Since the invention of agriculture 11,000 years ago, human population has trended up—but the boom may be drawing to an end. Birthrates are falling around the world; by the end of the century the number of people on the planet may top out and, in an unprecedented reversal, start to decline. Good news, right? The answer is not so simple. Growing populations are associated with progress; shrinkage has often correlated with cultural decline. One stark example comes from Tasmania, an island off southeast Australia. Nearly the size of Ireland, it was colonized 34,000 years ago by people with sophisticated toolmaking skills who came across a land bridge from Australia. By the 18th century, Tasmanians used simple technology, hunting with rocks and crude clubs. In 2004 anthropologist Joseph Henrich used a mathematical model of cultural evolution to tackle this mystery [pdf]. He concluded that the island’s population, about 4,000 in the 18th century, at some point fell below the level necessary for complex skills to be passed from generation to generation. Scientists increasingly think population size and density have had a big impact on human development at certain pivotal points. That continues in the modern world, as young people disproportionately produce innovation, generate economic growth, and finance social support networks for the elderly.

100,000 Years Ago: Artistic Behavior Appears

Most researchers date the origins of Homo sapiens to between 200,000 and 160,000 years ago in Africa. Yet for their first 100,000 years, modern humans behaved like their more archaic ancestors, producing simple stone tools and showing few signs of the artistic sparks that would come to characterize human behavior. Scientists have long argued about this gap between when humans started looking modern and when they began acting modern. University College London archaeologist Stephen Shennan has proposed that cultural innovations were likely due to increased contact among humans as they began living in ever-larger groups. Shennan adapted Henrich’s Tasmanian model to much earlier human populations. When he plugged in estimates of prehistoric population sizes and densities, he found that the ideal demographic conditions for advancement began in Africa 100,000 years ago—just when signs of modern behavior first emerge.

65,000 Years Ago: Stone Tools Spread

Population size could explain why the same stone tool innovations show up at the same time across wide geographic regions. Lyn Wadley, an archaeologist at the University of the Witwatersrand in Johannesburg, has worked at the Middle Stone Age site of Sibudu in South Africa, where she found evidence of two sophisticated tool traditions dating to 71,000–72,000 years ago and 60,000–65,000 years ago. Similar tools pop up all across southern Africa at around the same time. Wadley says early humans did not have to migrate long distances for this kind of cultural transmission to take place. Instead, increasing population densities in Africa may have made it easier for people to keep in contact with neighboring groups, possibly to exchange mating partners. Such meetings would have exchanged ideas as well as genes, thus setting off a chain reaction of innovation across the continent.

45,000 Years Ago: Homo Sapiens Takes Europe

A bigger population may have helped H. sapiens eliminate its chief rival for domination of the planet: the Neanderthals. When modern humans began moving into Europe about 45,000 years ago, the Neanderthals had already been there for at least 100,000 years. But by 35,000 years ago, the Neanderthals were extinct. Last year Cambridge University archaeologist Paul Mellars analyzed modern human and Neanderthal sites in southern France. Looking at indicators of population size and density (such as the number of stone tools, animal remains, and total number of sites), he concluded that modern humans—who may have had a population of only a few thousand when they first arrived on the continent—came to outnumber the Neanderthals by a factor of ten to one. Numerical supremacy must have been an overwhelming factor that allowed modern humans to outcompete their larger rivals.

25,000 Years Ago: Ice Age Exerts A Toll

By 35,000 years ago, H. sapiens appears to have had the planet to itself, with the possible exception of an isolated population of H. floresiensis—the “hobbit” people of Southeast Asia—and another newly discovered hominid species in China. But according to work led by University of Auckland anthropologist Quentin Atkinson, human population growth, at least outside of Africa, began to slow down around then, possibly due to the climate changes associated with a new ice age. In Europe, total human numbers may actually have declined as glaciers began to cover much of the northern part of the continent and humans retreated farther south. But population levels never dropped enough for humans to start losing their technological and symbolic innovations. When the Ice Age ended, about 15,000 years ago, population began to climb again, setting the stage for a major turning point in human evolution.

11,000 Years Ago: Farming Sparks a Boom

Farming villages first appeared in the Near East during the Neolithic period, about 11,000 years ago, and soon afterwards in many other parts of the world. They marked the beginning of a transition from the nomadic hunting and gathering lifestyle to a settled existence based on cultivating plants and herding animals. That transition helped catapult the world’s population from perhaps 6 million on the eve of the invention of agriculture to 7 billion today. Archaeologist Jean-Pierre Bocquet-Appel has surveyed cemeteries across Europe associated with early settlements and found that with the advent of farming came an increase in the skeletons of juveniles. Bocquet-Appel argues this is a sign of increased female fertility caused by a decrease in the interval between births, which probably resulted from both the new sedentary life and higher-calorie diets. This period marks the most fundamental demographic shift in human history.

Present: Global Population Peaks

Industrialized societies are experiencing a decline in fertility rates that exceeds the drop in mortality rates. Many developing nations are seeing even sharper fertility drops. As a result, the global population may plateau by 2100. There will still be billions of people to sustain technology, but culture and economics may see major changes.