At some point in human history, people began to cultivate food. When and where agriculture began is fiercely debated, but another mystery is why.
There are three core hypotheses why people would develop farming. Either they were starving and had to innovate. Or, they were living so well that they could afford the leisure to explore and experiment. Or, there is no common cause behind the domestication of plants and animals that occurred around the world (also known as the regional uniqueness hypothesis).
Now a paper by researchers at Colorado State University and Washington University in St. Louis published in Nature Human Behavior this week makes the case for the second hypothesis, showing that at different places where agriculture originated, the environmental conditions were improving, and would have enabled people to thrive.
The team wasn't trying to deduce where or when a human first thought to put a seed in the ground or leash a goat. Different crops and animals were domesticated in different times and places, explains senior author Michael Gavin, an associate professor at Colorado State. Discoveries in Israel show evidence of cultivation as early as 23,000 years ago, even though the people in that ancient village in the Galilee were still using wild-type grains. Animal domestication apparently began later, but over 10,000 years ago.
This paper asks why domestication of anything like that even happened.
The team looked at 12 different locations of early domestication (not cultivation) in different continents, at completely different times, searching for a common element. They found it.
Almost everywhere they looked in the past at the times agriculture developed, the environmental circumstances enabled a potential increase in population numbers.
The 12 points where domestication events originated were: Mesoamerica, the east coast of North America, two places in the South American northern lowlands, the Andes, the West African savannah, the Sudanic savannah, the Fertile Crescent, the Ganges in India, two places in China (a loess plateau and an area by the Yangtze River), and New Guinea.
The earliest known domestication in each of the 12 spots varies but by and large is considered to be between 11,000 years ago to 5,000 years ago.
Taming wheat and goats
The new study isn't archaeological itself, Gavin explains to Haaretz. The researchers deduced how people were living in a window of time, in each of the 12 spots, between the first signs of attempts to domesticate and evidence of physical change in the plant or animal species
"Why did agriculture begin in those places, at those particular times in human history?" the team asks.
They found that population density increased in 10 out of the 12 areas, the exceptions being the North American east coast and the Ganges in India, where a brief decrease was followed by a potential spike in population.
Scientists agree that the earliest cultivators still subsisted on hunting-gathering, augmented by what they managed to grow. The transition to a sedentary lifestyle based on cultivating plants and animals was gradual, taking thousands of years.
The road to agriculture began with low-level cultivation of wild plants. The early farmers would have selected the best plants and over time, the morphology of the plants changed.
Wild wheat for example disperses its seeds by wind; domestic wheat is both richer in protein and requires manual shaking of the sheaf (conveniently, grains don't go all over the place, only where the farmer wants). Grains found in the 23,000-year-old village by the Sea of Galilee were wild-type barley, wheat and goat grass, for instance. Animal domestication has also marked morphology in many cases, such as the sheep, bred to be woolly in mind and body; and the chicken, selected for meatiness. Goats seem to have retained their original personality.
Prehistoric hive mind
The suggestion that agriculture arose in times of plenty, when environmental conditions were improving and population density could increase, was reached by "hindcasting." The researchers' model doesn't look into the future, but the past.
Gavin points out that we don't know exactly how many people lived in the prehistoric communities where domestication developed, or know exactly how their environments changed. Theories of prehistoric populations and climate tend to be at a larger scale than is useful – continental, for instance.
But the researchers looked at 220 foraging hunter-gatherer populations that existed in recent times (between about 1900 to 1950), looked at the density of these populations, and created a model that predicted that density well.
What predicts potential population density of foragers? Environmental and social variables like mobility, Gavin says, adding, "Our colleagues in Brazil had good data for climatic variables going back millennia."
Then, factoring in paleoclimate reconstructions and other parameters - they hindcast population densities in the relevant areas from 21,000 years ago to 4,000years ago, covering the period in which domestication of plants and animals happened.
The model shows that environmental conditions were improving and the potential density of the human population was increasing, not at the same time but increasing at the time that the species were domesticated, Gavin says. "Conditions were better, so potentially, humans had more time to explore. When there are more people in a location, there are more potential heads, more potential to share ideas in the population."
In contrast, they found little support for the two other main hypotheses: that the environment worsened and people needed a new way of getting food during desperate times; or that there was no general pattern, but each case of domestication arose from unique social and environmental conditions. Well, it seems the environment was great and two heads really are better than one.