At almost the instant when humans started building cities, we figured out ways to put walls around them. The often violent history behind those walls is still affecting urban life today, in ways you may not realize.

Illustration by Mark Molnar

In the early 1960s, eminent scholar Lewis Mumford published a massive tome called The City in History. He argued that cities evolved largely as military entities, and their walls were the most obvious sign of their profoundly warlike character. Undeniably, early city walls were built as social armor, sometimes protecting a single settlement and sometimes an entire region. The Great Wall of China is the city wall writ large.

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Mumford's great insight was that these walls weren't just for defense. They were also an early form of surveillance technology. Authorities used these walls to watch who exited and entered through the gates. Their eyes were trained on the enemies within, as well as those without.

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But Mumford was also wrong about the origins of city walls.

The Truth About the First Walls

Human settlements grew walls long before the imposing fortifications of ancient Mesopotamia, Greece and Rome. And even during the period of monumental city wall building, those walls had many non-military uses.

Evidence of city walls dates back over 9,000 years ago to the Neolithic period, thousands of years before the rise of famously fortified ancient cities like Babylon and Athens. Almost none of these walls seem to have had a defensive purpose. Even the famous walls of Jericho, whose destruction is recounted in the Old Testament, were not originally built as a defensive perimeter. Instead, they were to prevent the low-lying city from being flooded by a nearby river during the rainy season. This fits with evidence excavated at the oldest parts of the site, which contains no remains of weaponry — but does show evidence of a build-up of silt and other debris from water at the base of the city walls.

Photograph of the walls of Jericho, via Braman's Wanderings

In a paper about the remains of Jericho, archaeologist Ofer Bar Josef concludes simply:

Given all the available data, it seems that a plausible alternative interpretation for the Neolithic walls of Jericho is that they were built in stages as a defense system against floods and mudflow.

Privacy and Neighborhoods

When we contemplate the origin of early city and village walls, it's important to remember that they were being built at a time when walls themselves were a relatively new invention. Hunter-gatherer groups began living year-round in settlements roughly 10-12 thousand years ago. This move from a nomadic life, where we owned nothing but what we could carry, set off what could be called a domestic revolution. Suddenly, people began building permanent hearths, planting farms, and constructing homes.

These villagers' ancestors may have had light tents, but Neolithic people had walls of mud, wood and thatch. They could hide from their neighbors. For the first time, people could begin to develop a sense of privacy. In Peter J. Wilson's book The Domestication of the Human Species, the anthropologist argues that humans first walls were probably a social or cultural development. They allowed people to develop a sense of individual and group identity in villages and cities that grew far beyond the size of any hunter-gatherer group. It's possible that humans needed walls to deal with the psychological stress of living in bigger groups; they gave people separate spaces where they could cool off from conflicts or share their feelings without social judgments.

In the years since Wilson's book came out, archaeologists have confirmed that many city walls appear to serve a social purpose rather than a military one.

In the Neolithic village Ilıpınar, located in the Anatolian region of Turkey, walls helped villagers consolidate their identity as a community. These people's biggest threat was not a military incursion, but fragmentation into hunter-gatherer groups. And indeed, it seems that Ilıpınar's inhabitants did eventually return to a semi-nomadic way of life. The village was slowly abandoned after several hundred years of permanent settlement. But first, it was occupied by people who only lived there for part of the year. It's as if they became partial nomads, then abandoned village life altogether.

Illustration by Miguel Salvatierra Cuenca

Early walls in cities were also used to enclose small groupings of homes rather than the entire settlement. Perhaps these internal walls were used to separate powerful groups from everyone else. Or maybe they were more like neighborhood boundaries that kept people from wandering into the pottery-makers' quarter and messing things up.

Either way, it seems that the city wall began as a way to separate "us" from "them" socially, and evolved later into a way to prevent our enemies from laying waste to our homes.

Warfare in the Gated Community

The great age of walled cities is absolutely associated with warfare. Starting with Bronze Age cities, we begin seeing the iconic walls from history books. Cities in ancient Greece, China, and the Middle East erect massive stone walls of fired brick to defend against their enemies. Medieval cities across Europe and Asia are known for their fortified walls, and for their great forms of military organization.

As an aside, however, it's worth noting that the fear of dissolution into nomadic life continued to haunt walled cities during the Middle Ages. The Great Wall of China was a defense against nomadic groups, protecting the Han Chinese from attack — and from being assimilated into the kinds of mobile communities that claimed those long-ago inhabitants of Ilıpınar.

Defensive walls around key cities remained the norm until the eighteenth century. Populations exploded beyond what the old medieval walls could hold, and weaponry had advanced to the point where a stone wall wasn't going to do much to prevent an invasion. Plus, battles were migrating from cities to heavily-defended forts outside residential areas.

But as the walls around cities fell, older forms of the urban wall rose again. In the mid-nineteenth century, a Prussian city planner named Julius Pitzman proposed a novel idea to the citizens of St. Louis, Missouri. The city was suffering from unchecked growth and lack of services for its residents. So why not create special walled neighborhoods within the city, entirely the property of the homeowners, with services like gas provided by private companies? The idea proved so popular that these walled neighborhoods sprouted up all over St. Louis and in its burgeoning suburbs. Pitzman wanted these places to be beautiful, and created elaborate, ornate gates for residents to drive through to get to their homes.

Gate into the "private city" of Washington Terrace in St. Louis, 1892, via Library of Congress

This was the beginning of what we now call gated communities. Their walls are mostly social, designed to shore up cultural and economic divisions rather than repel an invading army.

But in recent decades, neighborhood walls in cities have taken on a more ambiguous role in civic life. Perhaps most famously, the Berlin Wall was used to divide a city by military force. The wall wasn't to defend Berlin from outsiders; it was to defend one group of city residents from the other — or rather, as Mumford would probably argue, to allow the authorities to monitor who passed from one side to the other.

Today, the city of Rio is building a controversial wall between its low-income favelas and the rest of the city. Like the Berlin Wall was, this wall in Rio will be heavily policed. We're witnessing the militarization of internal city walls, as social divisions become armed conflicts.

When many of us witnessed the eruption of violence in the St. Louis suburb of Ferguson last month, we may have understood it as the culmination of decades of simmering social problems. It was. But it was also the tragic fallout from a decision made over a century and a half ago, when St. Louis embraced gated communities.

And if you crank the windshield of history back even further, it was just one result of an evolutionary path that began thousands of years ago, when we built the first walls to keep our neighbors out.

Annalee Newitz is the editor-in-chief of io9. She's the author of Scatter, Adapt and Remember: How Humans Will Survive a Mass Extinction. Follow her on Twitter, or email her.