I grew up laughing at Mel Brooks and Carl Reiner and their interviews with a 2,000-year-old man. (The greatest invention in history? "Saran Wrap.") I assumed, then, that the concept of a human life lasting two millennia was impossible. Perhaps it's not, though.

In an intriguing National Journal article titled "The Immortality Race," William Powers offered some thoughts on the possibility of humans living for 5,000 years -- and even if we fall short of that, he sees trouble for Social Security. True, those who could live to 5,000 probably won't be born for a very, very long time. But many longevity experts today predict that soon, in our lifetimes, it won't be uncommon for people to live to 100 and beyond. That change alone has some serious implications for Social Security, as the program has been designed around certain life expectancies for beneficiaries. If people suddenly begin living considerably longer, they'll be drawing a lot more money from the system, making it less reliable.

In a recent article titled "7 Social Security Myths," my colleague Robert Brokamp deftly explained that while Social Security may not be completely broken, it does appear to be unsustainable in its current configuration -- at least for those of us whose golden years are still several decades away. He quoted the Social Security Administration:

"Without changes, by 2042 the Social Security Trust Fund will be exhausted. By then, the number of Americans 65 or older is expected to have doubled. There won't be enough younger people working to pay all of the benefits owed to those who are retiring. At that point, there will be enough money to pay only about 73 cents for each dollar of scheduled benefits."

And Robert added: "Even without changes, you'll still receive approximately three-fourths of your scheduled benefit. That stinks for those of us who will be in retirement by then, but it's not as gloomy-and-doomy as we are often led to believe. As long as there are workers paying FICA taxes, the program will have money."

Phew! But still. any of us who may have been counting on Social Security to provide much of our retirement income should be looking elsewhere for solutions. (Let Robert help you, with our newly launched Rule Your Retirement newsletter, which offers easy-to-understand guidance. You can try a sample for free right now.)

Perhaps a more vexing problem for us Fools is what stocks to invest in for the long haul, if the long haul may be up to 5,000 years. I believe in buy-and-hold investing, but I'm not sure what I should hope to hang on to for 5,000 years. Will computers still be around then? Who knows. Perhaps it would be better to stick with firms such as Coca-Cola (NYSE:KO) and Procter & Gamble (NYSE:PG) for the really long haul, and invest in the likes of Microsoft (NASDAQ:MSFT) and Cisco Systems (NASDAQ:CSCO) for only the short haul -- say, 10 or 20 or 30 years.

Longtime Fool contributor Selena Maranjian owns shares of Coca-Cola and Microsoft.