"The Spaniards were much impressed with the productivity of manioc in Arawak agriculture in the Greater Antilles," historian Jonathan Sauer recounts in his history of crop plants. "[A Spanish historian] calculated that 20 persons working 6 hours a day for a month could plant enough yuca to provide cassava bread for a village of 300 persons for 2 years."

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By all accounts, the Taíno were prosperous — "a well-nourished population of over a million people," according to Sauer. And yet the Spanish, who ultimately colonized and ravaged the people, considered them primitive. The Taíno lacked the monumental architecture of the Maya or the mathematical knowledge of the Aztec. And most importantly, they were not organized in the type of complex, far-reaching, hierarchical social structure that is considered one of the hallmarks of civilization and was far more widespread in Europe and Asia.

Scholars have long puzzled over the different fates of the world’s peoples. Why, on the eve of the modern world, were some societies so technologically and politically complex? For centuries, leading intellectuals from Adam Smith to Karl Marx believed that agricultural abundance had propelled the rise of advanced civilizations. The Assyrians and Babylonians of ancient Mesopotamia, for instance, flourished thanks to their fertile farms, which fed an upper class that devoted itself to religion and empire.

In his 1997 bestseller “Guns, Germs and Steel,” historian Jared Diamond argued that the availability of nutritious and easily domesticated plants and animals gave some societies a head start. In the Middle East there was barley and wheat; in Asia there was millet and rice. “People around the world who had access to the most productive crops became the most productive farmers,” Diamond later said on his PBS show. And more productivity led to more advanced civilizations.

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But the staple crops associated with less-advanced peoples — like manioc, the white potato, the sweet potato and taro — weren’t necessarily less productive. In fact, manioc and the potato are superstar crops, less demanding of the soil and less thirsty for water. These plants still feed billions of people today.

Now, a provocative new study suggests the fates of societies hinged on a subtler problem with these plants. And if it’s right, it could dramatically complicate the popular theory of the agriculture-driven dawn of civilization that has appeared in textbooks for generations.

The study, published last year by economists at the United Kingdom and Israel doing novel work on archaeological and anthropological evidence, attempts to explain a strange pattern in agricultural practices. The most advanced civilizations all tended to cultivate grain crops, like wheat and barley and corn. Less advanced societies tended to rely on root crops like potatoes, taro and manioc.

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It's not that grains crops were much easier to grow than tubers, or that they provided more food, the economists say. Instead, the economists believe that grains crops transformed the politics of the societies that grew them, while tubers held them back.

Call it the curse of the potato.

How crops changed the world

The argument depends on the differences between how grains and tubers are grown. Crops like wheat are harvested once or twice a year, yielding piles of small, dry grains. These can be stored for long periods of time and are easily transported — or stolen.

Root crops, on the other hand, don't store well at all. They're heavy, full of water, and rot quickly once taken out of the ground. Yuca, for instance, grows year-round and in ancient times, people only dug it up right before it was eaten. This provided some protection against theft in ancient times. It's hard for bandits to make off with your harvest when most of it is in the ground, instead of stockpiled in a granary somewhere.

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But the fact that grains posed a security risk may have been a blessing in disguise. The economists believe that societies cultivating crops like wheat and barley may have experienced extra pressure to protect their harvests, galvanizing the creation of warrior classes and the development of complex hierarchies and taxation schemes.

"Since the grain has to be harvested within a short period and then stored for use until the next harvest, a visiting tax collector could readily confiscate part of the stored produce," the authors write. In ancient China, for instance, the famously bureaucratic government depended on a seasonal tax on grain harvests.

Using an anthropological database, the economists gathered information about the political sophistication of societies before the 1500s. They knew whether regions were organized by tribes, by chiefdoms, or by large, elaborate states. They also knew what the major crop in each society was.

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In ancient Africa, Asia and Europe, for instance, societies had access to a large catalog of different grains, including barley, sorghum, wheat and rice. They also had access to one root crop, the yam. And in the ancient Americas, societies had access to one kind of grain, corn and three different kinds of root crops — white potatoes, sweet potatoes and cassava.

These maps show a clear correlation between crop choice and political complexity. Societies that grew grain tended to have more hierarchical political systems — empires, even — like the rice- and wheat-cultivating kingdoms of ancient India. Tuber crops were associated with smaller, more local political units.

In many places, grains do not feed more people, acre for acre, than tubers. Potatoes, in particular, are an amazingly nutritious plant. The choice between cultivating cereals and tubers, where it existed, depended on local growing conditions. Societies tend to grow crops that yield the most calories. In some places, that meant potatoes. In other places, that meant wheat.

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When the economists examined that agricultural data, they found that more fertile regions did not necessarily yield more complex societies. The crucial factor wasn't the amount of food that a society could produce; it was the type of food they chose as their main crop — grain or tuber.

"The results are just amazing," says the University of Warwick’s Omer Moav, who wrote the paper along with Luigi Pascali, also of the University of Warwick; Joram Mayshar of Hebrew University; and Zvika Neeman of Tel-Aviv University. "It's not the absolute productivity of land; it's the productivity of cereals (grains) relative to tubers.”

Does the theory make sense?

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Consider, for a second, what that means. In Jared Diamond's version of events, certain regions were cursed because they were less efficient at growing food. Low productivity leads to low agricultural surpluses leads to less complex societies. According to the economists' data, the productivity of the land didn't matter. The curse was in the type of crop.

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The theory is not ironclad, of course. One problem is that most tuber-growing societies lived in the tropics, where there was also endemic disease that slowed the growth of complex civilizations. Anthropologists also point out that to the best of our knowledge, tubers were domesticated thousands of years after cereals, so societies that grew grains had a head start.

And then there is the case of the Incas, who oversaw an empire that grew both grain and potatoes. The Incas developed a way of freeze-drying potatoes by leaving them out at high elevations. This technology allowed them to treat potatoes like a grain: non-perishable, transportable and taxable.

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Still, the overall picture — the connection between grains and civilization — has intrigued top economists.

"It's an extremely original attempt to come to grips with a very old question: Why in some places we have the emergence of hierarchies — peasants and priests and nobles and so on," says Joel Mokyr, a professor of economics at Northwestern who teaches the paper in his economic history classes. "They use economic analysis to point out something which I think is basically correct — that it’s very important what kind of crop you’re growing, that it affects the kinds of political institutions that develop."

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"It's one of the most creative and exciting ideas I've heard in a long time," says Nancy Qian, an associate professor of economics at Yale. Qian points out that even today physics dictates the economics of potatoes, which are overwhelmingly consumed domestically because they're so heavy and expensive to transport. "Over 80 percent of the potato is water," she says. "It's a really local food."

Skepticism about the power of the potato

Some anthropologists and archaeologists have had mixed reactions to the argument, which they believe downplays the human element. They're skeptical that plants could fully explain why some civilizations prospered and others stalled.

In their book “The Creation of Inequality,” archaeologists Kent Flannery and Joyce Marcus present evidence that hierarchy and inequality predate plant domestication. Many foraging societies developed complex class structures in the absence of agriculture.

"For the last 50 years, archaeologists have been investigating the origins of inequality and the rise of the state and I do not know any archaeologists who will buy the idea that cereal agriculture was behind it," Marcus, a professor at the University of Michigan-Ann Arbor, wrote in an email. "Inequality [a hallmark of complex society] results from social and political processes, and is not an agricultural process."

Increasingly, anthropologists say that the key to understanding the rise of civilization is to study political and religious institutions. Many now believe that societies took up farming not out of necessity but for cultural reasons — to please a king or to satisfy their religion. T. Douglas Price, a professor of anthropology at the University of Wisconsin-Madison who studies the origins of agriculture, argues that farming was a conscious choice made by societies with pre-existing levels of political sophistication.

“"It's a much more complex question than they seem to realize," Price says.

Historians once believed that people were forced into farming because of overpopulation, but evidence shows that the hunter-gatherer lifestyle was more than enough to satisfy people's needs. Moreover, agriculture seems to have begun in places where there was already plenty of wild food and stable populations of people. There must have been some other motivation, besides desire for food. Bone records show the earliest farmers were shorter and sicker than their hunter-gathering peers. It appears that early farming was much more miserable than foraging. So why did people do it?

"Answers to such questions about the transition to agriculture clearly have more to do with internal social relations," Price wrote in a 1995 essay.

Don't be too hard on the potato, though

It's possible that everybody is partly right. Societies may naturally tend toward complexity, but the presence of easily stolen staples like grains might have accelerated that process in some regions of the world.

Robert Bettinger, an anthropology professor at the University of California, Irvine, is sympathetic to the economists' theory, which take after an idea from his own work on ancient hunter-gatherers.

Bettinger's research explores an ancient mystery: why people in pre-historic California mostly ate acorns instead of salmon. The fish was plentiful and easy to harvest, and the technology existed to dry and preserve the meat. But for some reason, societies for a long time focused on collecting acorns, which was more time consuming and less nutritious.

Bettinger argues that early societies avoided salmon because it is what he calls a "front-loaded" resource. It takes a lot of upfront work to hunt salmon and turn it into jerky; but after that, the dried salmon is easy to steal (or to freeload off your neighbor). Acorns, on the other hand, are "back-loaded." A lot of work is needed to turn a cache of acorns into a meal. They are bothersome to steal.

Once the tribes in California became less nomadic, Bettinger and his co-author Beth Tushingham write, more and more of them turned to salmon hunting. Because they stayed in one place, they could better defend their salmon stores.

"It comes down to the ability to expropriate others' labor," Bettinger said. "Some resources have a lot more labor represented in them."

In a way, agriculture is the ultimate front-loaded resource. It takes a large amount of work to till, plant and cultivate crops. So it makes sense that farming could only flourish among civilizations with a system of property rights (to prevent freeloading from neighbors) and a measure of protection against outside threats.

In this light, it's also reasonable to imagine that different kinds of crops imposed different pressures on societies — that grains, which were more vulnerable to theft, accelerated the growth of civilizations, while tubers held people back.

If there's truth in this theory, it would represent a tremendous irony. The potato may have been a curse in antiquity, but it has become a blessing in modern times.

A famous paper by Qian, an economist at Yale, and Nathan Nunn, an economist at Harvard, argues that the white potato revolutionized agriculture in Europe after being brought over from the Americas. It dramatically increased the amount of food that people could grow, particularly in places unsuitable for grain agriculture. Between 1700 and 1900, the world population nearly tripled; Qian and Nunn give the potato a large chunk of the credit.

"According to our most conservative estimates, the introduction of the potato accounts for approximately one-quarter of the growth in Old World population and urbanization between 1700 and 1900," the economists write.

Today, the potato is one of the world's most important crops. The USDA estimates that Americans consume about 126 pounds of potatoes a year.

China, the world’s largest potato producer, is trying to persuade its population to eat more of the tuber and rely less on rice and grains. Last year, reported China Daily, the country’s national planning agency director declared that “The potato will soon be Chinese people's newest staple food, after rice, wheat and corn.” As Bloomberg has pointed out, the potato, which requires less land and less water to grow, is highly attractive to a country like China, with it massive population and pressing environmental concerns.

If it ever had a sinister past, the potato has more than redeemed itself with its amiable blond starchiness — the perfect bearer of butter and comrade to ketchup.