Ask archaeologists where economic inequality comes from, and most will probably blame farming. The dawn of agriculture roughly 10,000 years ago, they say, precipitated a dramatic shift in the way humans lived and organized themselves. With the sedentary, agrarian lifestyle came surpluses to be managed, land to be controlled and free time to be spent on art, religion and other specialties. That led to the evolution of hierarchies, the explosion of cities, the emergence of taxation, politics, slavery, war.

All over the world, they say, wherever a person planted a seed, inequality inevitably sprouted.

A new study published Wednesday in the journal Nature offers a potential second culprit: animals. Specifically, large domesticated ones like cattle and horses — creatures that could pull plows and carry warriors, enhancing the kinds of activities that give rise to differences and discrimination.

That conclusion came out of an analysis of inequality in 63 societies spanning some 10,000 years of human history. According to Binghamton University archaeologist Randy McGuire, who was not involved in the study, it is the most comprehensive effort to quantify inequality in the ancient world. And it might have some lessons for people today.

In the modern world, economic inequality has long been described by a single number. If everyone in the community makes the same amount of money, this number, called the “Gini coefficient,” is zero. If one person has everything and all the rest are penniless, the Gini coefficient is 1. In the United States, the Gini coefficient for income inequality is about 0.39. In South Africa, it's 0.62. In Iceland, it's 0.25.

A few years ago, Washington State University archaeologist Tim Kohler wondered what he would find if he applied the formula to the periods just before and after the adoption of agriculture. Conventional wisdom would indicate that the Gini coefficients for farming cultures would be higher than those of hunter-gatherer societies. But by how much? And how might they differ across geography and time?

Since inhabitants of ancient societies weren't earning weekly paychecks, Kohler and his colleagues had to find a proxy measure. They settled on house size, the logic being that the richest people in a community would have the largest homes.

Then began the painstaking work of studying the layouts of 63 archaeological sites in North and Central America, Europe, the Middle East and Asia. They ranged from spare hunter-gatherer settlements to the small, 11,600-year-old Syrian villages of Jerf el-Ahmar to more modern metropolises like Pompeii and the Aztec capital Tenochtitlan.

As predicted, the Gini coefficients derived from measures of house size increased as societies evolved from settlements into states. This fits with an idea in economics called the “inequality possibility frontier”: As communities produce more surplus, they expand their capacity for inequality. In places that barely eked out an existence via subsistence farming, the inequality possibility frontier was low  because few people had a whole lot of spare time or resources to distinguish themselves from everyone else around them. But the more resources to go around, the easier it was for one group to control them.

Cultures don't have to meet the maximum level of inequality enabled by the amount of surplus they produce — that's why this concept is called a “possibility frontier” — but the sad fact of history is they often come close.

Kohler's analysis also revealed an unexpected trend: Societies in Eurasia and North Africa were a lot more unequal than ones in the Americas. Take, for example, Tenochtitlan in Mexico and the ancient Egyptian city Kahun. Each site represents a point in time roughly 6,000 years after agriculture arose in their region. But Kahun's Gini coefficient is close to 0.7 — roughly twice the value for Tenochtitlan.

“At that point we scratched our heads,” Kohler said. “What could be the cause of this differentiation?”

He looked at what Europe and Asia had that the Americas lacked and found his answer: large, domesticated animals. Horses, sheep, cattle, mules and goats are all native to the Eastern hemisphere. Societies in North and Central America had domesticated dogs and turkeys, and cultures of the Andes (which were not included in Kohler's survey) had llamas and alpacas, “but none of those animals are suitable for plowing,” he said. “And the breakthrough we think . . . was when people began to use oxen or cattle for plowing in the Old World.”

With a horse or an ox pulling a plow, a farmer can till a lot more land and plant a lot more crops in the same amount of time. This “extensification” of agriculture generated larger surpluses that allowed for larger cities and raised the inequality possibility frontier.

The new system encouraged people who had a beast of burden (or a dozen) to buy up more land, since they now had the capacity to cultivate bigger plots. These farmers could also rent out their oxen or mules to less fortunate neighbors. If someone had a horse, they could travel atop it, use the animal to carry heavy goods long distances or become a mounted knight and ride it into battle. There were countless ways to accumulate money and power if you had a good, strong animal to help.

Meanwhile, those who couldn't afford a draft animal might work just as hard as their ox-owning peers, but they would never earn as much. They wound up selling their land — or having it taken outright — and became laborers for a landowner rather than working for themselves. These conditions accelerated the stratification of Eurasian societies into classes of powerful landowners  and mounted warriors and poor, landless peasants.

Having animals around “increases the wealth of the wealthiest and decreases the wealth of the poorest,” Kohler said. 

Michelle Elliott, an archaeologist at the University of Paris who was not involved in the study, said she finds Kohler and his colleagues' argument compelling, but she'd like to see the Gini formula applied to more sites. She'd especially like to see it applied to Andean societies, where llamas and alpacas were used in farming but not as draft animals, which would be an interesting test of Kohler's theory.

“They’ve got this basic framework,” she said. “Now we need to fill in the blank spots with more data to see if this pattern holds up.”

The implications of his finding, Kohler said, go beyond academics, because inequality exists today for the same reason it arose at the dawn of civilization. “Agriculture extensification . . . has an interesting way of tying wealth to future income,” he said.

In other words, how much the inhabitants of ancient Egypt were able to earn depended largely not on their work, but on their wealth — the sum of all the physical and intangible assets they possessed, including livestock. If you owned a team of oxen you could plow more land, sell more crops, buy yet more oxen and more land, employ more workers, generate more profit. And you could pass all of that wealth — the oxen, the land — to your descendants, who in turn could use them to earn more in their lifetimes.

Today, ownership of a big bovine doesn't make you rich. But people profit from other kinds of wealth: nice houses that accrue value, fancy educations that lead to great jobs, bank accounts, hedge funds. Research suggests that the best way to understand inequality in today's society — including racial inequality — is not by comparing incomes but by looking at wealth.

From that perspective, the United States doesn't look so good. According to a 2015 report, the Gini coefficient for wealth inequality in America is 0.8 — higher even than Kahun in ancient Egypt.

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