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Key Theme 1: Patterns of Population

This theme comes first because the number of men and women in the world, the distribution of populations around the globe, and the migration patterns of people from one region to another have always had a large effect on all other types of change. The study of population size, density, and distribution is called demography.

One hundred thousand years ago there may have been fewer than a million human beings on the planet, indeed as few as 10,000. Today there are more than six billion. Compared to other large animals, humans have multiplied at an astounding rate in those 100 millennia. When children of today are thirty-something in 2025, they should expect to share the earth with about 8.5 billion people. From 10,000 years ago to 1750 CE, population world-wide increased on average by about 67,000 people a year. By the 1990s, that many new babies were crowding onto the planet every seven hours1.

Other large animals don't multiply in quite this way. When a new species evolves, it multiplies to fill the new niche for which it has evolved. For example, a bird species with a beak specialized for breaking a particular type of nut will multiply as long as it has enough trees to support its growing numbers. Once it has reached that limit, its numbers will be checked and from then on will only fluctuate modestly up or down. It is as if the bird population was limited to the number of individuals that the species' one new technology (its nut-cracking beak) could support.

Things are different with humans. Because of collective learning, they, unlike animals, have the ability to innovate over and over again. When the population seems to reach a limit, humans have often devised a new technology that has allowed them to settle a new region or find new ways to extract energy and resources from their surroundings. This in turn has allowed the population to grow once more. In short, human population growth has consisted of a series of leaps forward. New skills and technologies have made these leaps possible.

Let's divide world population growth into four major periods.

  • During the early paleolithic era, from about 1.5 million years ago (when the biological ancestors of humans first started making tools) to about 250,000 years ago, our hominid predecessors developed a few new technologies that allowed them to spread out of Africa and into the southern parts of Eurasia. Presumably, their numbers rose. These changes were not very striking, however, because other mammals, such as the ancestors of the orangutan, had made similar migrations earlier.
  • During the later paleolithic era, from about 250,000 years ago to about 10,000 years ago, things began to change. Over this period we see more and more new technologies appearing. These allowed early humans to explore more and more environments, first within Africa and then elsewhere. By modern standards, the pace of population growth was extremely slow. It was only rapid in comparison with the pace of change of other animals. In this era, growth was mainly the result of humans (or their ancestors) migrating into and occupying new regions of the world, not the consequence of increases in population density in particular places. That is, growth was extensive, rather than intensive. Numbers grew as humans settled more and more of the earth, but the size and nature of each community probably remained much the same.
  • The third period of growth was the agrarian age, when people first started farming. It lasted from about 10,000 years ago to about 1700 CE, when humans began to industrialize on a large scale. During this period, the population growth rate world-wide was significantly greater than in paleolithic times. Agriculture allowed humans to settle more densely, so growth now became intensive as well as extensive. Slowly, the very nature of human settlements began to change. As farming technologies improved, it became possible to support more and more people on a given area of land. Humans began to settle in villages and small towns. And then, about half way through this period, humans began to build cities.
  • About 300 years ago, global population began shooting up fast, and the rate has continued to accelerate ever since. Since 1700 CE, numbers have grown from about 610 million to nearly ten times that many. Right now, five babies are born in the world every second and only two people die. That makes a net gain of three new people every second. The number and size of cities in the world has also drastically increased in the past three centuries. The crowding of our planet has of course had an enormous impact on the way we live. But why has it happened? Answers may be sought in study of other transformations that may be related-industrialization based on fossil fuel energy, advances in medicine and public health, faster global transport and communication systems, larger and more efficient networks of economic exchange. Contrariwise, world population growth itself stimulates even greater innovation in technology, communication, and medicine to meet humankind's needs.

Human Population Growth
10,000 Years Ago to Now

Population Graph


Human Population Growth
1000 CE to Now

Population Graph 2

Population charts from David Christian, Maps of Time: An Introduction to Big History
(University of California Press, 2003), 209, 343


Population growth has by no means always been smooth. Over the long term, demographic dips have occurred, some of them severe. Recent genetic research in human DNA has shown that about 100,000 years ago the existing Homo sapiens population took a headlong dive. We can only hypothesize about the reasons for this decline, but it may have left as few as 10,000 adult men and women in the world. Numbers recovered, but it took thousands of years.

In the sixteenth century CE, to cite another example, the Indian population of the Americas may have dropped as much as 95 percent. The principal cause was contact with people from Eurasia and Africa, who carried a variety of infectious diseases previously unknown in the Western Hemisphere. In the twentieth century, wars, revolutions, genocides, epidemics, and famines have carried off tens of millions of people within periods of as little as a single year. None of these disasters, however, offset the accelerating population growth of recent times, even for a short time.

Also, changes in global population are not only a matter of raw numbers. How many people inhabit a country, how closely packed together they live, and how much moving and traveling they do are all factors that affect standard of living, changes in the natural environment, levels of crime, and many other aspects of daily life. The steady growth of cities in the world as a whole is one of the most important aspects of demographic change.

Seven thousand years ago the world had no cities. The earliest large centers arose in the Tigris-Euphrates, Nile, and Indus River valleys between 6,000 and 4,000 years ago, though only a tiny percentage of the world's total population lived in them. As recently as 1800 CE, only about 3 percent of the global population inhabited towns of 5,000 or more people. Today, nearly half the people on the planet live in cities. Tokyo, the world's biggest city at present, has more than 26 million inhabitants, a number about equal to the population of the entire planet just 4,000 years ago. Of the twenty-five biggest cities in the world today, only two are in the United States. Some cities have become big so recently that many Americans might have trouble locating them. Among the top twenty-five are Tianjin, Osaka, Karachi, and Lagos. Can students find them on a map?


A Crowded Metro Train in Moscow
R. Dunn

The 50 percent or so of the world's population that does not inhabit cities lives in rural areas, that is, in villages or in homesteads scattered in the countryside. About 25 percent of the population of the United States is rural; in Britain it is less than 10 percent. By contrast, the population of the African continent is about two-thirds rural. Country living does not mean, however, that people necessarily have plenty of room for themselves and their farms. Rural population density can be very high. On the island of Java in Indonesia, for example, there are about 2,000 people per square mile, but most of them are farmers, not city folk.

Travel and migration have been important to the story of population growth in the past 100,000 years. By about 12,000 years ago, humans inhabited all the major land areas of the world except Antarctica. All of them made their living by hunting, gathering, or fishing, Since then, people have continued to move and migrate, sometimes short distances, sometimes very long ones. The placement, or distribution of people in the world has been constantly changing.

Cities have always attracted newcomers from the countryside because cities have been centers of trade, manufacturing, religious worship, government, social support, and cultural variety. In ancient and medieval times men and women flocked to bustling cities like Babylon, Rome, Cairo, Hangzhou, and Teotihuacan. Today, people are migrating in huge numbers from countryside to city and from country to country. Most of them are looking for jobs and better standards of living, though they don't necessarily find those things when they reach their destination. About three-quarters of American children live in cities. What does city-dwelling offer that living in a small town or on a farm does not? What are the disadvantages of urban living today?

Finally, we should remember that the presence of such colossal numbers of human beings on the face of the planet affects the entire biosphere. Some animals, such as sheep, cows, rats, and pigeons, have flourished in association with human communities. Far more species, however, have been driven to extinction, or to the brink of it, as humans have taken up more and more of the space and resources that other species need to survive. So, as you consider other Key Themes, remember that in world history-whether 100,000 years ago, two centuries ago, or today-growth, decline, and movement in human population is often a causative factor in explaining changes in other aspects of life.

Click here for maps showing changes in world population from 1-2020 CE Demographic, Environmental, and Security Issues Project

Why Do We Need to Understand this Key Theme?

  • Conditions of population growth and density have a huge impact on how people live, work, and get along together. In the long run of human history, the presence today of more than six billion people on the planet represents a recent and drastic change. This fact, together with the constant flux in worldwide population movement and distribution, affects our daily lives in countless ways.
  • In today's schoolroom, students are more likely than ever to have been born in some other country or to be the child of recent immigrants. About 25 percent of the population living in the U.S. today was born outside its borders. This reality suggests the value of understanding not only differences in cultural beliefs and practices among America's population but also the patterns of global migration and urbanization that have become so complex in our own world.

Questions for Classrooms


Landscape Teaching Units that Emphasize Key Theme 1:

[In Development]


1 John R. Weeks, Population: An Introduction to Concepts and Issues, 6th ed. (Belmont, CA: Wadsworth, 1996), 34.