian. His interest in Egyptology had started at a young age when he visited Egypt with his mother, and he described this interest as a youthful romantic fascination. But it really turned out to be quite formative in terms of his later research path. After a couple of years immersed in studying these subjects, he started to gravitate towards medicine, which was the profession and research area of his father. He applied to medical school at Uppsala University with the intention of eventually pursuing a research career. During his full-time studies of medicine, he continued taking courses in subjects related to his other interests. For example, he taught himself the Coptic language. This is an historical language of the Christian Egypt. Knowing what we know today about the career of Svante Pebo, it seems quite clear that without this diverse academic interests and the environment that he created for himself in Uppsala, and his adventurous mind, we probably would not be here today. I think this is a great reminder that nurturing several interests in different topics can really have unexpected benefits. In 1981, after four years in medical school, Pebo joined the group of Per Pertusson, whose lab investigated how our immune system recognizes infections and how viruses interfere with this process. Per Pertusson's lab was world-leading and buzzing with activity. It was one of the first labs in Sweden who employed the new molecular biology techniques that had become so popular and had really revolutionized the field of medical research around the world. The possibilities seemed endless, and Pebo published in a relatively short period of time, not one but three papers in Cell, along with many other papers. He also became a close collaborator of Walter Schaffner. He had developed methods to express recombinant proteins in mammalian cells. This was quite new at the time, and something that was very useful in terms of addressing the questions that Svante Pebo was assigned to study as a doctoral student. But in parallel with his official PhD project, Svante Pebo started to wonder if he could not apply these powerful molecular biology techniques to study DNA in samples extracted from historical specimens. Thanks to his many contacts, for example he had become a close friend of the professor in Egyptology in Uppsala at the time, he obtained samples from Egyptian mummies and he extracted DNA from these. He did all this work outside of the normal hours and without the knowledge of his official PhD supervisor. This led to two major realizations. First, an encouraging one. It was possible to extract DNA from very old samples. Second, a more discouraging one. The DNA of that age was massively contaminated with other sources of DNA. It was chemically modified and highly degraded into small fragments. This led him down a path of extensive methods development. So it did not put him off, but instead he did the work that he knew was required for him to succeed in his goals of analyzing DNA from very old and even extinct species. After his PhD degree and a short fellowship in the lab of Walter Schaffner at the University of Zurich, Pebo moved to the University of California, Berkeley, where he joined the laboratory of Alan Wilson. This was another highly dynamic environment and really an epicenter of human population genetics and evolutionary biology. There Svante Pebo worked on a variety of extinct non-human mammals. He knew that this would help him develop the necessary methods as their DNA would not be confused with DNA from humans handling the specimens, which was the case if you worked with ancient human DNA. His timing in Alan Wilson was really perfect because PCR had just been invented. This greatly facilitated the analysis of small amounts of DNA. It also presented new problems that had to be solved. And throughout his career, his attention to detail has given him a reputation of being an exceptionally critical and careful scientist. The methods developed during this time and throughout his career form the basis for how laboratories around the world work with ancient DNA today. And this field is now flourishing. In 1990, he returned to Europe to become the professor of biology at the University of Munich. And in 1997, he was invited to become the founding director of a brand new Max Planck Institute of Evolutionary Anthropology in Leipzig, Germany. As the founder, he had the opportunity to shape the institute in a way that he believed would yield the most interesting science. He and his co-directors built a research institute that is truly cross-disciplinary and has been and is very successful. One of Svante Perbo's main research goals was to study the genetics of one of our closest relatives, the Neanderthals. In 1997, he reported the successful sequencing of the Neanderthal mitochondrial DNA. And in 2006, he announced a plan to sequence also the nuclear genome, which was very ambitious and seemed almost impossible to most people at the time. Given that Neanderthals went extinct over 30,000 years ago, this was an enormous technical challenge. But Perbo was now spurred on by the new developments in massive parallel sequencing, high-throughput sequencing techniques that were emerging around this time. I think this ability to adapt and to employ the most powerful technologies to address the questions that need to be addressed is truly a hallmark of Svante Perbo, as was his ability to collaborate and attract the expertise he needed in terms of his colleagues and co-workers and collaborators. His team now grew and it relied increasingly on co-workers with computational skills, as this was required to analyse the data generated by high-throughput sequencing. In 2010, Perbo reported the draft Neanderthal nuclear genome and 2010 turned out to be really an exceptional year. This was also the year when he discovered a new hominin, the Denisova. And this was the first time an unknown hominin was discovered from genetic data alone. A high-coverage Denisova genome was published in 2012 and there are now three high-coverage Neanderthal genomes, in addition to many, many lower-coverage genomes from Neanderthals. All of the high-coverage genomes have been generated by Svante Perbo and his team. And this information gives us a completely new reference point for understanding our evolution. Before this, we could only compare ourselves to chimpanzees, from which we diverged some five, six million years ago. So we now had a much closer reference point to understand how our evolution influences our physiology today. Svante Perbo further showed that Neanderthals and Denisovans interbred with Homo sapiens and that present-day humans of Eurasian ancestry carry archaic-derived DNA. Understanding the function of these gene variants is now an intense research area with implications for many areas of medicine. The availability of genomes from Neanderthals and Denisovans also gives us clues about what makes us uniquely human. And this is one of the main current interests of Dr. Perbo. It is now my great pleasure, on behalf of the Nobel Assembly, to welcome this year's laureate of the Nobel Prize in Physiology or Medicine 2022, Professor Svante Perbo, onto the stage to give his lecture, which is entitled The Neanderthal Genome and Evolution of Modern Humans. Well, let me then start by thanking the President of the Karolinska, thank Gunilla Karlsson-Helmstam for these very kind introductions, and of course also thank the Nobel Assembly at the Karolinska Institute for this prize. What I then wanted to do, if I find, there we go, is to start out by reminding you what you all are aware of, I think, that human evolution started in Africa and goes back many millions of years, and human forms then leave Africa around 2 million years ago to live also in Asia and in Europe. And then modern humans, much later, appear in Africa to 300,000 years ago and start spreading out of Africa less than 100,000 years ago, spreading seriously out of the Middle East to the rest of the world. And an interesting thing is that at that time, modern humans, that means humans with skeletons similar to us, that then eventually became the ancestors of everyone alive today on the planet, were not alone. So in Eurasia, in Western Eurasia, they encountered Neanderthals, and in Eastern Eurasia they encountered other forms of now extinct hominins that we know less about. So Neanderthals then, to the left on this slide, were these robust forms of humans. They appear in the fossil record something like 400,000 years ago, and then live in Western Asia and Europe until about 40,000 years ago, and they disappear generally in conjunction with that modern humans to the right here appear on the scene. And our research group is then obsessed, if you like, with Neanderthals since almost 20 years. And you may ask, why should we be interested in these forms of humans that are not here anymore? There are at least two reasons for that to me. One is that they are indeed our closest evolutionary relatives. So if you like, they defined present-day humans as a group. So if we're interested in what ways we are genetically and biologically unique, we should compare ourselves to them and study how we are similar to them and how we differ from them. Another reason is that they were here quite recently, until around 40,000 years ago. So say 1,400 generations ago, they were still here. So the interesting question is, of course, how are we related to them, what happened when we met, and so on. But if you then want to study them from a genetic perspective, you then need to retrieve DNA from bones that are at least 40,000 years old. And that work then, as we heard, goes back to the early 80s in Uppsala, when I was a graduate student, and goes back to studies of ancient Egyptian mummies like this one, a 2,400-year-old mummy from Berlin that we studied at the time, that look very well preserved morphologically. If you then look in a microscope on histological sections of them, you can, in the basal layers of the skin, see things that look like coloured dyes that intercalate in DNA. And about the time we got these results, then there was a group at Berkeley, Alan Wilson's group, that we also heard mentioned, that published the first DNA sequences from an extinct organism, from the quagga, a form of a zebra, around 100-year-old things. So I enthusiastically also tried to clone DNA from mummies, and got human DNA from them, and published them. And then over the next couple of years, it became increasingly clear that it wasn't as easy to do this as I had thought. So these first sequences were clearly not from the mummy, they were probably from a museum curator, or from me, or from someone else. And the reason for this is then that there are a lot of technical issues with these things. The first obvious one is, of course, that it's very little DNA that survived in such tissues, generally 100-fold or a million-fold less than in fresh tissues. This DNA is degraded to small average size, and it's chemically modified. And it occurs then in a vast excess of microbial DNA from fungi and bacteria that colonise the bones over tens of thousands of years. And one consequence of this is that those are tiny amounts of contaminating present-day DNA that wouldn't at all influence any study of present-day DNA, may totally overwhelm such experiments. So there was a number of years we worked on methods to overcome all these technical challenges, including this contamination issues, working in clean room conditions, etc, etc. And it became clear that you could reliably retrieve short fragments, particularly of multi-copy parts of our genome, such as the mitochondrial genome or repetitive parts of the nuclear genome. And much of this work was then done not with human remains, but of extinct animals, because you could then easily distinguish contaminating human DNA from, say, DNA from a giant ground sloth or from mammoths. But our interest was really in human evolution. And at that time there were two competing ideas about how modern humans related to earlier forms of hominins, such as Neanderthals, that can be depicted like this. One idea was that modern humans came out of Africa, replaced all other forms in Europe and Asia without any contribution from them at all. Another idea was that there was continuity from these earlier forms, from Neanderthals, for example, to present-day Europeans. And you could, of course, imagine different shades of this. You could be total replacement, 0% contribution from Neanderthals up to total continuity that some people advocated that modern-day Europeans, for example, were essentially modern-day Neanderthals. So after having worked a lot on these techniques and after a lot of negotiations, we got access in the early 90s to specimens from Neanderthals, and not just any Neanderthal, the type specimen found in 1856 in Neanderthal in Germany. We were able to take a sample from the upper humerus and retrieve then a part of the mitochondrial genome, quite particularly a highly variable part of the mitochondrial genome, quite cumbersomely doing PCRs, cloning these, identifying things that were consistently there as substitutions. And when one then estimated how this mitochondrial DNA was related to present-day mitochondrial DNAs, it became clear that all present-day humans, as was already known, went back to a common ancestor in the order of 100,000 years ago. And about half a million years ago was there a common ancestor with the mitochondrial DNA of Neanderthals. So that then suggested or showed that for the mitochondrial DNA, total replacement was what had happened. But it was, of course, clear already at that time that the full story of the genetic history of any group cannot be described just from the mitochondrial DNA that is maternally inherited, and there's only one locus that's inherited as a unit. That the full story would be, sorry, in the nuclear genome. And the possibility to go after that then came with high-throughput DNA sequencing technologies in the beginning of the millennium, where you could sequence millions and billions of DNA molecules. So you could imagine extracting the DNA from such a fossil, making sequencing libraries, sequencing everything that's in there, and in the computer identify what is human-like there. And it was a great effort to look through very many different sites to find good specimens that would contain enough endogenous DNA to do this. There was particularly one site in Croatia, Lindia Cave, and this specimen that turned out to be very useful, containing up to two or three percent endogenous Neanderthal DNA. There was also a lot of improvement of technology at the time, where we got much, much better in extracting short fragments of DNA and turning them into libraries. Sequencing technology got better, and indeed the size distribution is indeed very small in these things. And when we then started sequencing this fragment, one interesting thing turned up towards the end of the molecules particularly, where when one then looked at the substitution patterns, you found that C's cytosines at the end of the molecules very often appeared mutated to T. And that turned out to be the result of the most common form of modification in ancient DNA, which is hydrolytic deamination of cytosine to uracils, which are then by polymerases read as T. So this is then a problem. You have to take it into account when you align your sequences to present genomes, but it's also an advantage because and use it as a marker for molecules that are truly ancient, because it takes hundreds and thousands of years for this to accumulate at a pre-general frequency. So we used three bones from that site, sequenced a lot of molecules from them, aligned them to the human genome, and much of the bioinformatics, both at the time and currently in our work, is led by Janet Kelso and her group to sort of align these to the human genome, taking particularly the C to T substitutions towards the end of the molecules into account. And we then in 2010, as we heard, had a first course draft version of the genome, where a little over half of the alignable genome was available. So a big question then in the beginning was how were Neanderthals related to present-day people? So an expectation, if Neanderthals had contributed to present-day Europeans, would be that Europeans today should share more genetic variants with Neanderthals than Africans share with Neanderthals, as Neanderthals had never existed in Africa, so there was no reason to assume that they would have contributed to Africans. So to address that and analyze other aspects of the genome, we put together a big consortium. An important person in that work in Leipzig at the time was Ed Green. There was also David Reich at the Broad Institute and Monty Slatkin at Berkeley that were very influential in this. And one analysis then was to compare the Neanderthal genome here to genomes we sequenced to have similar patterns of errors in our sequences from different parts of the world. So you could then do a very simple analysis where you simply identify differences between two present-day genomes to start with two Africans and compare that to the Neanderthal and say, how often does a Neanderthal match that African or that African? And you will expect that to be 50-50 since there's no reason to assume that a Neanderthal would be more closely related to one African than another, and indeed that turned out to be the case. If you then take a European individual and an African individual and do the same analysis, we found statistically significantly more matching to the European individual, suggesting that there could have been a contribution. Surprisingly, we take a person from China and Africa, we again find the same thing, although there was no evidence that Neanderthals had ever been in China. And say in Papua New Guinea, where for sure Neanderthals had never been, we again find the same thing. So it turned out wherever you looked outside sub-Saharan Africa, you found this additional similarity to the Neanderthals. There were also other approaches in the analysis that suggested that there had been a contribution from Neanderthals and that you surprisingly found that everywhere outside Africa. So the idea that came from that was that when modern humans left Africa 70,000 years ago or so, met Neanderthals early on, perhaps in the Middle East, they mixed with Neanderthals and then carried this Neanderthal contribution with them out to the rest of the world. To the effect then that wherever people have roots outside Africa, something like one or two percent of their DNA comes from Neanderthals. But what then of course became very interesting was to have much better Neanderthal genomes. Genomes of high quality, high coverage, where you could see all parts of the genome to which you can align these short fragments you extract and believe heterozygous positions, see all variants. And what became very important for that was particularly a site in southern Siberia in the Altai Mountains, finds that were excavated by the team from Novosibirsk led by Anatoly Derivyanko and his group. So it was for example bones like this, a toe bone of a Neanderthal, and there were lots of technical improvements. Much of this being done by Matthias Meyer, who now leads a group in development of such techniques in our group. One example of this is for example a method to turn the double-stranded DNA you extract from a library, from a fossil into sequencing library. That relies on starting by separating the two strands from each other, turning each strand into a template for making double-stranded molecules in the library. So each double-stranded ancient molecule will have two chances to end up in the library. So if one strand is nicked or has some modification on it, the other strand has a chance to come in. And this and other improvements then made it possible to go from this very bad draft genome to high coverage genomes, so each positioning is covered 20-30 times. So today in the public domain there are three high quality Neanderthal genomes, there are more on the way, they vary in age and also cover quite a bit of the geographical distribution of Neanderthals. And you can then compare that to present-day people in much more detail. And what becomes obvious then in an example for one chromosome here, that if each line is a present-day chromosome 9, there are segments here that are identical or almost identical to Neanderthals. So this then adds up to about 1 or 2 percent of the genomes of people outside Africa as we said, but you also see here that different individuals carry different fragments of the Neanderthal genome. So you can jump from individual to individual and you can then retrieve something like 40, probably 50 percent or more of the Neanderthal genome in present-day people. What also happened then at the time was that among the thousands of bones that the Russian archaeologists found at the Niseva cave, they were very skilled to realize that a tiny little bone fragment might be of human origin. So they particularly pointed us to that bone, we were able to retrieve DNA from it and actually a high quality genome from that bone, and we were very surprised then to find that when we compared that genome to Neanderthals, it was clearly not a Neanderthal, it was also not a modern human, it was something else. And this ageism when groups diverge is very dependent on many assumptions, mutation rates and so on, but if we say that the divergence between modern humans and Neanderthals is something like 600,000 years, this divergence is about 400,000 years. So we called this group the Nisevans after the Niseva cave where they were found, and it became clear that they too had contributed to present-day people. You find no contribution in Western Eurasia, but wherever you look in mainland Asia you find a contribution of clearly under one percent, and interestingly out in Oceania a much bigger contribution of five, six percent. And it also has become clear from the work particularly of others that the Nisevans have an interesting population history. So if you look in Papua New Guinea for example, and this is work by Josh Aikie and his group now at Princeton, you can plot on this line similarity in fragments of chromosomes to the Neanderthal. You find this Neanderthal contribution quite close to the Neanderthal genome, and you find this contribution from the Nisevans quite distant from the Neanderthal genome. Identity would be up here and this is sort of quite distant. If they then look in East Asia, in China or in Japan, you find this Neanderthal contribution just as in Papua. You find this Nisevan contribution distant from the Nisevan genome, but in addition a contribution very close to the Nisevan genome we have sequenced. It turns out wherever you look in East Asia you find these two Nisevan populations quite distinct from each other that have contributed. So at least two Nisevan populations have contributed in Asia. There's indication that there are even more populations that have done so and that's a difference to Neanderthals that seem to be very homogeneous as a group. So if you just summarize what we think we know then from studying genomes on the history of Neanderthals, Nisevans and modern humans, they, the archaic humans, have an origin in Africa. They come out of Africa at least half a million years ago and evolve in Western Eurasia to what we call Neanderthals, in Eastern Eurasia to what we call the Nisevans, where the border between these groups was changed over time for sure. We don't know what it, where it has been always, but at least at some time then in this site in southern Siberia there were Neanderthals, at other times there were Nisevans there. Then modern humans have an origin in Africa, come out, meet Neanderthals early on and mix with them, continue to spread and mix more times. In the east they meet the Nisevans and mix with them several times, continue out to other parts of the world where there were no hominins at the time and these other groups then disappear with time but live on in the contribution to present-day people. You may then ask, are Africans special in a way in not having this component in their genome from other groups of now extinct humans? We have no genomes yet in Africa from other forms of humans but clearly modern humans appeared. Some were in Africa, spread across Africa and met other groups and if one mixed outside Africa one probably did so in Africa too and there are also indications actually that this may have happened. I think we will in the end learn that this happened in the same way in Africa. An interesting thing is that we now begin to get rather direct evidence of this mixture with modern humans and Neanderthals. The first such instance was in Romania at a place called Boasa Cave where cavers have found a mandible that looks like a modern human mandible. It's dated to 40,000 years ago so it's an early modern human that lived at the time when the last Neanderthals were still around. When we looked at the genome of this individual we were quite excited to find that this individual had mixed with Neanderthals and actually have long segments on the chromosomes here, half of chromosome arms often that are very similar or identical to the Neanderthal genome. This then shows that this person in its family tree, so to say, had a Neanderthal either six, five or four generations back. This individual had a Neanderthal ancestor. More recently in Bulgaria there is a site called Machu Kiru Cave where one finds technology typical of very early modern humans, initial Upper Paleolithic, and deep in the stratigraphy there one finds human remains that are around 45,000 years old and these are early modern humans, some of the very earliest we know of, and higher up 10,000 years later are also modern humans. We've been able to retrieve genomes from these three individuals and this individual and we then find that all the early individuals 45,000 years ago there had this contribution recently from Neanderthals, a number of generations back in their family history. If we look 10,000 years later in the same cave they look pretty much like people today having around 2% of Neanderthal admixture distributed in much smaller segments. The picture that is emerging is that the first modern humans that came to Europe mixed very extensively with the local Neanderthals. The picture I think that is emerging is that Neanderthals and Nissebans were probably incorporated into probably much larger modern human populations that arrived, that they were simply assimilated into modern human populations. That's at least part of the story. It may of course have been very different in different regions and at different times, but that is sort of part of the story. What I then want to discuss is what we can use the genomes from our closest relatives for in terms of looking at evolution along the human lineage here. We can of course now look in the genomes and find changes that are shared between modern humans and Neanderthals and different from the chimpanzees. These are things that happened up here before the divergence. We can also find things that are unique to Neanderthals and things that are unique to modern humans. Much of the examples I want to bring you is actually a collaboration with Hugo Seberg here at the Karolinska, who also spends much of his time with us in Leipzig. One of the things he noticed was that when he looked at ion channels in the Neanderthal genomes, there was one channel that stood out, a sodium channel, in that it has three amino acid substitutions that occur in all Neanderthal genomes we have access to at the moment. That's a lot. It's the only ion channel with three substitutions. This is an interesting channel. It's a channel that sits in the peripheral nerve endings and initiates the sense of pain. When he then expressed the Neanderthal version of this channel and the modern human version in frog oocytes or in human cells, he could show that for a certain stimulation, the Neanderthal version of this channel let through more of a current. I'm not a neurobiologist, but one could then show that this was not because the channel opens quickly, but that this inactivation is less quick. It remains open once it is activated for a longer time. You can also then look at these three substitutions and show that none of them seemingly has any effect, but two of them is enough to elicit effect, the two intracellular amino acid substitutions there. What we can then do is make use of the ever bigger biobanks that are available. In the UK biobank with 360,000 individuals, it turned out to our surprise that it's actually present. This version, the Neanderthal version, is present at a low frequency there. It turns out then that this, what we thought was a Neanderthal-specific thing, has actually come out over to modern humans and exists in a low frequency. That then allows you to use a questionnaire information from people in the UK biobank. There are 19 questions that have to do with all types of pain, back pain, stomach ache, headache, and so on, and see what it associates with. The most obvious thing that pain in your life associates with, unfortunately, is increasing age. The older you are, the more aches and pains you have, which is trivial. You have more medical problems. But more relevant then is that people that carry this Neanderthal variant here do report more pain in their lives. If you relate this to this age effect that we can perhaps more associate something with, it is as if you were eight years older in terms of pain in your life if you carry this Neanderthal variant. So we can of course from that not say that Neanderthals experienced more pain in their lives. We all know that the sense of pain is modulated, spinal cord and very much so in the brain. But it's still interesting that people who are heterozygous today, this is so rare, so of course everyone we have in the UK biobank is heterozygous, they do report more pain in their lives. And Neanderthals were homozygous for this. So maybe we need to rethink a bit our view of Neanderthals as these brutish, insensitive individuals. Maybe they were actually very sensitive. So another example I want to bring then has to do with reproductive biology. It has to do with progesterone that you know is a hormone that has many functions, but one major function is that it's produced by the corpus luteum after ovulation and prepares endometrium for a possible pregnancy. And what we're interested in here is the receptor, the progesterone receptor. It was already known that there is a variant of the progesterone receptor that exists that's associated with pre-term birth, so premature babies. What is striking is then that this variant exists hardly at all in Africa, but does occur in populations outside Africa. Wherever you see this distribution, it's very likely that it actually comes from Neanderthals. When people noticed this, it was then even suggested that this has to maybe select a disadvantage of Neanderthals, that premature babies that are of course at risk to die. However, what is also becoming possible now is that we have thousands and now over 10,000 genomes of present day of modern humans over the last 20,000 years. So we begin to be able to follow the frequency of variants across time by looking in present day human genomes. So if you have this little movie here where we just have the archaic genomes in red, we have carriers of this risk variant for pre-term birth, black, other ones are gray, and we walk forward in time from 15,000 years ago towards present day, around 7,000 years ago, you will see how this variant almost explodes in frequency, which makes no sense if it is a risk factor for pre-term birth. So that then caused us to look again in the UK Biobank, and if you look at the associations there now with the modern alleles, the non-Neanderthal variant, what we find is that this is significantly associated with bleeding early in pregnancy, with miscarriages, and it is negatively associated with numbers of full sisters and actually brothers, but not significantly so. So this is then quite surprising, the non-Neanderthal version is actually a risk factor for miscarriages. So it turns out then that the Neanderthal version here is indeed associated with pre-term birth, but it's also protective against miscarriages and results in more live births, and we're beginning to understand the function of that also, in that if you look what carriers of this Neanderthal version express more of the receptor, so you can imagine that you have more of a progesterone effect if you have more of the receptor, and it is sort of compatible with the recent observations that if you give progesterone to women who have experienced several miscarriages, you can prevent or increase the likelihood of a live birth, compatible with that more progesterone effect, either by giving more progesterone or having more of the receptor may be good. And this is sort of the pattern that many genetic variants that are already known turn out to come from Neanderthals when we now have the Neanderthal version. So one example of this is cytochrome P450 genes that sit in association with each other in the genome here that are well-known variants clinically. They turn out to come from Neanderthals here to be very close to the Neanderthal genomes, and one of these variants here is associated with less efficient sort of metabolism of ibuprofen, so the half-life of ibuprofen is a lot longer if you are carrying the Neanderthal version. The other gene is perhaps even more relevant in that it's associated with decreased metabolism of warfarin, so the therapeutic dose is a lot lower, higher doses may be toxic. Another case of this that sort of surprised us very much was in the pandemic that we still deal with, where you of course know that it's a big, many people are infected with the virus, become not very sick at all, but some become very severely ill. We also know the risk factors such as old age, male sex, or diabetes, but there are clearly also genetic factors that influence this. All these other risk factors are not enough to fully explain why certain people become so ill. So Hugo was involved in this international consortium to do an association study of severe disease, and when the first results in 2020 came from that, there was one clear risk factor of chromosome 3, and it shocked us very much to then see that these risk factors, the haplotypes associated with risk, were very closely related with our Neanderthal genome, particularly one of the genomes from Croatia there, whereas the protective variants were quite distant from that. So this risk factor has clearly come over from Neanderthals to present-day humans and exists in some humans today, and it's quite a strong genetic factor to be associated with risk. Say in 2020, if you were hospitalized and were not a carrier of this, you had a 7% risk of dying, whereas you had perhaps twice as high a risk if you were a carrier of the Neanderthal variant. Unfortunately, this region is very complex. It's hard to realize what is going on, and we and others are intensely studying what may cause this difference in risk here. What is interesting is the distribution across the world of this risk factor. We can, from that, first of all, estimate how many extra deaths you had in a pandemic due to the Neanderthal contribution, and that is at least a million or probably much more. But you also see that it's almost absent, this variant, in East Asia, and it's very frequent in South Asia, in India or Pakistan or Sri Lanka. That's up to 50% carriers, and it's somewhere intermediate in Europe. So this clearly suggests that there's been something that has been negatively selected in the past in the East here and South Asia positively selected. So probably this has other factors that have influenced this in the past. Clearly, this variant has other effects. Hugo again noticed that a million base pair downstream of this is a gene that many of you have heard about, CCR5, a co-receptor for HIV. And carriers of this Neanderthal variant express less of CCR5. And indeed, if you look in cohort studies, people who carry this Neanderthal risk variant for COVID have a reduced risk of becoming infected with HIV. So this locus then has different effects. It increases your risk of becoming severely ill if you're infected with COVID. If you're exposed to HIV, it decreases your risk of getting the infection. This just illustrates the fact that genetic variants have different effects, are a double-edged sword, and these are, of course, recent infections. COVID is with us for a few years, HIV for just a few decades. Must have been other things in the past that influence these big frequency differences. So it's sort of clearly even a multi-edged sword. Before leaving Neanderthals, I should say they are not really bad in this pandemic. There is now another variant, one is found on chromosome 12, that is also associated with severe disease. In that case, a Neanderthal version turns out to be protective. In that case, one knows very well what goes on. There are genes in this region encoding an enzyme that makes an unusual little oligonucleotide that activates the double-stranded RNAs that degrades viral genomes and is also protective in other RNA infections. But unfortunately, the effect size is much smaller. So whereas the chromosome 3 about doubles your risk of becoming severely ill, the one on chromosome 12 decreases by about 20%. So in the end then, I also want to discuss genetic changes that happen on the lineage to modern humans. So things that occurred in the last half million years of human evolution and spread to be present in many or almost all people today. And we are very interested in those changes because we think that some of them may then be involved in functions that are unique to modern humans. And I do think that modern humans are special in some ways, because Neanderthals and Denisovans existed for hundreds of thousands of years. They probably never got more numerous than 100,000, 200,000 individuals at any one time. With modern humans and after some time, at least from 70,000 years ago, technologies start changing very rapidly and become regionalized in different parts of the world. There are some indications that Neanderthals had art, but figurative art that we can immediately recognize what it depicts comes and becomes frequent in certain parts of the world, such as southern Europe, with modern humans. And of course, most fundamentally, probably modern humans become very numerous, spread around the world, replace other forms of hominins and start spreading over open water and colonized every part of the world where you can live. So an idea is that some of this sort of readiness to absorb and develop culture and technology may hide in those changes here. So you can then list them, of course, list things that became frequent in the human lineage in the last half million years, just to give you a sense for what we may do with those things. If we look on one amino acid change, such change, which is in an enzyme, glutathione reductase, there's an amino acid substitution there. And this enzyme then reduces oxidized glutathione. So it's important in taking care of oxidative radicals that are produced in all our cells by metabolism. But this enzyme, when there is no oxidized glutathione available, produces itself some oxidative radicals. So when people here at the Karolinska synthesized the Neanderthal version of this and the modern human version, you can see that the Neanderthal version is actually more leaky and produces more oxidative radicals. And you can again go to these biobanks and find that in certain populations, this ancestral version was contributed on Neanderthals and occur in a low frequency, for example, in the UK biobank. It's very rare there, but you can then look for associations with disease in present-day people or other phenotypes. And you can then see that this variant is associated with some diseases that have to do with inflammation or have a component of inflammation, such as arteriosclerosis or inflammatory bowel disease. I think this indicates where we are going with this currently, that you can use these genomes from our closest relatives to identify things that are human specific, to find things that are Neanderthal or Denisovan specific. And you can use this gene flow from these groups, where they introduced either Neanderthal-specific things or archaic things into humans to then study their effects in people alive today. But you can also, of course, do experiments, for example, by ancestralizing human cells back by CRISPR technology and study their effects in tissue culture, and even make animal models that decumanize or ancestralize for different things of these things. And I then, just in the end, want to say that all this work, most of this work, was done in the Max Planck Institute in Leipzig, and as representatives of generations of postdocs and graduate students and technicians who have helped with this, I show the current group, and also the group that since a few years work in the Okinawa Institute of Science and Technology. In the end, of course, also want to acknowledge the institutions that have made this work possible and the funding sources. And I thank you for your attention. Thank you so much for this beautiful lecture. As we did here in the beginning, there has been three years now, and this has been the first Nobel lecture. We now have the opportunity to honor a few more laureates from the previous years. Perhaps we'll get them up here on the slide in a minute, but could I invite the president of Karolinska Institutet together with the laureates up on stage, please. So let me just introduce David Julius, Arden Patapoutian, Charles Rice, together with Svante Pääbo. A big hand for these present laureates. Thank you.