Today, I have the pleasure of chatting with Kyle Harper, who is a professor and provost emeritus at the University of Oklahoma, and the author of some really interesting books: The Fate of Rome, Plagues Upon the Earth, Slavery in the Late Roman World, and an upcoming one called The Last Animal. The reason I wanted to have you on is because I don't think I've encountered that many other authors who can connect biology, economics, history, and climate into explaining some of the big things that have happened through human history in the way you can. The most recent reason I wanted to have you on is I interviewed David Reich, the geneticist of ancient DNA, and some of the questions we were discussing, he kept emphasizing this overwhelming and surprising role that diseases have had in human history, not just in the recent past, but in his work going back thousands of years, tens of thousands of years. He said, 'You gotta have Kyle on.' I emailed him afterwards, asking 'Who should I interview next?' And he said, 'You gotta have Kyle on'. You have this graph in The Fate of Rome. You show human population over the last few thousand years. I assume that these two downspikes are both the bubonic plague, Yersinia pestis, right?
And so this is not like some small little nudge you can see, the overwhelming—I mean, other than the hyper exponential growth in human population—the overwhelming, not just one, but the overwhelming two major features in human population going back the last 10,000 years, is this one bacteria, right?
One of the things you discuss in the book is that the collapse of the Roman Empire was a result of this one particular event.
The period that I normally work on is from the High Roman Empire, the glory days of the Pax Romana in the first or second century, through what we call the late antique or early medieval period, the sixth or seventh century. At the beginning of this period, Rome dominates this Mediterranean empire. It's what you think of when you think of ancient Rome. It's the largest city in the world. It's the center of this huge network. By the end of this period, the city of Rome has 50,000 to 100,000 people. It's a tenth or twentieth of its former size. I think we now can say pretty clearly that environmental factors like climate, but also especially diseases, play a part in that really big transformation. While there's a problem because we don't have the same kind of modern government mortality statistics that we do for COVID or even for the last century or century and a half, we have to piece together from clues. But it's pretty clear that the bubonic plague events, whether you're talking about the Black Death of the 14th century or the plague of Justinian in the sixth century, these events are capable of causing death rates, temporarily, that are just orders of magnitude beyond what we're accustomed to. Even in these ancient societies, the reason why these were so shocking, in a world where the death rate is always pretty high—probably several percent of the Roman population, three, four percent a year may be dying in a normal year—for them to just be utterly shocked by the death rate already tells you that it's some multiple of what they're accustomed to.
Yeah. I think you discuss in the book the possibility that the death rate might have been close to or even over 60% wherever the Black Death hit. This is literally the most significant thing.
Yeah. It's mind-blowing. In the case of the Black Death in the 14th century, it's pretty clear. It kills 50, 60% of the population in entire regions. We don't necessarily think that it killed 50, 60% of the whole continent, although that's actually not impossible. But even the fact that it's killing 50% of the people in cities, in provinces, in countries, is just beyond the damage that other plagues do.
Right. And do you think that were it not for this 60% mortality event plus the fact that we haven't even discussed yet, this super severe cold snap, do you think that the Roman Empire might have otherwise just kept going? You discuss there were these two previous other big pandemics. The empire still survives. I think Will Durant had this quote that the Roman Empire fell for longer than most empires have lasted. Do you think it'd be similar to China maybe? Maybe a dynasty collapses, but fundamentally, the same sort of cohesive nation reemerges?
Yeah. That's a great comparison. Not just decline and fall of dynasties, but also geographic changes in the configuration, that parts get added and parts get cleaved off, but you still kind of think of it as fundamental continuity in the core. That to me is a very plausible counterfactual. Justinian, the emperor in the 540s—he reigns from the 520s to the 560s—he's on a path of success. He's retaken Africa. He's mostly retaken Italy when the plague hits. To me, a very plausible counterfactual is that a more or less Mediterranean core of the Roman Empire could have survived east and west. It does sort of survive in the east, but even including really all of Italy, Africa, and probably Spain. That would have been a very reasonable outcome of the sixth century if you hadn't had this kind of random shock. The Roman Empire would keep going. Remember, it does, and it calls itself the Roman Empire until the 15th century. But we would think of it as maybe more the Roman Empire if it still included the western Mediterranean and was this major, powerful, urbanized polity that resists invasion from the southeast, as happens in the seventh century. So the answer is yes, I think that the Roman Empire absolutely could have had another turn as the thing we kind of mean when we say the Roman Empire: this pan-Mediterranean, powerful, urbanized empire.
Yeah. Okay, one of the things I found really interesting was you were discussing the firsthand accounts as this big—and feel free to explain the cold snap as well as it's happening—but the firsthand accounts of people who are experiencing this, some of whom come from this burgeoning Christian faith, which already lends itself to millenarianism and apocalyptic thinking. I'm curious basically how did different people try to make sense of this once in a 1,000-year event that's super intense?
Clearly, people have to try and explain, within the elements of a worldview that they have, how something like this could happen. They don't have modern science. They don't have germ theory. They don't think of it in terms of a biological event or climatic event. Since that's come up and you've invited, I'll say a little bit about that. This is one of the other really exciting frontiers where we're learning new things about the human past that we just didn't know 10 or 15 years ago. In this case, we now have really cool paleo climate data that helps us understand that this period of the sixth and seventh century was also a period of really abrupt and significant natural climate change. We're all familiar with anthropogenic climate change, that carbon emissions stay in the atmosphere, trap heat. Humans are changing the climate. That's a big problem and we can talk about it if you want. I just want to clarify, that view is not incompatible with the reality that the climate does also change for natural reasons on every timescale, from long geological timescales to much shorter timescales. We live in the Holocene. The last 11,700 years have been pretty stable, pretty warm. It's an interglacial. We're literally between Ice Ages right now. It's been really stable in the big picture. Yet even within that stability, there are smaller scale climate variations and climate changes. Because we need to understand how the earth system works, how the climate system works in order to be able to model what's happening, we need an empirical record of what the climate has done. For historians, this is great news because now we have a huge number of sometimes even pretty high resolution climate reconstructions for historical periods across the Holocene. We now know, like we did not know this 20 years ago when I started graduate school, that the Roman period experienced some really abrupt episodes of climate change. In this case, the sixth century, we know the cause: there was a series of really significant volcanic eruptions. Volcanoes are a very powerful short-term climate forcing mechanism. They eject sulfur into the stratosphere. It aerosolizes and creates a reflective shield that scatters the radiation entering the atmosphere. It leads usually to short term cooling. In this case, you had a series of really significant volcanic eruptions that cooled the climate for several decades. In some ways, the later series of eruptions even like a century and a half. It wasn't just a little bit cooler, it was like a degree to two degrees cooler, which we all kind of know now: two degrees isn't weather, this is climate. Two degrees doesn't affect your day, but two degrees globally is a pretty different globe. All of a sudden in the late Roman world, it's much, much cooler. Probably areas that have been wetter are now drier. Places that are drier may be wetter. It changes the hydrological cycle as well, which is more complicated. In addition to the shock of the plague, you have this simultaneous and probably not unrelated shock to the climate system. We know that it was essentially challenging for agriculturalists. When the sun is blocked and it's really cold and the wheat doesn't grow, your society then starves. The Romans get this wham bam double shock of climate change, famine, and plague. So back to how people explain this. Apocalyptic thought is one of the principle ways people frame it. To them, nature's going crazy. Huge amounts of the population are dying of this horrible sudden disease and the crops don't grow. You don't have microbiology and you don't have climatology. So you explain it with the resources of the worldview you have. There's a huge burst of apocalyptic thought in the sixth and seventh century, which is always kind of there. You mentioned that Christianity is eschatological. It is, yes, fundamentally, but that comes out in different ways with different emphases in different time periods. This is a period, the sixth century, when there's a really sharp emphasis on eschatology in Christian thought.
I found your early chapters in the book about what the Roman economy was like in this happy period quite interesting. There's a bunch of questions I have about this. If you read Gibbon, writing in the 1770s, I think he says that the happiest time in human history was this period you're talking about. That was true at least according to him as of a couple centuries ago. This was still peak civilization. You discuss the complexity of the Roman economy. The fact that millions of tons of wheat and other products have to make their way to Rome and the trade networks and everything. You basically say they were experiencing productivity gains. The wages were increasing, population was increasing, but they were still not at the level at which it was plausible that, say, for these climactic and biological factors, they might have had an industrial revolution. I'm curious why you think... paint a picture for me of what the Roman world looked like as of this happy period and why that still counterfactually couldn't have just saved us a thousand years of history if they were on the right track.
First of all, I think this is the sort of question that historians ought to worry about all the time: why didn't the Roman economy catalyze the takeoff? In some ways it was so precocious for its time period, and it seems not utterly impossible. The Roman world is still a pre-industrial economy so agriculture is the dominant sector. The majority of people work in agricultural pursuits and productivity is low. They don't have modern mechanized traction. They don't have modern synthetic fertilizers. They don't have the modern green revolution yields, all the things that have made agriculture stupendously productive. The primary sector is fairly limited in terms of its productivity because of the limitations on technical inputs. We can think of the inputs to an economy are going to be capital, labor, and ideas. What the Romans have is people. They have some investment, but they don't have technology. They don't have ideas. It's a late Iron Age civilization. I do think there's productivity growth and that productivity growth comes from markets, from trade where you get comparative advantage. In Egypt, I'm really good at growing wheat. You can make glass in Syria. And then we'll trade. The urbanization of the Roman world certainly facilitates that. Cities are these hubs of productivity and exchange. There's some technology. If you look really hard over five or six centuries, there's certainly economies of scale where the production process and manufacturing is moved from artisanal to industrial scale. But there's no takeoff because they don't have science. They don't have research and engineering that drives continuous productivity gains. I think they go precociously far in a pre-industrial setting where you take trade really far. They have good institutions in terms of strong property rights. There's relatively reliable contract enforcement. There's financial markets. They have the most advanced financial markets in the world before the 17th or 18th century. There's impersonal financial intermediation. It's not like you have to know me and come ask me for a loan if you wanna build a ship and go trade something. There are banks that take money from depositors, keep balances, and then lend out to debtors who want to go and do entrepreneurial things. They have so much potential, but there's no spark. You never see these sustained productivity increases. I would just say ultimately it's because the Romans don't have technology improvements that are really self-sustaining. The reason they don't have that is because they don't have science. Their science sucks. I'm offending some of my colleagues, I'm sure. Galen is great. Ptolemy's incredible. I love Pliny the Elder's encyclopedia, but if you look in the big picture, the contribution that the Roman Empire makes to our knowledge of how nature works and then the applied technology that comes out of that is really pathetic for five, 600 years. They go as far as you can with Smithian advantages to market exchange and specialization, to banks and finance. But without the kind of creative destruction of new technologies that improve productivity, you're eventually gonna run out of improvements.
If you're Augustus or some other Roman emperor and you're like, 'Look, we've got this big good economy, but I wanna see productivity gains,' and you wanna make it happen somehow, is there something from a top down perspective you could have done? In Britain, the government subsidizes the royal arts and so forth and the Longitude Prize.
That's exactly what I was gonna say. This happens first in France and then Britain, but you get royal societies for science where you're doing really... I would say there's like three things that are essential there. One is the promotion of what we would call basic or fundamental science. It doesn't all have to be immediately practical or commercialized. But you're promoting deep knowledge of nature. Two, you're doing it in an empiricist way. This is something very important in the 17th century that the Romans by contrast don't have, is the spirit of Francis Bacon that we need to ground our knowledge in experiment and observation, not just believe whatever authorities or Aristotle said. That's very much the spirit of places like the Royal Society: we don't take things on anybody's word, especially Aristotle's. You need basic science. You need empiricism, rigorous and self-correcting. Third, you need a sense of useful knowledge, and that's the other thing that really comes together in the 17th century: not just the basic and abstract science, but the application, and the 17th century language for that is useful knowledge. That is something that doesn't ever get wired together in the Roman Empire. There are tinkerers and engineers, but they're not talking to the mathematicians and the physicists. If you were from on high to design self-sustaining innovation, I think you would want to bring those elements into proximity. Augustus, unfortunately for them, didn't do it. Probably good for the world. The Romans are pretty nasty people in a lot of ways. I definitely am of the opinion that the high science matters, that Isaac Newton is not a tinkerer. He's not building pumps. But the guys who are are his friends. They're in and around the Royal Society, and they're absolutely... Look at Denis Papin, who was a French engineer who was very much in the circle of Leibniz and the very high abstract mathematics, trying to build vacuum pumps. The proximity of high math, high science, very abstract with what is ultimately gonna be the sectors that lead to mechanization where then you can harness this source of energy that is there all along but hasn't been tapped in coal. That's what catalyzes the big positive cycle of positive feedbacks. What the Romans don't have is that. What the Romans do have is the kind of specialization, and now that we look for it, it's there. When you look at food processing, which is a huge sector, the way that they built mills, there's definitely improvements. But there's never the catalytic change where you get runaway positive feedbacks.
That's right. A previous guest of mine, Nat Friedman, I don't know if you saw this, launched this challenge called the Vesuvius Challenge. This library at Herculaneum in 79 AD was buried under the ashes of Mount Vesuvius when the volcano erupted; speaking of volcanoes. Now they figured out with modern techniques how to read the burned scrolls and it is supposedly the biggest library of classical text ever. It would double the amount of classical text we have. As a scholar of ancient Rome, what would you personally find most fascinating? What are you excited to find from this data?
I'm super interested in the history of math. What happens after Euclid? It's very hard to say because you get these really interesting people that pop up, like Diophantus who's later, in the early Roman Empire. There's still really interesting math going on. Euclid is incredible. The Greek experiment in math and science is the one that I think had the better chance of sparking sustained takeoff. And it didn't. It'd be interesting to know more about why. Why did things stall? Because these people... Euclid is not just a towering genius who comes out of nowhere. He's very much a product of the culture and the questions that are being asked in the generations before. It just sort of feels like after him, you fail to get that kind of sustained continuous progress and advance. Maybe back to that big question that we were asking before: what prevents the kind of breakthroughs that we see in the modern world?
What is the population of Greece during their golden age? Of the greater Greek world or individual city-states like Athens?
We think of Athens as being a couple 100,000 people. Not massive.
So I wonder if the Greeks had the science but not the people to sustain a modern economy. Or not modern, but even a sort of industrial economy. And then the Romans had the people but not the science.
Yeah. There's probably something to that, that there's just not the critical mass of educated people, of sheer cognitive power to keep it going.
Let's turn to another one of your books, the one about slavery in the Roman world. I did not realize before I read that one how much Rome was a slave society. I guess that just isn't a salient thing in a conventional understanding of Rome. Why don't you paint us a picture of how much slavery was involved in that world?
Slavery tragically is a really important institution throughout history. We sometimes tend to think of it as a distinctly modern phenomenon, but that actually misses the deeper picture. In fact, it obscures the importance of modern slavery because modern slavery is uniquely important and it's uniquely tied up with certain kinds of market exchange and certain kinds of production, certain kinds of racial ideologies. There are things about modern slavery that are really important to understand are different, but not just because slavery is there. Slavery has this longer history and slavery is more important in some societies than others, and we want to try and understand that, to ask why and then what implications does that have for understanding those societies? Rome is one of those societies. Slavery is really a prominent institution in Rome from the late republic. As the Romans conquer other parts of the Mediterranean, they start taking captives as slaves en masse, and they build an economy that really relies on slave labor in important sectors of the economy. Plantations where commodities like wine, olive oil are produced for market exchange that allow landowners to amass enormous amounts of wealth. Slavery becomes this really important institution that's entangled in the development of the Roman economy from maybe the third or second century BCE, and then with ups and downs and really important changes along the way for centuries and centuries.
So you're pointing towards from the supply side, all the Roman conquests lead to all this surplus labor that they can make use of, and on the demand side, these cash crops.
Exactly. I'm very big proponent of the idea that you have to have both. You have to have a source of slaves. After the conquest stops, the Romans figure out other sources of slaves. If anything, the demand is equally or perhaps even more important because if there's not a mechanism, if there's not institutions that let you turn this kind of exploitation into cash flow, the institution's not gonna go very far. It really is the institutions, the presence of markets where you can take labor and turn it into profit that's the most important element.
One of the things I find interesting is in the age of colonization we're used to thinking about slavery in terms of race but also religion and other things which more obviously demarcate free and slave populations. In the Roman world, it doesn't seem that that's clearly the case, yet there's no abolition movement the way that emerges out of England in the 19th century or maybe even before that. The reason that's mysterious is, if you were literally descended from slaves, if you were like, 'My grandfather was a slave but then we were freed,' and they're basically just like you, you would think that there would be more of a sense of... not everybody would be an abolitionist, but at least some people would be writing about abolition. With Christianity and so forth burgeoning, they didn't seem to have a problem with it. Why is there no abolition movement despite the heterogeneous nature of the slave population?
It's sort of disturbing in a way, isn't it, that humans have the ability to convince themselves that it's okay to own other human beings as property through a variety of different kinds of ideological justifications. You see even in the ancient world there are different models that people use to say that slavery's okay. Aristotle develops a theory of natural slavery that actually some people deserve to be slaves by their very nature and that it's actually good for them to be in bondage. What's really interesting though is that that doesn't actually ever seem to be the dominant ideology. The Roman ideology of slavery is not racialized. It's not like the Romans think that the Greeks or the Germans are some fundamentally separate kind of human that justifies their exploitation. The Roman ideology of slavery is really rooted in the law of property and status. They think that slaves are people who've been conquered and rather than killed, they've been spared, and they've been sold into the condition of being somebody else's property. This seems to mentally explain to them where their slave system comes from and why it's justifiable. You have different kinds of criticism of the slave system from within, but remember, most of what we have written is from the slave ownership class. I don't think the slaves were themselves believing this ideology, and there must have been sort of what we would think of as abolitionist movements or spirit that we just don't have really good records of. It is this curious thing that the Romans are able to build this huge system that's really brutal and really violent, but has this kind of flimsy ideology where they tell themselves these stories. The deeper lesson of that is that humans can create these systems of belief that will exclude others and justify almost any form of exploitation and convince themselves that it's okay.
I hope your next book about The Last Animal discusses the potential parallels with factory farming.
There's some... there's a pretty gruesome chapter, I'll say that.
I don't know if you mentioned, what numbers you say, I think it was like 10 to 20% of the population under the Roman Empire was enslaved. Given that large a size of a slave population, it's surprising to me that there are so few slave revolts not only in Rome, but even throughout history. There's Spartacus in 71 BC, then there's the Haitian Revolution. If 20% of the population is enslaved, how is this sustainable? If you're running a farm and there's 4,000 slaves and then the next farm over also... why aren't there more slave rebellions? Why not? How did they do this?
They have a really elaborate system of repression. They're worried about it. Probably the parts of Roman society where there are 20, 30, 40% slaves are pretty limited to certain regions and certain time periods. Partly because once you cross some kind of threshold, the challenges of repressing direct violent resistance… It's a system of exploitation. That means there's always a mix of carrots and sticks, to put it crudely. The Romans extract people's labor partly through physical violence, but also partly through systems of manumission that try and incent people to obey and not to rebel in order to earn their freedom. They're using everything from literal chains to enticements to try and keep rebellion from ever coalescing in a way that can turn into collective violence. It's a little bit challenging for us to look back. We know in Pompeii, the slave population is huge. It must be 30%. Not all of these people would've been plantation workers who were lashed every day and worked to the physical bone. A lot of them are nurses and textile workers and maids and tutors and all sorts of things that are sort of quasi-embedded in households as well, where there's always this weird psychological dimension too. Part of the strangeness of slavery is how deeply embedded in domestic institutions it is as well. There are ideologies in which the paterfamilias is sort of the father and the master. That tries to brainwash people against resistance. The important thing to recognize is it's just a pervasive system that tries to colonize people's minds and pervasively tries to keep them from resisting.
I wonder if we can close the loop with the question we began with, which is, why didn't Rome have an industrial revolution? I don't know if it's a plausible explanation that cheap slave labor reduced the incentives for mechanization and engineering and other crafts or if not. I don't know.
It's definitely an argument that's been made. Aldo Schiavone was an Italian historian who argued that. It's kind of a neo-Marxist tradition that argues this. It's an interesting argument. I don't buy it at all. The good version of that argument would just be that the Roman Empire is using slaves in many of the most forward elements of the economy too. We tend to think progress and economic growth and innovation is good, and we know slavery's bad, so we tend to think that those things don't go together. But in reality, it's the most economically advanced sectors of the Roman economy that had a high degree of organization and productivity that tended to employ slaves. In the Roman world, you could make the argument that if the labor in those sectors had been free, there would've been more opportunities for positive feedback loops. The way the argument's usually made is just that the Romans got rich without really thinking about productivity. They just wanted to extract labor, extract wealth rather than create wealth. That's not a terrible argument, but ultimately I don't think it's the system of labor that keeps the Roman world from industrializing. There are lots of sectors in the Roman world where slavery is not a dominant institution, and it's not like they're more productive or flirting with some kinda breakaway. So it's an interesting argument, but not one I've ever found all that persuasive.
Final question about Roman slavery. What did Gladiator get right and wrong about? Would they just abduct you in front of your house and make you a slave?
You mean the first one? The first one got right that when you're making a movie you should worry more about making a good movie than a certain amount of history, but actually the first one's a great movie. If it was completely historically accurate, it would've been much more boring. So I'm not gonna be critical of that movie. It plays very loose with the facts of high politics around Commodus and the creation of this character. But who cares? Russell Crowe's incredible.
But on slavery in particular.
On slavery in particular, I think actually that's one of the strong suits of the movie. You see this completely exploitative system that brings people from very different parts of the world who have very different backgrounds. The system of urban spectacle is very real. The use of slave labor in that is certainly a part of it. So the movie actually gets some really important things about that right. That makes it totally forgivable that it has to create a kinda high politics storyline.
Okay, I think that covers all the questions about Rome. We can get back to your most recent book about human history and plagues. What do you make of the general argument that people have often made that we were living in a sort of Eden before agriculture? Especially given you've explained that all these diseases that we're sorta stuck with are actually quite new. If we take that perspective seriously, was life before human population exploded and we had agriculture just much more pleasant, at least in comparison?
Homo sapiens is 200,000 to 300,000 years old. We emerge in Africa and disperse, multiply, but we spend 90, 95% of our history as foragers. People who are hunter-gatherers, who take energy from wild food sources, rather than sedentary farmers who've domesticated plants and animals and live a sedentary lifestyle where you're enslaved to this wheat or rice, but it gives you reliable calories. That is along with the Industrial Revolution, and then whatever this thing we're about to go through, the biggest change in the history of our species. The shift from foraging to farming affected everything. It affected our beliefs. It affected our genetics. We're all basically genetically different, adapted to live in a different kind of environment with different kinds of diets. It affected our societies, it affected inequality, it affected culture in every possible way, and of course it affected our health in really basic ways. It affected our labor regime; doing the same kind of labor over and over every day is very different from running around as a hunter, chasing deer or whatever, which sounds quite nice. It changed our labor regimes. It changed our diet, most of all. Hunter-foragers tend to eat high protein, high fat-ish diets, with no refined carbohydrates, but limited carbs. It's a very varied, highly varied diet. Sedentary farmers tend to eat more monotonous diets and they tend to be dependent on grains and starches, so a very narrow spectrum for your calories. Changes in labor regime, changes in the diet, and then changes in lifestyle, being sedentary and living in big populations that then puts you in proximity to other humans, puts you in proximity to human waste. Feces are a major conduit of infection. It puts you into proximity to the air they breathe, which is conducive to respiratory diseases. This transition, which takes thousands of years, is more of a process than an event. It has massive implications for human health, including the infectious disease environment that we inhabit. It's not like hunter-gatherers were living in paradise. The infectious diseases that they had were seriously burdensome, they sucked, and probably most people died of infectious disease. Malaria is a really old disease. Lots of diseases existed in the Pleistocene, in our Paleolithic past. It's not like it was Eden. There is this idea that the transition from foraging to farming... Jared Diamond called it humanity's biggest mistake. Certainly these changes entailed some things that were not net positive for humanity and one of them is that it definitely increased the infectious disease burden. Simply as our population multiplies and as we're in contact with feces and as we're sharing the air through which respiratory pathogens can spread, diseases are constantly trying to take advantage of this. That's just how nature works. Energy is scarce. Everybody's trying to steal it from everybody else, including microbial parasites. The disease burden of humans over time definitely increases. The burden of infectious disease on humans goes up over time. Very broadly across these thousands and thousands of years, the diseases suffered by people by the time of the Roman Empire are absolutely much worse than what had been the case in Stone Age times.
James Scott has an interesting theory in Against the Grain. I don't know if it originates with him, but he argues that one of the reasons that the early agriculturalists were so successful... And David Reich, if you've seen his stuff about The Yamnaya 4,500 years ago conquering all of Eurasia, but before them, the Anatolian... the initial farmers are the ones who displaced the initial hunter-gatherers across Europe and Asia. He argues that initial wave was so successful because of these first diseases that the farmers had created the conditions to engender. Basically, the relationship these farmers had with respect to the foragers they were taking over from was similar to the relationship the Europeans had to the Native Americans, where inadvertently, the disease is just a significant player in why you were able to dominate them. I don't know how plausible you find that.
I mean, first thing, it's important, I think you were starting to get at this, that there's never a generation of humans that has the opportunity to make this choice once and for all, like, should we stay hunting mammoths or should we become sedentary farmers with basically torturous dentistry and die by diarrhea? This happens over thousands of years through an evolutionary process where nobody can... It's a story of unintended consequences. The mammoth are gone partly because we killed them all. People start... the first livestock that are domesticated are goats. Nobody says, 'Hey, let's become goat farmers.' The goats are wild. They're ibexes. People are hungry, and so they start managing them to only kill the males to make sure that they can reproduce, and they start penning them and they start killing the wolves who are trying to attack them. Over very long periods of time, this becomes this tight mutualistic relationship where all of a sudden we're goat farmers. But no generation makes that whole decision for anybody. That's part of it, is that it's unintended consequences that are made in very incremental steps. Two is, I definitely agree that there's some kind of cultural selection here where the farming groups are simply so much more adapted to extract energy efficiently from the environment. It's all about energy. You want to multiply, you want to grow, you want to survive, it's all about energy. Foragers require huge landscapes to extract enough energy to feed themselves and grow and reproduce, whereas farmers per unit of land can extract such higher rates of energy that then can be, through photosynthesis, turned into edible sugars that we can metabolize. Those populations are just growing faster, that they quote unquote out-compete the hunter-gatherer population say of Europe that are largely but not completely displaced. Now, on top of that, just the energy story alone is a big piece of it, but then on top of that, you probably do have some kind of population difference in the exposure and possibly even immunity to infectious diseases. I definitely think that early farmers, the first farming societies that are starting to live sedentary lifestyles where you have aggregations—these are not cities, these are villages—but still, that's more than a hunter-gatherer band. Your childhood is then going to be constant exposure to a series of pathogens. Those kinds of populations, when they're then migrating into Europe, are probably carrying these pathogens with them that may have had a kind of further effect that on top of just being able to extract more energy and multiply faster, drives up the mortality of the existing populations.
Yeah. The point you made about fertility is interesting. I vaguely remember reading that it's not just the fact that the energy density is lower, it's that you're also moving around a lot. Because of that, you're spacing out kids much more so than if you were just in the same place. I think the actual fertility for foragers is sort of like reasonable—I don't know if sustainable is the right word because I don't mean in an ecological sense but more so—it keeps your population constant.
