Last month, the entire human race changed adjectives. No longer are we a rural species; as of May 23, 2007, humanity is urban.

This news comes to us from the United Nations Populations Fund: for the first time in our history, the majority of our species lives in towns and cities. In fact, researchers at two American universities claim that May 23 was the tipping point — the very day the 3,300,000,001th person left his or her farm for life in the big city.

But when people crowd into cities, problems of sanitation follow. I describe the relationship in my book: “As a population’s density grows, so too grows the threat from its accumulated poop. {A population’s} safety depends on being able to destroy poop faster than it can produce it.” With that in mind, the UN’s report should bode ill for the sanitary health of our species.

But according to UNICEF, the trend is the opposite: as worldwide population density has increased, we humans have actually gotten better at managing human waste.

In their assessment of world sanitation and drinking water, UNICEF reported that 58% of the world has adequate sanitation as of 2002. That’s up from 49% in 1990.

As horrified as I am that 42% of the world — 2.6 billion people — don’t have access to decent sanitation, I still see in those numbers a staggeringly positive trend. One would think that the world’s population growing by 950 million people in a twelve year period would overwhelm whatever sanitary solutions civilization had thus far devised. But the results have contradicted expectations: fewer people go without sanitation today than did in 1990, despite the fact that almost a billion more people were added to the mix.

What does it mean to have sanitary coverage? It doesn’t mean that you have a toilet that flushes into the sewers. While globalization is spreading the Western-style toilet to the far reaches of the Earth (a trend I hope to examine in my next book; and as the Western-style sitter replaces the Eastern-style squatter, will Western-style rates of hemorrhoids and colon cancer be far behind?), toilets and sewers aren’t the only receptacles UNICEF deems appropriate for humanity’s business. According to UNICEF , “access to sanitary means of excreta disposal is estimated by the percentage of the population using improved sanitation facilities. Improved sanitation facilities are those more likely to ensure privacy and hygienic use.” The facilities UNICEF looks upon with approval include:

• A connection to a public sewer
• A connection to a septic system
• A pour-flush latrine
• A simple pit latrine
• A ventilated improved pit latrine

Unimproved sanitation facilities are technologies like public or shared latrines, open pit latrines, and bucket latrines.

Human beings, at least those on a Western diet, produce an average of a half-pound of poop and two quarts of urine a day. A latrine servicing a hundred people thus accumulate fifty pounds of poop and fifty gallons of urine every twenty-four hours; at this rate, unimproved sanitary facilities can easily lapse into a health threat. Poop is a vector for viruses, parasites, and bacteria, and poop attracts insects and vermin who carry more of the same. Before toilets and sewers grew widespread in the late nineteenth and early twentieth centuries, poor sanitation killed millions. Modern cities don’t much suffer typhoid or cholera anymore, but in many less developed places, outbreaks still occur.

In my nascent career as sewage eco-warrior, I’ve given a lot of criticism to toilets. “They waste water,” I’ve said. “They demand an expensive sewage infrastructure,” I’ve said. “They don’t allow us to safely reclaim our waste as fertilizer,” I’ve said. I’ve imagined a bathroom utopia based on a yet-to-be-invented technology in which our waste is kept out of the sewers to avoid contamination and perhaps even harnessed to generate electricity. But with every criticism I deliver I try to give toilets their due: they represent a tremendous step for humanity. This year’s shift from rural to urban would not be possible had we not shifted from cesspool to toilet one hundred years ago. The externalities the toilet has created are worth the lives it has saved. Our next step is to reduce these externalities without sacrificing the gains we’ve achieved in sanitation, convenience, and ease-of-use.

The UN’s population report expects another 1.7 billion people to make the urban shift by 2030, warning of increases in poverty, violence, and pollution as they do. UNICEF underscores these warnings with their analysis of the world’s access to clean water and sanitation. But UNICEF’s report provides much hope to the exploding masses of humanity: despite all the problems facing our species, we’re finally starting to get better at the basics.

For the first humans who gave up the nomadic life and settled into towns and villages, the problems quickly piled up. Quite literally, I mean: stinking, fly-covered piles of you-know-what. Sedentary humans rapidly learned that civilization and sanitation are inseparable — without laws and taboos regulating the disposal of human waste, stench and disease overwhelm civility and decorum.

Today we rely on toilets, sewers, and sewage treatment plants to keep us safe from our waste. But despite millions of lives saved, this sanitary model is not the paragon of human achievement. Rather, it’s a jury-rigged series of fixes applied to salvage an infrastructure designed in accordance with flawed science. It’s far from the ideal sanitary model. But in the short term, we’re stuck with it.

Which means that while civilization’s last great sanitary leaps came from London and Washington, the next is more likely to come from Lagos.

The economic concept of path dependence explains how the set of decisions one faces for any given circumstance is limited by the decisions one has made in the past, even though past circumstances may no longer be relevant. Sewers, for instance, rely on a certain amount of water being flushed down toilets to push the waste through the network of pipes. Though the technology exists to drastically lower the 1.6 gallons per flush our toilets currently use, we’re locked into using that ridiculous amount of water for fear that our sewers might grind to a stagnant halt without it.

Path dependence also explains why it wasn’t until 1972 when Washington finally decided it was time for America’s sewers to end their then century-long practice of channeling raw poop straight into America’s waterways.

Until the late 19th Century, most Westerners deposited their poop into backyard cesspools. This sanitary model was completely overwhelmed by growing population densities during the Industrial Revolution, subjecting civilization to seven great cholera pandemics and millions of deaths as a result. In the mid-1850s, a London doctor named John Snow discovered that cholera was spread by fecal contamination of water. Cities across the world slowly turned to sewers as the best means to remove waste from the population that creates it, led by London in 1859.

But to the scientists and engineers of the time, it was enough to simply direct a city’s sewers to outflow directly into running water. This seems stupid today, but it was in perfect accordance with the science of the time, which held that “the solution to pollution is dilution.” By the time Germ Theory rolled around and science realized that raw waste, diluted or not, was a vector for pathogens, we were way too invested in sewers to give up on them.

So America turned then to purification plants to sanitize municipal drinking water, enabling upstream cities to continue dumping sewage while helping downstream cities avoid that pesky dysentery. Not only was this system unable to scale to every single point of inflow, it did nothing to alleviate the environmental havoc being wreaked on the waterways. But the country was unwilling to give up on sewers, which meant the only choice was to jury-rig a treatment system: sticking multimillion dollar scrubbing plants at sewage outflow points to sequester organic matter from the 32 billion gallons of water we flush every day. There are over 16,000 such plants today; and since 1972, the government has invested a further $250 billion dollars in this sanitary model.

When the weather is nice, this model seems to work. (A steady rain in cities with combined wastewater/stormwater sewers overwhelms treatment plants, forcing them to discharge their untreated flow straight into the water.) Still, in spite of its general adequacy, this sanitary model is hugely expensive, wasteful, and energy intensive. It’s far from ideal. Rather, given our situation, it’s the best we can hope for.

“The best we can hope for.” Such a phrase is the hallmark of path dependence.

Do I have something better in mind? In my book, I speculate on the future of what, to me, is the most promising alternative: fuel cells that turn poop into power. Though it’s admittedly decades away, this is my vision of utopia: your poop is collected in your basement and harnessed to power your household.

And if you went “ewwww!” at the thought of collecting poop as a resource, then you’re suffering ideological path dependence. The flush toilet has locked you into an ideology in which poop should only flush down a black hole and disappear forever. That’s not to say I envision a future in which you shovel poop into your fuel cell like coal into a furnace, of course; but, as I wrote in the New York Times, civilization needs to learn to view poop not as waste but as a resource.

This is the one sanitary advantage held by a city like Lagos. Lagos is, in places, a failed city with a failed sanitary infrastructure. While this means Lagos suffers from terrifying sanitary conditions and periodic cholera outbreaks, it also gives them an opportunity. Just like many developing countries have leapfrogged expensive land-based telephone infrastructures straight to the efficiency and cost-effectiveness of mobile phones, so too can cities like Lagos benefit from *not* being locked into sewers and sewage treatment plants.

Because, unlike us, change won’t have to be justified against abandoning the billions already invested and the ideology already assimilated. Instead, when their turn finally comes, they’ll be able to learn from our century-plus of trial, error, capital outlay, resource drain, and energy consumption, and leapfrog that path altogether.

While the Supreme Court has spent this term worrying about silly things like abortion, the death penalty, and workplace discrimination, this country just saw one of the most important poop-related court decisions since 2002, when a deputy sheriff was charged with felony vandalism after leaving fake poop in a judge’s chambers. Because last week the Weld County District Court in Greeley, Colorado ruled that poop is protected by the First Amendment.

Here are the facts of the case. On May 31, 2006, Kathleen Ensz of Greeley decided that she was sick and tired of receiving campaign literature from U.S. Representative Marilyn Musgrave. So she took a three-inch-long lawn ornament from her German Shepherd, stuck it inside one of Musgrave’s pamphlets, and dropped the pamphlet outside of the Greeley building in which Musgrave’s office was located.

Overreaction being the better part of valor, Musgrave’s people called the cops. And in accordance with the post-9/11 Pansying of America, the cops immediately assumed it was Bowel Qaeda, labeling the turd-o-gram “suspicious” and possibly even “explosive.” Tens of thousands of taxpayer dollars were immediately committed to the investigation, which began and ended with reading the address label still pasted to the campaign brochure.

But taxpayer expenditure didn’t stop there. The DA decided to charge Ensz with “depositing a noxious substance with the intention of interfering with the use and enjoyment of a building,” ensuring the full resources of the state of Colorado would be employed on this quest for justice. After all, this wasn’t just a bag of doodie thrown on some nerd’s porch in a teenage prank — this was a direct attack on a member of Congress.

Nor was Ensz just some teenage prankster. She’s vice chairman of the Colorado Senate District 13 Democratic Central Committee. And, as the Denver Post points out, both Musgrave and Ken Buck, the district attorney who brought the charge, are Republicans.

In his closing statement at the trial, Chief Deputy District Attorney Christian Schulte said jurors should convict Ensz of “criminal use of a noxious substance” because she “intended to deprive Musgrave or other people of the use and enjoyment of their property”. Ensz’s defense argued that this case was an attempt to chill free speech, and that it was only being brought for political reasons.

“It was, very simply, my personal protest of Marilyn Musgrave’s representation,” said Ensz. “It was a political statement.”

Jurors agreed with the defense. Ensz was acquitted, and the precedent was set: poop enjoys the protection of free speech.

This is a very thorny issue. On the one hand, mine is a book that clearly relies on free speech protections for its survival. A less permissive society would block it in a heartbeat. (Did you know that PoopReport.com is banned in Saudi Arabia?) So I’m against any implication that poop is not subject to the same protections as, say, puppy dogs or flowers.

But at the same time, my platform against turd terrorism is unequivocal: I are staunchly opposed to it. From an upper-decker to an all-out bathroom bombing, there is absolutely no justification for intentionally using poop to render a restroom or any other room unfit for human use. Poop should flow from your butt to your toilet. That is my party line. Even something as minor as placing a turd in campaign literature: I cannot condone it.

But what if Ensz had been found guilty? What if it was suddenly *illegal* to leave poop anywhere but in a toilet? What would that mean for the poor, incontinent poopers whose stories of woe provide so much entertainment? Nearly all of us have had an unfortunate McAccident. Imagine how much worse things would be if, in addition to the social stigma that accompanies Underwear McSlurry, one also faced legal repercussions if it dripped onto the floor?

So while I cannot condone Ensz’s use of poop, I applaud the court’s decision not to punish it. Because otherwise the experience of facing a ticking chyme bomb would be fraught with even more horror. (And, possibly, our nations roads would be even more dangerous.) I’m proud that America retains the freedom to eat at Taco Bell without worrying about legal consequences.

What is required, then, is some sort of legal framework distinguishing poop-as-speech from poop-as-terror. Because poop cannot be wholly subject to the protections of free speech, lest the turd terrorists in the world think themselves free to commit their unspeakable acts. And don’t be complacent about this: there are evildoers in this world. There are men and women who find joy in brown spackle, and fruition in your horrified response to it. Deviant miscreants with excrement: they’re out there, and the rule of law and order is necessary to protect us from them.

Thus, in respect to the wisdom of the Weld County court, I propose this benchmark to differentiate between free speech and fecal jihad: ease of cleaning.

If one can simply remove the offending matter with no more protection than a rubber glove, then it’s free speech. This standard protects Ensz’s method of political discourse: by ensconcing poop in a campaign brochure, her point was made but a skidmark was not. Her message was communicated, and then easily removed.

But as soon as cleaning is required, then the perpetrator has gone too far. If your act necessitates a scrubbing with bleach, then you have violated another human being’s inalienable right not to have to clean up poop that doesn’t belong to them. If the recipient of the message has to break out a squeegee or a paint scraper, then the line demarking turd terrorism has been crossed.

In the wake of Ensz vs. Weld County, I believe the courts need to clarify this point. It’s free speech if it sends a message. It’s turd terrorism if it requires a mop. This distinction will help us resolve the next iteration of Brown vs. the Board of Education — that is, the next time some jerk tosses a bucketful of humanure in the direction of a PTA meeting.