Instant Genius - How human disease changed history, with Dr Jonathan Kennedy

Episode Date: April 2, 2023

Over time, we have become infected with various viruses, bacteria and other microorganisms, which have had a huge impact on our evolution and history. In this episode, Dr Jonathan Kennedy, a reader in politics and global health, tells us about the close links between disease and colonialism, how infection shaped the migration of humans out of Africa, and what we can all learn from the COVID pandemic. Learn more about your ad choices. Visit podcastchoices.com/adchoices

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So you can experience exceptional sound at home. Music just as the artist intended. Visit name audio.com to learn more. Hello and welcome to Instant Genius, a bi-sized masterclass in podcast form. I'm Alice Lipscomb, Southwell, the managing editor at BBC Science Focus magazine. In this episode, I talked to Dr Jonathan Kennedy, a reader in politics and global health at Queen Mary University of London. He's just written a book, Pathogenesis, which explores the impact of disease on human history.

He tells you about the close links between disease and colonialism, how infection shaped the migration of humans out of Africa and what we can all learn from the COVID pandemic. Hello, my name's Jonathan Kennedy and I'm an academic. My day job is at Queen Mary University of London, where I'm a reader. So that's a slightly archaic job title somewhere between senior lecturer and professor. And I'm reader in politics and global public health. And you've just written a book called pathogenesis. So just to start off, can you explain to our listeners, what exactly pathogenesis is? The book basically aims to transform the way that we think about history. Most histories see the natural world as a kind of stage on which humans play out

their roles. But I think, you know, if you look at science over the last few centuries, the more we learn about the world, the more we learn that the natural world isn't just some inanimate stage. In fact, humans are part of this massive system, an ecosystem, and we're, of course, a big part of that, but we interact with a variety of other animates and inanimate parts of this system, including infectious diseases. And so basically what pathogenesis, the book does, is it goes back through history, and it looks at really quite a large number of cases from the extinction of the Neanderthals all the way up to COVID really to show how infectious diseases, pathogens have really played a crucial role in history, have really been a driving force in some

of the major political, social and economic transformations in the past. And I think the first thing that really amazed me when I was reading the book is that the only reason humans don't lay eggs and we give birth to live young is because of a virus. Now, how could our evolution have been different if that virus had never infected? Yeah, I mean, this absolutely blew my mind because my background is in history and sociology. And, you know, I have always had an interest in science. I studied biology and chemistry at A level. And I'm married to a doctor and I teach in a medical school. So I do have an interest in these things. But around about the time of the COVID pandemic, I started to read a lot more about microbes and

their impact on humans and human evolution. And this was, one thing that really, really kind of blew my mind, the idea that retroviruses reproduce by inserting their DNA into our genome. And if they manage to infect either our sperm or our egg cells, then these genes are passed on to all subsequent generations. So when you look at the human genome, you actually see that something like 8% of all our DNA comes from these infections. So it kind of raises some pretty, pretty crazy, crazy questions. But as you said, humans have been able to acquire kind of wholesale from virus infections, various capabilities that we think as being fundamental to being human. So one of these is the ability to give birth, because if we look at how the

placenta binds to the womb, the cell lining is very similar to the way that a virus would bind to a cell when it's trying to infect it. And this makes sense in a way because, you know, it's really phenomenal that mammals are able to carry young inside their bodies. It's much, much safer than laying some eggs and leaving them somewhere and possibly being chased, chased away. But, you know, it also causes problems for the immune system because you basically have a genetically different parasites. You know, we could see our young as a parasite growing inside our body. So how do we avoid the immune system reacting and destroying this. And this is only because we have inherited these genes from a viral infection that basically allows the placenta to bind to the mother.

And there's other other examples of capabilities being acquired from retroviruses too. So the other one that really, really struck me when I was researching for the book was the ability to form memories. So the way in which kind of memories pass from one cell to another seems to be very, very similar to the way that viruses pass their genetic information from one cell to the other. And it seems that this gene is inherited from another viral infection. And in fact, scientists have bred mice where they've removed this gene and the mice don't seem to be able to form memories. So yeah, it really kind of, I found that really, really surprising as a layperson delving into the kind of science of retrovirus infections. Yes, we definitely tend to think of

viruses as bad things, don't we? But in those cases, it seems that they've actually been really useful and have really shaped our evolution in that sense. Yeah, yeah, exactly. And, you know, viruses play an enormously important role in the whole of the ecosystem. There are viruses everywhere that there's life on the planet, basically. So someone once calculated that if you put all the viruses on Earth end to end, they would reach for 100 million light years, which I find it hard to comprehend that as a distance. But most of these viruses aren't infecting humans, aren't capable of infecting humans.

Only about 200 or so are capable of infecting us. Most of these are so-called bacteriophages, so from the Greek to devour. So they eat bacteria. And in fact, again, it's another fact that really stunned me, but these viruses kill about 20% of all bacteria every day. So they play a really important role in the natural world, basically maintaining balance and making sure that no one species of bacteria or no one strain of bacteria becomes

too dominant. And thinking about history a bit more here as well, one thing that was really interesting was that you said how disease could have kept Homo sapiens in Africa for longer and kept the Neanderthals in the Mediterranean for longer. And there was a sort of poison antidote effect where they couldn't really mix because they would end up getting ill if they did, kept them in their sort of separate regions for a really long time. Yeah, yeah. Again, this was, this was a really fascinating chapter to write. And I guess, you know, it starts with one of the great mysteries of the Paleolithic era. You know, basically if we go back to something like 60,000 years ago, the world was very, very

different to the way it is today. Homo sapiens had perhaps been around for 150, maybe even 250,000,000 years, but we lived exclusively in the African continent, and there were a variety of other human species on the planet, so Neanderthals in Western Eurasia, Denisovans in Eastern Eurasia, and a variety of other human species in Southeast Asia. So, I mean, I like to think about it almost as being quite similar to Middle Earth, to, you know, kind of Tolkien's world, you don't just have one species of human, you have all sorts of different human. And then all of a sudden, around 50,000 years ago, this changes. Homo sapiens burst out of Africa and really very quickly spread throughout the whole of

the so-called old world and even get as far as Australia. And all other species of human seem to disappear. And there's a variety of theories for why that is. If we go to something like Sapiens, the incredibly famous, famous recent, or not so recent book about the history of mankind, the argument is that our species went out because we were the wisest, because we were more intelligent and capable of kind of out-competing other species of humans, which is important because Neanderthals were certainly bigger and stronger, stronger than us.

But, you know, there are certain problems with this theory, not least the last couple of years. You know, there's been a lot of research that shows that Neanderthals are really seemed to be, you know, kind of very intelligent and capable of all sorts of complex behaviors similar to our own species. So they appeared to have produced some pretty basic cave art. They appear to have sailed between islands in the Mediterranean. They very clearly were able to cultivate fire. They even seemed to have been able to use kind of mycidinal plants to treat various, various kind of maladies. So this doesn't really make sense, but the theory that I think is most promising is that it was infectious diseases that that wiped out the Neanderthals. You know, I'm sure your listeners will know that certainly

people of European, Asian and Native American descent have between 2 and 4% Neanderthal DNA in our genes. And I think it's really important to point out that the genes that we retained from Neanderthals are not just random. They are genes that gave us an advantage or gene variants that gave us an advantage as we were migrating out of the African continent. And so some of these relate to skin and eye color, others to body hair, because obviously it would be a big challenge for our species having evolved for hundreds of thousands of years in Africa to arrive in cold, dark, dark Europe. But also, a lot of these genes seem to be related to the immune system.

And this makes sense because Neanderthals and Homo sapiens were perhaps separated for 500,000 years. We lived in Africa. They lived in Western Eurasia. And we would have both evolved to survive in different disease environments and developed immunity to the pathogens that we came across in the areas that we lived. And then, you know, when we first started to meet 100,000 or so years ago, this would have really kind of created a massive, a massive problem because certainly Neanderthals wouldn't have developed immunity to pathogens that we carried and vice versa. And so it's kind of a similar process to what we would see, for example, with the arrival of Europeans in the Americas,

when it just devastated the indigenous population, when the population of the Americas fell by 90% within 100 years of the arrival of Columbus. It would have been something like that. But basically, as we know, we know now that Homo sapiens and the Andatars interbred and exchanged genes. And scientists call this the poison antidote model of adaptive introgression. And the idea is that when Homo sapiens and the antitals met, they, you know, gave each other a poison because they would exchange pathogens that they didn't have resistance to, but also when they reproduced, they would provide each other with the antidote because they would exchange genes. And so, you know, they wouldn't have to evolve immune responses through the painstakingly slow process

of Darwinian evolution by natural selection. They would gain these gene variants, you know, immediately through reproduction. And so this happened both ways. but the reason why homo sapiens won out was because we had evolved in tropical Africa. And even today, the closer one gets to the equator, the more of the sun's energy reaches the earth. And so this means that there's more vegetation, there's more animals that feed that vegetation, and there's more microbes that live on the vegetation and the animal life. So you tend to get more and more deadly infectious diseases. sapiens were able to overcome Neanderthal pathogens faster than the other way around.

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And you touched on it briefly there and I know a lot of people will be familiar with the fact that when Europeans went into South America, then the native population just got wiped out, essentially. We brought all these germs with us. So in what other ways is disease linked with colonialism? What are some other examples of that? Yes, I think it's really interesting to compare the difference between the colonization of the Americas and the attempt colonization of the African continent. So Europeans colonize the Americas incredibly easily. You know, you see kind of Hernan Cortez, the Spanish conquistador, go to Mesoamerica in the early 15, 1520s and conquer this vast empire, the Mexico or the Aztec civilization with about a thousand,

a thousand troops and not just conquer it to kind of the Spanish ruled it for centuries afterwards. And then you see an even more remarkable incident 10 years later when Francisco Pizarro goes to the South American continent and conquers the great Inca Empire that, you know, ran more or less the whole length of South America. And he did this with 168 troops. And traditionally, we've thought that kind of germs might have played a role, but also, you know, as Jared Diamond says, the conquest of the Americas by the Spanish would have been inevitable anyway because we had better, better weapons and we had guns and steel. But I mean, I think this is really, really unlikely if we look at how much difficulty, you know, the American-led alliance had

trying to really kind of achieve very modest aims in Afghanistan the last 20, 20 years. They sent 130,000 troops, the most advanced weaponry the world has ever seen. They spent trillions and trillions of dollars and, you know, ultimately nothing changed. So I think we really have to challenge this hubristic idea that European society was better developed and that's why we defeated the indigenous populations of the America. It's very clear to me that infectious diseases were by far the most important factor. And if they have to have to be a lot of, hadn't acted as an kind of unwitting secret weapon, then the European conquest of the Americas would not have been possible. And we can see this when we compare what happened in the Americas

to sub-Saharan Africa. And these days, we kind of train to think of sub-Saharan Africa as a poor place. But in the middle ages, it was actually seen as very, very wealthy. It was the source of the vast majority of gold that flowed into Europe. If you look at medieval maps like the Atalan Atlas, you see rivers of gold flowing through West Africa. You see Mansa Musa, the Malian Emperor, sitting on a gold throne and handing out discs of gold to Berber camel herders. And so Europeans certainly coveted these natural resources. They wanted to colonize Africa, but they couldn't manage. And a big part of that is the fact that we were part of the same trade networks. as I said, gold went over the Sahara to Europe.

And so the same kind of old world pathogens that infected Europeans and that Europeans have developed immunity to also affected the vast majority of the African continent. But I think another important factor is the role of malaria and the role of yellow fever. And so this, I guess, kind of worked as a defensive force field that made it almost impossible for the Europeans to conquer West Africa. Even in the 18th and 19th century, something between 40 and 70% of Europeans, or European would-be settlers,

would die within a year of arriving in West Africa, mostly from malaria. And if you go further inland to places like Mali, the death rate would be even higher. It would be kind of 300%, which is hard to get your head around. But basically that means that you'd, on average, you'd survive for about,

four months before being killed. So the British at the time referred to the region as the white man's grave. And I think, you know, often we forget that even as late as 1870, so that's centuries after the Americas were conquered, the vast majority of Africa was still independent. You had the French colony of Algeria in the temperate north, and in the temperate south, you had the British and Dutch colonies in what's now South Africa. But the but the vast majority of the continent was not colonized. And it was only towards the very end of the 19th century with the scramble for Africa that you have this colonization of the region.

And this was made possible by quinine. So Europeans had known about quinine for centuries, but they only began to use it systematically towards the end of the 19th century. And this lowered death rates to a level that made the colonization of the African continent. possible. So how come the local population in Africa wouldn't have been as affected by malaria and yellow fever? Is it because they would have been exposed to these illnesses from a very early age and they would have built that immunity to it? And yet when European people went down there, they just didn't have that inbuilt immunity almost. Yeah, yeah, exactly. So the epidemiology of

malaria and yellow fever are slightly different, but the outcome is the, the outcome is the same. So malaria killed and it still kills large numbers of young people in West Africa. It still kills several hundred thousand young children a year. But if one's exposed repeatedly, then one's body builds up resistance. And so adults from the region have only quite minor symptoms when they're when they're reinfected. So malaria doesn't really seem to, like the impression one will get as a European in traveling to West Africa in the 1500s or the 1800s would be that African adults aren't really affected by this. And with yellow fever, basically, if you're infected as a child, you tend to get pretty minor symptoms, and once you've been infected, you are immune for life. But when you're

infected as an adult, the symptoms are much, much more serious. And a third of people who develop symptoms will die. So again, you have the same general effect that it would seem like it doesn't affect adults that have grown up in the West African region, but for the European would-be settlers who were coming and being infected for the first time, they would die in their droves. Now, one disease that many people will be familiar with is the plague. We learn about it in school. We know it appeared during the medieval times and also in the 1600s in London. But according to your book, it appeared much sooner than that in human. history like 5,000 years ago?

I knew the recent history of infectious diseases pretty well, but, you know, it was something that really kind of I was stunned by when I started looking at the latest research by scientists who have kind of basically tried to extract DNA from human skeletons and then not just identify the human DNA, but also see what... what kind of microorganisms were in the blood at the time that people died. And that gives you a pretty good impression of what they would have died from. So the earliest sample of plague bacteria is from about 5,000 years ago in Western Sweden. So it was extracted from the dental pulp of a 5,000-year-old skeleton.

And I mean, this is fascinating in and of itself that we can kind of look so far back into the past and see what people were dying from and people were getting sick from. But I think what's really interesting is when we relate this to other kind of other facts and other sources of information and we can really build an idea of what was going on around this time. So first of all, we can compare this to other samples of Yersinia pestis, plague bacteria found in ancient skeletons across Eurasia. And we get this kind of impression that, the whole of the landmass was affected by what might have been a really quite devastating pandemic about 5,000, 4,500 years ago.

And it's also really interesting when you compare this with other sources of information. So we know that around this time, certainly in Western Europe, the population collapsed. So we know this from archaeologists who've basically totted up the number of archaeological finds throughout history and find a big slump around this time. We know this from scientists who have basically looked at peat samples and have worked out that around this time you get far more seeds from wild plants than you do from domesticated crops. And so this suggests that around 5,000, 4,500 years ago, there was perhaps a Neolithic Black Death that wiped out a large proportion of the population.

And to add to this, some recent research that looks at the kind of big migrations of humans coming into Western Europe demonstrates that around this time you have a really, really big change in the genetic makeup of the population. It seems like there was a big migration of herders from the Western Steppe region who basically surged through the surge through the, surge through the, region. And this is really important because these Western step herders, they seem to have brought Indo-European languages, and they also account for a large proportion of the gene variants carried by particularly northern Europeans. So it's really incredible to think that we can

still, if we walk around, you're in Bristol or I'm in London, if we walk around the streets, we can still see and hear the consequences of this plague epidemic. that occurred 5,000 years ago. So as a species, what can we do to ensure we survive the threat from diseases in the future? Well, I think it's really important to invest in science and technology. And certainly if we look at the speed at which a safe and effective vaccine or several safe and effective vaccines were developed during the COVID pandemic, this is really remarkable, even miraculous.

But I think it's also really important. to remember that science and technology on their own are not enough. We also have to look at political, social and economic problems. If we think about the COVID pandemic, for example, it seems like, you know, the initial spread in China of COVID was inhibited by the secrecy of the Chinese Communist Party, so the lack of transparency. And then if we look at the devastation that COVID caused in the UK and the US in particular. This was the result of pretty bad decisions by politicians not to lock down at the right time to just let the virus rip through the country.

And we can also look at who was affected. And not everyone was equally at risk of getting sick and dying from COVID. Certainly poor people and people from certain minoritized ethnic groups were at much higher risk of getting ill and dying. And so I think we have to realize that the problem with COVID wasn't just the COVID virus. It was that our society had created a habitat through which the COVID virus was able to prosper. And if we deal with these issues of poverty and lack of transparent government that really kind of helped the COVID pandemic to thrive, then we can help avert a future pandemic as well.

Thank you for listening to Instant Genius. That was Dr Jonathan Kennedy talking about how disease has shaped human history. His book, Pathogenesis, is out on the 13th of April. The latest issue of BBC Science Focus magazine is out now. Pick up a copy in store or visit sciencefocus.com. This podcast is sponsored by name, audio and focal. The texture and emotional depth of music

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