William Cronon - 460 Handout 3 - Migration, Disease, and Death

Lecture #3: Migration, Disease, and Death

Outline:

I. Prelude: The Black Death

The migration of organisms among ecosystems is a central theme in environmental history world-wide...an insight that probably doesn't seem so novel in 2020 as we continue to experience the widening consequences of COVID-19.

Disease is fundamental to environmental history: the bodies of human beings serve as environments in which disease organisms reproduce and thrive. Each of us has our own internal ecosystem, and the ecosystems that are ourselves are embedded in a web of social, economic, political, cultural, and biological relationships not just with other people but with many other animals and organisms.

The movement of epidemic diseases that we'll trace in this course is of course not limited to the trans-oceanic migration of disease organisms from the "Old World" to the "New World" that is the central story of today's lecture.

Among the most famous of all epidemics in world historywas the Black Death, beginning in Europe in 1346. It had massive consequences whose effects lasted for decades, even centuries.

As far as we know, the plague originated in the foothills of Himalayas, Burma, and Mongol caravans were the vehicles transporting it westward across the steppes of Central Asia.

The old story: plague can be endemic to rats and other rodents, transmitted by fleas living in rodent community. Three organisms are thus involved: rats, humans, and the infectious agent, originally known as Pasteurella pestis (named for Louis Pasteur), now renamed Yersinia pestis.

The movement of plague west from Burma to China to Europe infected humans and rat populations with fleas as the vector transmitting the organism to both.

The plague had differential effects on humans and rats: it could be akin to a childhood disease for rats, often with limited mortality; but in humans, it could have mortality rates as high as 30%-90%.

There's recently been a historiographical controversy over whether this familiar story is accurate. Samuel Cohn and others have argued that rats and fleas could not have caused the Black Death, for several reasons:

mortality rates of modern plague significantly different from those of Black Death;
seasonal cycles of infection of plague and Black Death are different;
virulence and transmission rates don't seem to match.

Cohn hypothesized that some unknown viral agent was the true cause of the Black Death, not the modern plague. He also speculated that perhaps the agent was anthrax.

Perhaps the lesson we can draw most confidently from this is that it's very hard to diagnose or isolate the epidemiological agents of a disease 700 years in the past.

Also, the words and understandings that inform our own thinking today about this disease are only partly relevant to our understanding of its history in the 14th century (and how that history was understood by people of the time). There's a way in which it doesn't really matter whether what happened in Europe in the late 1340s was caused by rats, fleas, and the plague bacillus, or by some other constellation of biological agents.

Whatever the agents, the consequences are not in doubt: four years after this epidemic started in Europe in 1346, 1/3 of the population of that continent was dead: 25 million people were dead. If we were all sitting in a room together, you could look around and imagine what it would be like if every third person you see were to die from this disease. (This may help put the terrors of COVID-19 in perspective, though of course each new pandemic illness has its own unqiue terrors.)

Broad conclusions we can draw from the story I've just told for the purposes of environmental history in general might include the following:

human migrations are always accompanied by other creatures, some of which are lethal for their human hosts;

we share many of of our most important illnesses with other animals (epidemics in humans are called epizootics in animals);

disease can be endemic to some areas, but absent in others...and therefore epidemic when it occurs:

changing transport technologies turn trade corridors into disease vectors (war can play a role as well);

the late Middle Ages were a time of increasing contact between Europeans and rest of world:

The spread of diseases to parts of the world where they had previously been absent was fundamentally linked to this process, both imported into Europe and exported out of it.

II. Virgin Soil Epidemics

It's not generally a successful evolutionary strategy for disease organisms to wipe out their host populations, since they put the ecosystems in which they themselves reproduce (our own human bodies) at risk if they do so. This is why many diseases (including COVID) move between different host populations that are different affected by the resulting infections, just as rats and humans experienced the plague differently.

The antibodies of our immune systems serve as defensive tools for host populations: diseases trigger us to generate new antibodies as pathogents reproduce inside us, leading to a perennial race in which diseases seek to reproduce and transmit themselves to new hosts before the host population can generate sufficient antibody defenses to stop the spread.

It's importance here to notice that disease immunities are often at least as historical as they are genetic: a population's history of exposure to a given disease allows mothers to transmit antibodies to offspring in utero and through breast milk, and the shared collective antibodies of host population significantly affect overall rates and severities of infection.

We call this last attribute "herd immunity": the collective pool of antibodies that protects the population as a whole from disastrous epidemics.

As we now move for the rest of the lecture to the western hemisphere, the crucial observation with which to start is this: native populations of the Americas lacked any historical immunological experience of many Old World diseases: measles, smallpox, chickenpox, mumps, malaria, yellow fever, etc., were all absent from the western hemisphere at the time Columbus made his voyage in 1492.

As a result, those populations were comparatively healthy, but they were also deeply vulnerable and very much at risk when exposed to disease organisms migrating from Eurasia and Africa.

Why did the ancestors of today's Native Americans lack immunity to these disease organisms?

Here I'll add the caveat that for the sake of the next block of argument, I'll assume that these ancestors migrated to North America across a land bridge at the Bering Strait, the most commonly accepted archaeological explanation for how humans reached the western hemisphere. This hypothesis is at variance with the creation stories of many native peoples who believe that they originated close to their original homelands in the Americas. Whichever of those narratives you yourself belief, most (though not quite all) of the biological implications I sketch here could still apply.

If the ancestors of present-day Native Americans did in fact migrate across a land bridge at the Bering Straits at the glacial maximum 18-20,000 years ago, the potential epidemiological implications of this could have included the following:

their migration via cold northern environments of the Arctic that were inhospitable to illnesses originating in warmer climates may have encouraged the disappearance of those disease organisms;
their migration in small bands meant host populations that were too small to serve as self-sustaining hosts for disease organisms, which died out as a result;
the fact that they brought no domesticated animals to act as additional host pool for diseases diminished the likelihood that some diseases could accompany them;
a migration 18-20,000 years ago obviously could not be accompanied by any disease organism that only appeared in other parts of the world after that migration was complete.

As a result of all these factors, migrating Indians shed Eurasian illnesses as they migrated (if they had ever been exposed to them)--and thereby lost any historical immunity they might have had to those diseases.

The introduction (or re-introduction) of disease organisms into populations with no antibody experience with those organisms produces what is known as a "virgin soil epidemic," about which you're reading in the article by Alfred W. Crosby this week.

Such epidemics can be especially devastating not just to infants, but even to mature adults in their prime reproductive years, age cohorts that otherwise are often better able to survive illness than the very young or the very old.

It's not that individuals are more or less susceptible to infection, but rather that the population as a whole has statistical vulnerability: it has zero herd immunity (the condition that described the entire human population of planet Earth when COVID-19 began to spread in 2020).

Classic virgin soil epidemics have often occurred on relatively small islands, or in isolated populations: the smallpox epidemic in Iceland in 1717 a famous example.

Note also that emerging immunity to one invading infection provides no protection to other invading infections with different antibody profiles, with the result that repeated waves of epidemics became part of the biological experience of native populations throughout the Americas.

Repeated waves of infection continued even into the 20th century, as exemplified in the twentieth century occurred during the construction of the Alcan Highway during World War II in 1942-3.

III. We Were Born to Die (Mayan lament)

The earliest trans-Atlantic disease migrations were non-virulent endemic illnesses--diarrheas, dysentaries, respiratory illnesses, sexually transmitted diseases--capable of surviving the voyage on board ship without burning themselves out in their European and African host populations.

It was more difficult for more virulent organisms to make their way across the ocean, since they had to keep finding new hosts throughout journey in order to reach the other side of the Atlantic.

1518: smallpox appeared for the first time in Santo Domingo (modern-day Haiti and the Dominican Republic). One-third to one-half of the natives died in just few months.

1521: smallpox aided Cortes in his conquest of Tenochtitlan (modern-day Mexico City) , capital of the Aztec empire.

Repeated epidemics, with many different diseases, brought centuries-long population decline for native peoples throughout the Americas.

There were at least 35 separate recorded episodes of smallpox alone between 1520-1898.

Among the best documented and most horrific was the Mandan epidemic when an American Fur Company steamboat brought smallpox up the Missouri River to the Mandan villages in 1837. You can read more about this episode in Wikipedia if you're interested: https://en.wikipedia.org/wiki/1837_Great_Plains_smallpox_epidemic

IV. Counting Up the Victims

The myriad demographic effects of these epidemics have only come to be understood by scholars over the past few decades.

Early population estimates followed James Mooney's Aboriginal Population of America North of Mexico (1928), which estimated that the total population of Indians in all of North America at the time Columbus made his voyage was just 1.1 million. Mooney's classic text can be downloaded here if you're interested:
https://repository.si.edu/bitstream/handle/10088/23978/SMC_80_Mooney_1928_7_1-40.pdf

W. W. Borah and S. Cooke begin gathering data in the 1930s and 1940s to try to estimate the scale of mortality especially in Mexico & California, and concluded that death rates were far higher than originally imagined.

In 1966, Henry Dobyns applied Borah and Cooke's findings to the entire hemisphere, assuming an ultimate mortality rate from epidemic disease of 90%, and calculated that the pre-Columbian North American Indian population was 10-12 million, while the population of the entire western hemisphere amounted to 90-110 million.

Dobyns' arguments generated much controversy and his estimates were likely too high, but scholars today all agree that native populations in the Americas suffered terrifyingly high mortalities in the wake of being exposed to Eurasian epidemics.

For our purposes, the key conclusion of these scholarly debates is that human migration from Eurasia and Africa produced declines in North American Indian populations from highs of 2-10 million people to less than 1 million. Even by the smallest estimates, Amerindian populations in North America were cut in half.

The epidemics had dramatic social, economic, political, and cultural consequences, which we'll explore during the next couple weeks.

V. The Continuing Migration of Disease and Death

The migration of disease organisms reproduced Old World disease environments not just for native populations, but for Europeans as well.

South Carolina is an important case in point.

The forced migration of enslaved peoples out of Africa brought a number of tropical and subtropical diseases to the Americas. Among these were malaria, yellow fever, filariasis, yaws, and elephantiasis, all of which became common to the Carolinas during this period. Only sleeping sickness among the major African diseases failed to establish itself, since its carrier the tsetse was not available to help in the migration.

African diseases in South Carolina favored enslaved people of African descent in wet, lowland rice-growing areas where malaria became endemic. The recessive sickle cell gene, which is more common in Africa than elsewhere because it offers some protection against malaria (and so is selectively favored in regions where malaria is endemic, even though it causes sickle-cell anemia in individuals unfortunate enough to carry two of these recessive genes) meant that although enslave Africans suffered from the emerging disease environments of the Carolina lowlands, they were less vulnerable to malaria than their white owners were, which produced settlement patterns in South Carolina where plantation-owning whites often lived in urban Charleston, leaving the management of their plantations to overseers. The result was an unusual degree of geographical separation between plantations and their masters, contributing to the black majority population (and widespread white fears of slave uprising and extreme pro-slavery views of whites) that characterized South Carolina on the eve of the Civil War. Again, one should never assign too much causal responsibility to biological factors in any historical situation, but it's perhaps suggestive that South Carolina was the first state to secede from the Union in 1860, and the first to fire on Union forces in the shots on Fort Sumter that began the war.

Even the plague is now endemic in the American West, where you can easily find signs in roadside oases warning you not to interact with ground squirrels and other rodents that are infected with it. It arrived in California at the start of the 20th century in trans-oceanic steamships which, like the carvans that transported the plague in the 14th century, accelerated the speed at which disease organisms could move world-wide.

The arrival of air travel only accelerated that process in the middle decades of the twentieth century, as we've seen with the horrors of HIV-AIDS, West Nile virus, Ebola, and now COVID-19. For especially striking graphic illustrations of this process for COVID-19, see these remarkable animated map presentations in The New York Times:
https://www.nytimes.com/interactive/2020/03/22/world/coronavirus-spread.html
https://www.nytimes.com/interactive/2020/us/coronavirus-spread.html

The world-wide environmental history of epidemic disease has not ended, and likely never will.