long . lines and ripples

The Long Prehistory of Animal-to-Human Disease Transmission

As a coda to my previous post, another chapter of James C. Scott’s Against the Grain that is worth a look is his third chapter: “Zoonoses: A Perfect Epidemiological Storm.” For a book that argues that agriculture generally worsened the human condition, the topic is a natural fit: all the evidence suggests that some of the most pernicious diseases that are still with us today (e.g., influenza) came into being less than 10,000 years ago, when humans began to live in the constant presence of cultivated plants and animals. Beyond the general relevance of the topic in 2020, the zoonosis problem is getting a lot of airtime right now. A few days ago, I heard a short bit on NPR about it, and the New York Times just ran a long article in the magazine about zoonotic diseases.

This chapter offers an argument about what changed in humanity’s distant but still discoverable prehistory (10,000-20,000 years ago). Here’s a paragraph that condenses what Scott argues:

The disease ecology of the late Neolithic was not simply a result of the crowding of people and their domesticates in fixed settlements. It was rather an effect of the entire domus1 complex as an ecological module. The clearing of the land for agriculture and the grazing of new domesticates created an entirely new landscape, and an entirely new ecological niche with more sunlight, more exposed soils, into which new suites of flora, fauna, insects, and microorganisms moved as the previous ecological pattern was disturbed. Some of the transformation was by design, as with crops, but much more represented the second- and third-order collateral effects of the domus’ invention.


Between 10,000 and 5,000 BCE, agriculture spread to several major civilizations, and yet the world’s population increased from just four to five million. Given the advances in agriculture and food-production capacity of the most technologically advanced civilizations during this time, this low level of population growth is surprising–especially compared to the population explosion that would happen over the next several millenia after 5,000 BCE. But why did the first 5,000 years of agriculture see a relatively modest (+20%) increase in the human population? Boiled down, Scott’s answer is that agriculture brought with it a continuous rain of disease that decimated human populations until they developed basic immunities.


The importance of sedentism and the crowding it allowed can hardly be overestimated. It means that virtually all the infectious diseases due to microorganisms specifically adapted to Homo sapiens came into existence only in the past ten thousand years, many of them perhaps in the last five thousand.They were, in the strong sense, a “civilizational effect.” These historically novel diseases–cholera, smallpox, mumps, measles, influenza, chicken pox, and perhaps malaria–arose only as a result of the beginnings of urbanism and, as we shall see, agriculture. Until very recently they collectively represented the major overall cause of human mortality. It is not as if presedentary populations did not have their own parasites and diseases, but such diseases would not have been the crowding diseases but rather diseases characterized by long latency and/or a nonhuman reservoir: typhoid, amoebic dysentery, herpes, trachoma, leprosy, schistosomiasis, filariasis.

Scott, Against the Grain, 101-102

And here’s an interesting tie-in with the current crisis. Scott claims that an important early spreader of zoonotic disease were migrating birds:

Before extensive human travel, migratory birds that nested together combined long-distance travel with crowding to constitute, perhaps, the main vector for the spread of disease over distance.

Scott, Against the Grain, 101

Birds generally spread out to find unclaimed territory both before and after migration, but during the migration itself, more bunching can occur. But it is the ability to fly makes them uniquely suited to spread pathogens over a long distance. How many other animals can claim that?

As it turns out, there is at least one more important winged animal that spreads a lot of disease: our only other flying mammal (which has been in the news a lot lately), the bat. Ferris Jabr describes the uniqueness of the bat last week in the New York Times article I cited at the beginning:

Rabies, Ebola, Marburg, SARS, MERS, Hendra, Nipah: Bats are a definitive or probable source of many of the most lethal zoonotic viruses to enter human populations. Why? There are many reasons. Bats are an ancient and diverse lineage: Nearly one in four mammal species is a bat; as a group, they have been co-evolving with a vast array of viruses for around 50 million years. Many bat species are social: They roost in large numbers, huddle together for warmth, groom one another and suckle their young, providing numerous opportunities to circulate pathogens among themselves. Bats are highly mobile, sometimes traveling dozens of miles between roosting sites or migrating hundreds of miles seasonally, taking their viruses with them.

Bats also have a unique immune system, most likely as an adaptation to a talent no other mammal can claim. In order to fly, bats must significantly increase their metabolic rate, which creates dangerous molecular byproducts, such as reactive ions that damage cells and DNA. During flight, bits of fractured DNA escape the nuclei of bat cells and drift about, resembling the presence of viral invaders. In most animals, all that havoc and misplaced DNA would provoke a robust immune response and chronic inflammation, needlessly harming healthy tissue. As a result of these pressures, bats have evolved several countermeasures, including tempered inflammatory reactions. In turn, these adaptations have made them more resilient to actual viruses and less likely to initiate the kind of overzealous immune response that often kills other infected animals.

Ferris Jabr, “How Humanity Unleashed a Flood of New Diseases” (The New York Times Magazine, June 17, 2020)

Bats continue to be great at spreading disease–far better than birds–for the reasons listed above. But the best evidence now does not suggest that bats brought the coronavirus to a community. The current thinking that Jabr’s article summarizes at the begining is that a commercial hunter captured a bat in a cave or thereabouts, and brought it to a crowded market, where it was kept in cramped conditions around a lot of other animals awaiting slaughter or sale. And while the Chinese “wet markets” may be notable for the diversity and exoticism of their animals or sale, the general problem of crowding between unfamiliar species is exactly what Scott describes as so new and dangerous about the start of agriculture itself:

The neolithic was not only an unprecedented gathering of people but, at the same time, a wholly unprecedented gathering of sheep, goats, cattle, pigs, dogs, cats, chickens, ducks, geese. To the degree they were already “herd” or “flock” animals, they would have carried some disease-specific pathogens of crowding. Assembled for the first time around the domus, in close and continuous contact, they quickly came to share a wide variety of infective organisms. Estimates vary, but of the fourteen hundred known pathogenic organisms, between eight and nine hundred are zoonotic diseases, originating in nonhuman hosts. For most of these pathogens, Homo sapiens is a final “dead-end” host: humans do not transmit it further to another nonhuman host.

Scott, Against the Grain, 103

I am struck by the last sentence here. Humans aren’t even effective at passing on animal-borne disease to other species, only to one another. Their infection is an accident, collateral damage of the disease’s path to sustainable transmission between non-human hosts. Nonetheless, some of our most devastating diseases can be attributed to this dead end of human-animal transmission:

The list of diseases shared with domesticates and commensals at the domus is quantitatively striking. In an outdated list, now surely even longer, we humans share twenty-six diseases with poultry, thirty-two with rats and mice, thirty-five with horses, forty-two with pigs, forty-six with sheep and goats, fifty with cattle, and sixty-five with our much-studied and oldest domesticate, the dog. Measles is suspected to have arisen from a rinderpest virus among sheep and goats, smallpox from camel domestication and a cowpox-bearing rodent ancestor, and influenza from the domestication of waterfowl some forty-five hundred years ago.

Scott, Against the Grain, 103-104

Scott’s arguments in this chapter are simple and prescient. Where you have crowds of people and animals, you will have new diseases, every time. And given that China has some of the densest concentrations of human and animal life on Earth, it is not surprising that Scott that sees some regions in China as the living example of his argument. We are living it now:

The generation of new species-jumping zoonoses grew as populations of man and beasts swelled and contact over long distances became more frequent. It continues today. Little wonder, then, that southeast China, specifically Guangdong, probably the largest, most crowded, and historically deepest concentration of Homo sapiens, pigs, chicken, geese, ducks and wild animal markets in the world, has been a major petri dish for the incubation of new strains of bird and swine flue.

Scott, Against the Grain, 105
  1. “domus” = Latin for a house, especially an upper-class Roman dwelling. Scott uses the term broadly and metaphorically to suggest the whole range of changes to human life that accompanied permanent settlement.[]


Ferris Jabr, "How Humanity Unleashed a Flood of New Diseases". New York Times Magazine, June 17, 2020. Link

James C Scott, Against the Grain: A Deep History of the Earliest States. Yale University Press, 2017.