Anyone who's passed the age of 35 knows that we're not built to last. Many of us will slog it out to 76 years, the expected lifetime for American males -- females get two thousand extra days -- but even when young, our bodies barely work, and that marginal situation only worsens as the decades drone on.

It's worth noting that a lifetime of four-score and seven is a new problem for our species. If you lived in Egypt two hundred generations ago, perhaps with a gratifying job as stone chiseler in Giza, you wouldn't worry much about career burnout. You'd be dead by age forty. The up-side was that the Pharaoh didn't have problems with social security.

Life expectancy took a big jump during the Victorian era, when civil engineers fitted out cities with sewers and water pipes. But more improvement is on the horizon: Some people expect lifetimes to double in the coming century as we learn more about our biological makeup. That's nice, but why stop with a mere factor of two?

Gerontologists like Aubrey de Grey figure we can cure death altogether, and in the not-too-distant future. If you're bummed about missing out on this impending medical development, there's always cryonics -- which offers a doubtful promise of time-shifting your life into the 21st century by putting your body on ice today.

I used to be a big fan of living forever, although I soon learned that not everyone agreed. One guy told me he didn't relish the thought of endless dental hygiene appointments. At parties, I found that men were often enthusiastic about immortality, but the women were less so. A physician I know suggests this is due to women's reluctance to confront an infinite future of dealing with boorish men, hitting on them until the heat death of the universe.

Rapid turnover is nature's way of making sure that a species can keep up with changing circumstances and survive the long haul. But since humans have gone beyond basic biology, why not re-engineer ourselves for a lifetime without an end point? Or at least for one where we outlast the Roman Empire?

Well, it turns out there are problems... even beyond the tedium of boorish men.

Let me first state that if we can pull this off -- cure death -- it's self-evident that we'll also obliterate the debilities of aging. You'll be healthy to the end. Nonetheless, there are countless gotchas for any descendants that have made themselves as indestructible as zombies.

First off, they'll need to engineer a major societal revamp. You can't have kids every two years forever: we don't have the real estate. And of course, marriages would have an expiration date.

A myriad of other social structures would also have to be rejiggered: Imagine the frustration of waiting for a tenure slot at the local college which, even after millennia, is still stuffed with its original faculty.

Other difficulties are neither obvious nor tractable. For example, today more than 30,000 Americans die annually on the roads. That means you have a 50 percent chance of being taken out in an auto accident if you live for 3,600 years. So if we extend our lifetimes to thirty or forty centuries, using a car becomes an existential threat. You won't do it.

That may make you a permanent homebody, sitting at your desk playing video games as the eons tick by. Not a pretty picture, and probably not a fragrant one either. Over the course of 3,600 years, you'd have a 4 percent chance of dying in the tub, so bathing will be rare. And if you get hungry, you won't drive to the grocery store -- you'll walk.

Regrettably, you might not find any groceries. Farming is one of the most dangerous jobs around, and any farmer who lives long enough to fear riding in a car has had a more-than-even chance of being killed in the back forty. Incidentally, that's about the same death rate as mining coal, so we'll need to get those wind turbines built if you want electricity at home.

Here's the problem in a nutshell: if we extend human lifetimes a lot -- to millennia, rather than centuries -- all the small risks you heedlessly take every day will have a devastating cumulative impact. Most jobs will become unattractive, because just about any occupation becomes, eventually, a deadly occupation. We'll automate nearly everything we can, and stay at home immersed in a virtual world.

To accommodate this new lifestyle, software for our amusement will become more and more compelling. I mean, for how many centuries can you remain jazzed by "Grand Theft Auto"? I figure that "Roman Orgy III" would quickly be available for Xbox. Humans might become nothing more than protoplasmic containers for their nerve endings, since virtual experience will be the only kind of experience we'll have.

Sure, this is an over-the-top scenario, but there's something to be noted here: our society is made possible by the relatively short timescale of our lives. Extending our life spans a little is merely problematic. Extending them a lot demands a whole new paradigm. Otherwise, our future will be ugly and tedious, punctuated only by video games, dental appointments, and the occasional boorish lout.

