When the Ice Age ended around 10,000 BC, the world became warmer and wetter, and the climate became more stable. Carbon dioxide levels increased, which increased plant productivity. The stage for agriculture was now set—and this time the actors were ready as well.
Although there had been other interglacial periods in the past, early humans had never developed agriculture then. We suspect that increases in intelligence made agriculture possible, but the route may have been indirect. For example, the invention of better weapons and hunting techniques, combined with other technologies that let humans make better use of plant foods, could have led to lower numbers, or even extinction, of key game animals—which would have eliminated an attractive alternative to farming.
Farming appeared first in the Fertile Crescent of Southwest Asia. By 9500 BC, we see the first signs of domesticated plants: first wheat and barley, then legumes such as peas and lentils.3 From there farming spread in all directions, showing up in Egypt and western India by 7000 BC and gradually moving into Europe and India. Around 7000 BC, rice and foxtail millet were domesticated in China. Animals were domesticated on a similar timeline, with the Middle East in the lead. Goats were tamed around 10,000 BC in Iran, sheep about 1,000 years later in Iraq. Both the taurine cattle we're familiar with in the Middle East and the humped zebu cattle in India were domesticated around 6000 BC.
Agriculture came later to the rest of the world. In some cases it spread by a geographic expansion of farmers, in others through hunter-gatherers adopting already-existing methods of agriculture, and in yet others by hunter-gatherers independently inventing their own forms of agriculture. In Europe, agriculture was spread by Middle Eastern immigrants and by native Europeans learning to grow Middle Eastern crops such as wheat and barley. In sub-Saharan Africa, geographic barriers and climatic differences blocked adoption of most Middle Eastern crops and domesticated animals. There, agriculture appeared around 2000 BC and was based on locally domesticated crops such as sorghum and yams. The story is similar in the Americas, where the Amerindians were almost entirely cut off from the rest of the world and had to domesticate their own crops. (Some of those, such as maize and potatoes, are among the most important crops in the world today.)
Agriculture comprised what was surely the most important set of innovations since the expansion of modern humans out of Africa, resulting in changes in human diet, disease exposure, and social structure. Another consequence (one of great evolutionary significance) was a huge population boom. Human numbers had already been on the increase since the advent of behavioral modernity, partly as the result of migration into the far northern regions of Asia, over the sea into Australia, and across a land bridge into the Americas—all places that archaic humans had been unable to settle—and partly because of improvements in food production technology (such as nets and bows). An educated guess puts the total population of the world 100,000 years ago at half a million, counting both anatomically modern humans in Africa and archaic humans (Neanderthals and evolved erectus) in Eurasia. By the end of the Ice Age some 12,000 years ago, there may have been as many as 6 million modern humans—still hunter-gatherers, but far more sophisticated and effective hunter-gatherers than ever before.
Farming, which produces 10 to 100 times more calories per acre than foraging, carried this trend further. Over the period from 10,000 BC to AD 1, the world population increased approximately a hundredfold (estimates range from 40 to 170 times). That growth in itself transformed society—sometimes, quantity has a quality all its own. And as we have pointed out, this larger population was itself an important factor in evolution.
The advent of agriculture changed life in many ways, not all of them obvious. It vastly increased food production, but the nutritional quality of the food was worse than it had been among hunter-gatherers. It did not materially increase the average standard of living for long, since population growth easily caught up with improvements in food production. Moreover, higher population density, permanent settlements, and close association with domesticated animals greatly increased the prevalence of infectious disease.
The sedentary lifestyle of farming allowed a vast elaboration of material culture. Food, shelter, and artifacts no longer had to be portable. Births could be spaced closer together, since mothers didn't have to continually carry small children. Food was now storable, unlike the typical products of foraging, and storable food could be stolen. For the first time, humans could begin to accumulate wealth. This allowed for nonproductive elites, which had been impossible among hunter-gatherers. We emphasize that these elites were not formed in response to some societal need: They took over because they could.
Combined with sedentism, these developments eventually led to the birth of governments, which limited local violence. Presumably, governments did this because it let them extract more resources from their subjects, the same reason that farmers castrate bulls. Since societies were generally Malthusian, with population growth limited by decreasing agriculture production per person at higher human density, limits on interpersonal violence ultimately led to a situation in which a higher fraction of the population died of infectious disease or starvation.
All these changes generated new selective pressures, which is another way of saying that humans didn't fit the new environment they had created for themselves, so the species was under pressure to adapt. Because of the newness of the environment, genetic improvements were relatively easy to find—definitely easier at this point than finding ways to become better hunter-gatherers. Modern humans had been adapting to their hunting-gathering lifestyle for a very long time and had already exhausted most such possibilities. Adaptation to the farming life was doable, but as always, it would require concrete genetic changes.
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