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Since the invention of agriculture 11,000 years ago, human population has trended up—but the boom may be drawing to an end. Growing populations are associated with progress; shrinkage has often correlated with cultural decline.

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. One stark example comes from Tasmania, an island off southeast Australia.

In 2004 anthropologist Joseph Henrich used a mathematical model of cultural evolution to tackle this mystery [pdf].

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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.

Numerical supremacy must have been an overwhelming factor that allowed modern humans to outcompete their larger rivals.

—the “hobbit” people of Southeast Asia—and another newly discovered hominid species in China.

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.

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.

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.

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.

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