A few days ago, for the first time in our history as a species, the human population of the planet Earth became predominantly urban, not rural. Some bold researchers pinned down this epochal moment of passage to May 23, 2007. The date was of course a polite statistical fiction, based on a United Nations estimate of how fast people worldwide are shaking off the dust of the countryside and moving into town.

In any case, nobody stood up to ask the important question: What does it mean to become a city-dwelling species?

One school of thought has always treated cities as the antithesis of nature. Maybe that’s because cities originated in part to shelter people from scary beasts and other blessings of Mother Nature. Or maybe it’s because ardently embracing glass and concrete seemed like a good way for newcomers to distance themselves from their shrubby suburban past. A friend who aspired to become an urban sophisticate used to pretend that the trees in Central Park made him ill. Even Jane Jacobs, the great theorist of urban living, once disparaged modern nature as a “tamed pet” which do-gooders wanted to foist on the city “so the city might get some nobility, purity and beneficence by association.”

But the evidence increasingly suggests that these attitudes are misguided. Even city dwellers need nature, and what they get from it is their sanity. (Nobility? Purity? O.K., they’re still somewhere down the to-do list.) In the public housing projects of Chicago, for instance, studies have consistently shown that trees make for healthier neighborhoods, with more people spending time outdoors and more kids playing in creative ways. Housing projects with trees also have about 7 percent less crime than their treeless counterparts. Partly that’s just a function of having more eyes outside watching. But domestic violence also drops, suggesting that there’s something deeper going on here. Human nature exhales in the presence of leafy things.

You wouldn’t use a word like “sylvan” about any of these neighborhoods. They’re still just public housing projects. But “what’s relatively green for” a given individual “is better than what’s relatively built,” says Frances Kuo, an environmental psychologist at the University of Illinois at Urbana-Champaign. Planting trees may thus trump police technology as a way to make safe, livable neighborhoods.

Trees also apparently help heal the sick. For one study, researchers took kids with attention-deficit disorder on walks (“forced marches,” Kuo jokes) through one of three settings: a park with plenty of foliage, a downtown with lots of hard surfaces and a residential neighborhood. At least in the short-term, the park walk turned out to be as good as, or better than, pharmaceuticals in helping such kids concentrate on their work.

Other studies have shown that patients recovering from surgery in a room looking out on trees need far fewer painkillers than patients in rooms looking out on a brick wall. Open-heart patients in rooms with nature scenes on the wall have lower blood pressure and smoother recoveries than patients with blank walls or abstract art. (There are apparently times when a Franz Kline painting, say, can be visually toxic.) Rooms with lots of natural daylight also reduce patients’ sense of pain.

None of this ought to be particularly surprising. When they build new cages, zoos go to great lengths to recreate the natural habitats of the animals destined to live there. Oddly, though, that sort of consideration has rarely figured in the discussion when architects and urban planners have designed for human inhabitants. And yet for millions of years, right up until this past May 23, our species largely evolved in the presence of trees, open spaces, plenty of sunlight and the things – like flowers, foliage and water views – that promise the resources we need to live. When people get stuck instead in windowless offices, says Judith Heerwagen, an environmental psychologist in Seattle, they compensate by putting up four times as much nature-themed décor as they would if they just had windows.

But cities and architects are beginning to overcome their recent aversion to nature. Next year, at 42nd Street and Sixth Avenue in Manhattan, for instance, Bank of America will open a big new office building canted almost reverentially toward Bryant Park, just across the intersection. The ceiling of the lobby will match the height of the trees in the park, so the investment bankers entering elevators on a second-floor mezzanine can take in the view.

“Even if you don’t want to look at nature, we’re making you do it,” one of the architects jokes. Huge windows and reflective ceilings will bring natural light into the trading floor, a space known elsewhere as “the pit.” Heerwagen will be studying the results to test the popular, but unproven, hypothesis that “green” design can make workers healthier, happier, more productive, and less inclined to absenteeism.

Cities are also getting into the act, with “urban ecology” programs that often pay for the plants, but get local residents to wield the shovels and watering cans.

Baltimore has the Parks & People Foundation, Los Angeles has Tree People, Washington, D.C., has Casey Trees, funded with a single $50 million donation. And in New Haven a few years ago an AT&T field manager named Charles Nixon started planting around the house he had purchased, partly as a way to distract passersby from the 17 bullet holes in the façade. Since then, he and his neighbors, working with a local program, have spread their green empire over a six-block area they think of as the Beers Street neighborhood. Crime has gone down, and owner occupancy has increased.

“Back in the day, it was a rough, rough neighborhood, littered with drugs and thugs,” says Nixon. “But it got better since we started planting.”