We didn't evolve to sit. Darwin wants us to walk.

Your mother was right: You shouldn’t just sit around all day.

But that’s what Americans do; by one estimate, the average American adult spends about 70 percent of each day just sitting—at home, at work, in a car. In the last few decades, we have become an impressively sedentary people.

In 1950, Americans holding high-activity occupations that involved being upright and mobile and using large muscle groups (telephone lineman, farmer, milkman) narrowly outnumbered those with low-activity sitting jobs (telephone operator, accountant, insurance adjuster). By 2000, twice as many Americans had low-activity jobs as high. Agricultural jobs, for instance, dropped from just over 12 percent of the total labor force in 1950 to less than 2 percent a half-century later.

While more of us sit at work, all of us sit at home, and we sit more than ever. Consider television, the most studied stand-in for sedentary time. “We become what we behold,” media critic Marshall McLuhan wrote. “We shape our tools and then our tools shape us.”

Until the late 1940s, the television was a novelty. In 1950, only one home in ten had a television; now you’ll find a television in ninety-nine homes out of a hundred. And the television is an aggressively invasive species, spreading quickly throughout households. The average American home now has 2.24 televisions, and two homes out of every three have three televisions or more. Depending on what statistics you believe, the nation may currently be home to more televisions than people.

And the amount of time we spend with our glowing screens has increased at about the same rate the screens have increased in number. For years, American television watching has grown steadily by about one percent a year. Since television entered our homes, every decade we’ve added about thirty-six more minutes of television to our daily media diet. According to a 2009 Nielsen survey, the average American watches about 151 hours of television a month, or about 5 hours every day.

The average American will spend about nine years of his or her life watching television, and most of that will be spent sitting or lying down. That nine years of American Idol and dispiriting football games will not only be beyond recovery, it also appears very likely that those who spend that time parked in front of a television will come up short in the actuarial life table, as well—they’re likely to die about two years earlier than they would without television, thanks to their aggressively sedentary lifestyle.

The long and short of it: Darwin wants us to walk; everything in our evolutionary history has proven it. And yet we do the opposite and sit all day— at home, at work, in cars — fighting the very thing for which nature selected us. Some researchers call what’s happening “progressive sedentariness.” Which is just a top-shelf way of saying that we have sitting disease.

Sitting is, quite literally, killing us.

Adapted from “The Last Great Walk” by Wayne Curtis. Copyright © 2014 by Wayne Curtis. By permission of Rodale Books. Available wherever books are sold.

About the author

Wayne Curtis a contributing editor to The Atlantic magazine and the author of the new book, “The Last Great Walk: The True Story of a 1909 Walk from New York to San Francisco and Why It Matters Today.” He lives in New Orleans.