The Peopling of the Americas

Suppose that 10 people (5 breeding couples) arrived in North America about 15,000 years ago. How long would it take to fill up the Americas with people?

For the sake of simplicity, let’s assume that their population doubled every 30 years. That is about the current rate of growth of the African population. It seems reasonable that a small population of hunter-gatherers in a virgin continent could grow at that rate. Hunting would have been easy, because the large animals had no experience of human beings and no reason to be particularly afraid of them.

If their population doubled every 30 years, then it would multiply by roughly 1000 every 300 years (2¹⁰ = 1024). Let’s consider their progress in 300-year increments, assuming that they multiply by 1000 in each increment.

In the first 300 years, their population would have grown from 10 to 10,000. That’s a pretty small population. They would have made little impact on the environment, aside from an increase in fires in the areas where they lived. They would have spread a long way from their entry point, moving south for the climate if nothing else. They would also have split into about 100 small bands, scattered around the central and western parts of North America. There would be little conflict between those bands. They would have a common culture, and could move to avoid conflict if necessary.

By year 600 of our experiment, the population would be 10 million. They would have spread throughout the entire continent of North America, and probably into South America as well. By this time, most of the large animals would already have been hunted to extinction or reduced to small remnant populations in refugia such as mountains, islands and dense forests. All the good hunting grounds would be occupied, but there would still be plenty of subprime real estate available, such as forested or mountainous regions. Conflict would be increasing, because moving away from the neighbors had become more difficult. Life in general would be getting harder.

By this time, the environment would have been dramatically altered, mostly by fire. Humans like to burn things, and they usually burn away forests and dense undergrowth, creating a more savanna-like environment. Our ancestors evolved in an open woodland habitat, and we prefer that type of environment. (That is why almost every city park is an open woodland, not a dense forest or a treeless expanse.) Hunter-gatherers have always burned the landscape to open it up and create forage for the animals that they like to eat. It’s even possible that this massive burning caused some global warming. Regular burning decreases the amount of carbon stored in the biosphere, and increases the amount stored in the atmosphere.

Between year 600 and year 900 of our thought experiment, things would get very interesting. If the population kept doubling every 30 years, it would reach 10 billion by year 900. That’s larger than the current population of the Earth. It is much larger than the Americas could support by hunting and gathering. In fact, it’s probably more than the Earth could support in the long run, even with intensive modern agriculture. So, that didn’t happen. Something else did.

There were few diseases in the Americas that affected humans. Any diseases that were adapted to humans would have been shed on the long journey into the Americas, because diseases usually need large populations to exist. Diseases can spread from one species to another, but that is easier if the two species are closely related. Humans were not closely related to any animals in the Americas. The only primates were the New World monkeys of Central and South America, which are very distant relatives. So, the human population of the Americas had few diseases before Columbus showed up.

Human populations are controlled by three major factors: war, disease and famine. Famine usually causes war, so war and disease are the two proximate factors that control human populations. Without disease, the human population in the Americas would have been limited almost entirely by war. As the competition for food sources increased, the Native Americans would have become very warlike. Eventually, their population would have stabilized at a level that was maintained by constant warfare.

This is just a thought experiment, but it is probably close to what actually happened.

Most large animals in the Americas disappeared rather suddenly around 13,000 years ago. Horses, camels, ground sloths and elephants (mastodons and mammoths) all disappeared, with the exception of two small camel species living in the mountains of South America (vicuñas and guanacos). Many of the large predators also went extinct, including lions and saber-toothed cats. Bison survived, but they evolved into a smaller, short-horned form, probably because large feline predators had been eliminated, and horns were useless against human predators. The evidence suggests that the initial peopling of the Americas had profound effects on its flora and fauna.

After the initial explosion, the human population of the Americas probably didn’t grow much for thousands of years. It slowly increased as new methods of food production were discovered. Agriculture was invented in the Americas thousands of years ago, and civilization emerged in a few areas. However, civilization appears to have been less stable in the Americas than it was in Eurasia and North Africa. In the Americas, there was a cycle of civilization rising and falling. It would emerge in places where intensive agriculture was possible, flourish for a while, cause population growth, degrade the environment, and then collapse. That was probably due to a lack of disease.

Disease might be necessary to make civilization stable. Without major diseases, war is the main factor that limits the population. War destroys more than human lives. It also destroys societies and physical capital, such as buildings. By contrast, disease takes human lives without destroying social or physical capital. I believe that endemic disease acts as a stabilizer for civilization, allowing it to persist for longer periods of time. Without disease, civilization causes rapid population growth by increasing the food supply and decreasing violence. Population growth then causes environmental degradation, scarcity, hunger and eventually catastrophic warfare and collapse.

I believe that the absence of major diseases made civilization less stable in the Americas, and thus prevented it from reaching the same level of complexity that it reached in the Old World.

The Native Americans caused the first great ecocide in the Americas by wiping out the megafauna. But they were on the losing end of the second great ecocide, which started when Europeans arrived in sailing ships with their livestock, crops, weapons and diseases. In a strange twist of fate, horses returned to their ancient homeland in sailing ships as domesticated animals.

When Europeans discovered the Americas, they found the natives to be strong, healthy and extremely warlike. The natives had some practices that seemed strange to Europeans, such as burning the landscape every year. The Native Americans did not live in peace and harmony with one another and with nature. They lived in a state of perpetual violence between societies and between humanity and nature.

Our culture has a mythical view of the Native Americans as a wise and virtuous people who lived in harmony with nature. This view is simply false. Their population expanded until it was limited by hunger and war. They drove many species to extinction shortly after their arrival. They killed animals and each other to the limits of their abilities. They extracted as much energy from the landscape as their level of social, technological and economic complexity allowed. In many places, they degraded the environment with intensive agriculture. They had local population explosions and crashes.

The other modern myth about the Native Americans is that they were nomadic hunter-gatherers, living in small numbers in a landscape that was mostly wilderness. By 1500 AD, most Native Americans were farmers, and had been practicing agriculture for thousands of years. For meat, they did rely mostly on hunting, but they didn’t wander around in a wilderness spearing wild animals. In most places, they managed the landscape by regular burning, and they used efficient hunting methods, such as driving game into kill zones. Likewise, their “gathering” involved managing the landscape in various ways to produce reliable crops of wild foods. They built structures to harvest wild foods, such as fish weirs, that were used for generations. They had complex methods of food production, preservation and storage.

The native population of the Americas crashed rapidly after European contact, due to disease. In Mexico, the native population fell by roughly 90% between 1500 and 1600. Other crashes occurred in other places, but are less well documented. Diseases spread into North and South America from the points of European contact, killing millions of people. Early European explorers found dense populations of native people. Later explorers found an almost empty landscape. European colonists, such as the Mayflower pilgrims, settled lands that had been depopulated by disease. To some extent, the American wilderness was created by European diseases.

If you’re interested in this topic, I recommend two books: 1491 by Charles C. Mann, and Ecological Imperialism by Alfred W. Crosby.

What is the point of this brief population history of the Americas?

One is to correct our modern myths about the Native Americans and primitive cultures in general. Human beings had a big impact on ecosystems before modern civilization and even before agriculture. The Native Americans were not “noble savages” living in “harmony with nature”. That is a myth. The “harmony with nature” was a quasi-stable state in which human populations were limited by hunger and war.

The initial peopling of the Americas caused a wave of extinction of large animals, and profoundly changed the landscape. The human population probably grew rapidly and reached its limit within roughly 1000 years. Without any significant diseases, the population was then controlled by war. Agriculture and civilization were independently discovered in the Americas, but civilization never reached the same level of complexity as it did in the Old World, probably because complex societies are unstable without disease. Native American civilizations tended to expand and then collapse when they exhausted local resources.

Another important point is that populations are ultimately limited by competition for resources. It doesn’t take very long for a population to exceed the carrying capacity of any environment. During a 300-year period, the human population can easily multiply by a thousand. Such population explosions are rare in history, because the population is normally limited by famine, disease and war. Periods of growth occur when humans discover a new continent, a new way of life such as agriculture, or a new type of energy such as fossil fuels. They last until a new limit is reached. Good times are transient, because they cause population growth.

Finally, progress is not inevitable. It is a modern myth that human history is a story of progress, and progress is an inevitable consequence of human nature. That is a very biased and distorted view of history from the perspective of history’s recent winners, not from the perspective of its many losers. Human nature does generate progress from time to time, but there are long periods of stagnation in between. Progress is not inevitable, and when it happens, you might be on the losing end of it.