Let’s say 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?

Well, of course that would depend on their birth rates and death rates. For the sake of simplicity, let’s assume 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. The large animals had no experience of human beings and no reason to be particularly afraid of them, so hunting would have been easy.

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 and it 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. Neighboring bands would probably be mostly friendly with each another. They all spoke the same language, had the same legends, and could move to avoid conflict if necessary.

By year 600 of our experiment, the population would be 10,000,000. 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 already be occupied, but there would still be plenty of subprime real estate available, such as forested or mountainous regions. War would be on the increase, because moving away from your 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 savannah like environment. Our ancestors evolved in an open woodland habitat, and we prefer that kind of environment (which is why almost every city park is an open woodland, not a dense forest or treeless expanse). Hunter-gatherers have always burned the landscape to open it up and create forage for the animals they like to hunt. It’s even possible that this massive burning could have affected the Earth’s climate to some degree. Regular burning reduces the amount of carbon stored in the biosphere and increases the amount stored in the atmosphere. The spread of Homo sapiens into the Americas and into northern Eurasia at the end of the last Ice Age may have caused more rapid warming than would have otherwise occurred.

Between year 600 and year 900, things would get very interesting. If the population kept doubling every 30 years, it would reach 10,000,000,000 by year 900 of our thought experiment. That’s a larger population than exists on the Earth today. It is much larger than the Americas can support by hunting and gathering. In fact, it’s probably more than the Earth can 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. Humans were not closely related to any animals in the Americas. Their closest relatives in the Americas were the New World monkeys of Central and South America, but those monkeys are distant cousins, far more distantly related than the Old World apes and monkeys. So, few diseases jumped from animals to the human population. Thus, the human population of the Americas was probably almost completely disease-free 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. At some point between year 600 and year 900, they would have adopted a very warlike way of life. Eventually their population would have stabilized at a level that was maintained by constant warfare.

This was just a thought experiment, but it is probably fairly 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 (vicunas 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.

The human population of the Americas was probably fairly stable for thousands of years after the initial explosion reached its limits. It gradually 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 more cyclical pattern of civilization rising and falling. Civilization would emerge in places where intensive agriculture was possible, flourish for a while, cause population growth, degrade the environment, and then collapse. I think that was mainly due to a lack of disease.

In an environment without major diseases, war is the main factor limiting population. War, of course, is very destructive — not just of human lives, but also of the social order and physical capital, such as buildings. Disease, by contrast, 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 perpetrated 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 they arrived in the Americas. They killed animals and one another 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 those who came on the Mayflower, settled lands that had been almost emptied of people by disease. The population decline in the Americas returned some of the land to a wild state. The reduction in burning allowed forests to grow back in many areas, pulling carbon dioxide out of the atmosphere and perhaps contributing to the Little Ice Age: a cold period that began in the 1500s and continued until the 1700s.

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. Our conception of the Native Americans as “noble savages” living in harmony with nature is nonsense. The “harmony with nature” was a quasi-stable state in which human populations were ultimately limited by their methods of food production and proximately limited by war. The initial peopling of the Americas caused a wave of extinction of large animals, and profoundly changed the landscape by increasing burning. The human population probably increased rapidly and then reached its limits 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, perhaps because complex societies are unstable without disease. There were many local population explosions and collapses.

Another important point is that human beings are not magic. We are life forms shaped by and subject to the laws of nature. Populations are regulated by premature death. 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. It is only on rare occasions, when humans discover a new continent, a new way of life such as agriculture, or a new resource such as fossil fuels, that we temporarily escape from that condition. Peace and prosperity are not stable. War and scarcity are.

Finally, progress is not inevitable. One of our modern myths is that human history is a story of progress, and that 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 progress doesn’t seem that great if you are on the losing end of it.