In honor of Indigenous Peoples Day, I’m here to talk about everyone’s favorite topic: Disease.

When I was a kid, science told us that “Indians” walked here across a “land bridge from Asia” about 13,000 years ago. And science was wrong. We now know that the land migration that took place 13,000 years ago wasn’t the FIRST populating of the Americas, but the LAST migration that would occur here for about 12,500 years.

Which meant that civilization here developed with essentially no contact with Europe, Asia or Africa until the 10th century AD. And except for one doomed Viking colony in Greenland, there’s no other significant contact between the Old World and the New until the late 15th century.

Here’s why that matters: The humans who settled here during and immediately after the last Ice Age left the Old World about 6,000 years before humans in places like Iraq, India, Egypt and China started settling down to grow various grains in the same fields, year after year after year.

We think of these places as “cradles of civilization,” and being sedentary civilized people ourselves, we generally agree that civilization is a good thing.

But here’s what they didn’t teach us in school: For the people who actually grew those grains, early civilization was a massive step backwards. Compared to their “barbarian” neighbors, who generally lived in self-governing nomadic or semi-nomadic bands, “civilized” life meant forced labor, restricted movement, taxation, and — here’s the point — more or less constant poor health, interspersed with deadly plagues.

How bad was this Old World disease problem? Here’s how author James C. Scott puts it: Despite the innovations of agriculture, it took 5,000 years for global population to rise from 4 million people 12,000 years ago (or 12kya) to 5 million people 7kya. That’s anemic 25 percent growth over the 5,000 year period that included the birth of agriculture and civilization.

So why wasn’t the population increase larger? After all, over the next 5,000 years, ending around 1 AD, global population went from 5 million to 100 million. What was the hold up?

The answer, of course, is epidemic disease, and the cause is two-fold: 1. Domesticated animals living in close contact with humans, incubating germs across species; and 2. Crowding in the early Old World cities, which included no infrastructure for sanitation.

In other words, even though Neolithic agrarian life led to somewhat higher reproductive rates, it also led to MUCH higher mortality via emerging diseases.

This unhappy balance held human population in check for millennia, and what finally broke that balance wasn’t some new medical innovation, but simple evolution: Over thousands of years, the Old World humans who survived the animal-borne plagues of Mesopotamia, Egypt and other early civilizations passed their immunities down to their children.

Meanwhile, for whatever reason, the civilizations that emerged in the Americas avoided the Old World’s inter-species pathogen problem. Which might help explain, for instance, why some scientists now estimate that New World population in 1492 was larger than the population of Europe.

The more we learn about our ancient past, the more clearly “the Colombian Exchange” between the Old World and the New World comes into focus. White people in the 20th century liked to think of it as a clash of civilizations, but from a 21st century perspective, the meeting of Old and New looks like the single greatest apocalypse in human history. Indigenous Americans gave Europeans the potato, and Europeans gave indigenous Americans smallpox.

Going to school in the 1970s, I learned that the Americas were thinly populated by occasionally friendly heathen savages who couldn’t compete with European technology. Now we have evidence that North America was thinly populated in the 17th and 18th centuries because plagues spread by previous generations of European explorers had reduced the indigenous population by about 90 percent.

No one has solid numbers, but no matter which estimate you prefer, the reduction in indigenous population between 1492 and the settlement of places like Jamestown and Plymouth and New York reads like some dystopian science fiction novel. We’re not talking about one world-ending plague, but one world-ending plague after another, decade after decade.

The Plymouth colony, for instance, WAS LITERALLY BUILT ATOP THE RUINS OF AN INDIGENOUS VILLAGE THAT HAD BEEN ABANDONED THE PREVIOUS YEAR FOLLOWING A PLAGUE THAT HISTORIANS NOW ESTIMATE KILLED MORE THAN 90 PERCENT OF THE INHABITANTS OF COASTAL MASSACHUSETTS.

No modern plague kills 90+ percent of any population. But between the 15th and 18th centuries, smallpox and other Old World diseases wiped out entire civilizations in the Americas. Perhaps nowhere was this more poignant than in North America, where a population of perhaps 7 million people (Central and South America being far more populous) lived so well on a landscape so beautiful and abundant that 16th century English sailors wrote enviously about the natives’ health, strength, beauty, nobility and vigor.

We can only imagine how history might have been different if indigenous Americans had sent their own diseases back to Europe, with similar effects. But this was a one-way apocalypse, born of literally shitty hygiene in Mesopotamia, nursed through smallpox epidemics in Egypt and centuries of Black Death in Europe.

So when you think about the indigenous American experience today, imagine this: Ever since the end of World War I, occasionally friendly, occasionally hostile aliens from another planet have been wandering around Earth, making everyone sick.

Sometimes these new alien diseases kill three out of every 10 humans. Sometimes they kill more than nine out of 10. But every year there’s a new alien plague, and more aliens. Every year, for the past century, they’ve come and they’ve gone, and every year millions of people died.

You’re one of the lucky ones, but you’re living in the ruins of a civilization destroyed not by conquest, but by inexplicable pestilence.

And one day the aliens show up again, as usual. Only this time they start building houses.

And armories.

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