Eureka: Who Lives Longest?

A Swedish baby born in 1800 had a life expectancy of just 32 years. We know this because Sweden was one of the first countries to keep extensive records of births and deaths and, by 1800, had a reliable national system that allowed this morbid statistic to be calculated. That baby's life may sound nasty, brutish and short, especially for a nation advanced enough to keep such detailed records, but before you imagine 19th-century Swedish teenagers suffering the regret and ennui of midlife crises, consider this: that same year, a 20-year-old Swede could reasonably expect to live another 37 years.

Life expectancy is an average, and it fluctuates with age as the risks we face change throughout our lifetimes. Both those facts make it a frequently misunderstood statistic. High infant-mortality rates depress the figure substantially. This can lead contemporary observers to the false conclusion that most humans died quite young, even in the not-so-distant past. (Were you ever told, as a petulant teenager, that you'd have been considered middle-aged in medieval Europe?)

" 'Life expectancy' is this term that entered public lingo without public understanding of what it really means," says Andrew Noymer, associate professor of public health and sociology at the University of California, Irvine. Our hypothetical Swedish baby's 1832 expiration date is, of course, nothing of the sort. It's a way of expressing, statistically, that lots of babies and small children were dying in 19th-century Sweden. By simply surviving childhood, a young Swede could expect a relatively long life — and if he was lucky, a proper midlife crisis.

But so could Fred Flintstone. In the last decade, scientists have concluded that humans have lived into older adulthood since 30,000 years ago, during the Upper Paleolithic (part of the era more commonly known as the Stone Age). Michael Gurven, a professor of anthropology at U.C. Santa Barbara who has studied modern hunter-gatherer and horticultural tribes, found that people in these societies who survived childhood lived about as long as 19th-century Swedes did — into their 50s and beyond. His work is one clue that suggests Enlightenment Age Europeans could have had the same longevity as our ancestors who painted caves and hunted the woolly mammoth.

Before the Upper Paleolithic, early humans really did die young, most before their 30th birthdays. Then, during the late Stone Age, there was a significant increase in the number of people living into older adulthood. The scientific and technological advances that made the modern era possible are well known to us, but the Upper Paleolithic was also host to a flourishing of early human culture.

Rachel Caspari, a paleoanthropologist at Central Michigan University, studies the life spans of ancient humans, their ancestors and close relatives — together, known as hominins. In 2004, she and a colleague examined teeth from 768 hominin fossils representing three million years of primate evolution.

Looking at wear and other signs of aging in the teeth, Caspari split the fossils into groups of old and young adults, creating rough approximations of ancient demographics. Examing a span from between 100,000 and 30,000 years ago, Caspari found about four old adults for every 10 young adults. But beginning around 30,000 years ago, during the Upper Paleolithic, this reversed: Caspari counted 20 old adults for every 10 young adults.

This demographic shift coincided with an explosion of cultural production: clay figurines; carvings made of bone, wood and stone; cave art and jewelry making; and complex burial practices. According to Caspari, it was longer human life spans that seem to have made this flourishing possible. Having more time on earth allows our species to progress.

The most recent rapid increase in human life expectancy started around 1880 in Europe, North America and Japan. Now we might be approaching another age of great technological upheaval, as stem cells and gene therapy offer the potential to extend lives to unprecedented lengths. But life expectancy measures only gains that refer to a whole population, and in the United States, rising inequality has become a drag on this most basic measure of human progress.

Currently, life expectancy at birth in the United States is roughly 79 years, and it's the same at age 25, but our gains have slowed considerably. The U.S. ranks 51st in the world for life expectancy at birth. Around 1950, we were ninth. Furthermore, what improvements we do see are not spread evenly across the population. For example, between 1990 and 2008, white women with college degrees picked up three and a half years of life expectancy, while those without a high-school diploma lost five years.

Meanwhile, researchers are experimenting on mice, trying to isolate the mechanisms that cause aging, in an effort to slow (or stop) them. These mice have been injected with stem cells and subjected to various forms of DNA tampering. Perhaps someday, thanks to these mice, we'll be able to extend human life to Old Testament lengths — for those who can afford it. Perhaps not. Scientists disagree about whether there's a biological limit to human life expectancy, but it does seem possible that we're approaching a sort of sociological limit here in the U.S.

Justin Denney, assistant professor of sociology at Rice University, has studied the ways that certain health issues disproportionately affect the most disadvantaged people in this country. For example, he told me other studies have shown that wealthy, well-educated smokers outlive poorer, less-educated smokers, even if you control for the amount of tobacco consumed. The biggest barriers to improving life expectancy in the United States are societal issues, Denney says. You can't get a shot to undo the effects of growing up in a lead- and asbestos-contaminated row house. There's no laser surgery to fix the detrimental effects of generations of chronic stress.

Perhaps one day there will be a stem-cell treatment that can double one's life expectancy by slowing the aging process, making a centenarian as spry as a quinquagenarian. But if such a serum benefits only the few who can afford it, our national life expectancy will hardly budge — it's an average, after all. In 19th-century Sweden, the figure was dragged down by infant mortality. Today in the United States, it grows slowly because of the premature deaths of the less fortunate. Thanks to this vast inequality, even a high-tech fountain of youth would hardly move the needle.

"Look at the countries with the highest average life expectancy," says Denney, referring to places like Japan, Australia, Canada and, yes, Sweden — nations that distribute their health resources more evenly. "Ultimately," he says, "life expectancy is a measure of quality of life."

Maggie Koerth-Baker is science editor at BoingBoing.net and author of "Before the Lights Go Out," on the future of energy production and consumption.