No, humans don’t have a natural lifespan of 38 years

By Faye Flam

Biologist Benjamin Mayne certainly generated a lot of media attention with his research showing that the natural human lifespan is 38 years. If he’s right, then the implications are huge — for starters, we would have to rethink our entire health care system. That system is based on the (perhaps unrealistic) assumption that deaths not attributable to accident or violence are due to disease, and that all diseases can be conquered with enough medical research.

What if, instead, we’re programmed to die before we’re even old enough for a midlife crisis?

Not so fast. The 38-year limit comes from a system Mayne and his colleagues created to apply across vertebrates, from the live-fast-die-young forest shrew (2.1 years) to the slow-lane Greenland shark (400 years).

The researchers calibrated their ageing clock using animals that have been kept in captivity and whose maximum lifespans are recorded in a database. Humans have a maximum known lifespan of about 120 years, but this was excluded from their calibration data for being too much of an outlier. According to the paper, which was published in Nature Scientific Reports, “this does not reflect the variability [of] the true global average lifespan (60.9–86.3 years).”

So if we have a record of 120, and “true global average” of 61-86, how could we have a natural lifespan of 38? In some ways, that number makes sense, given that our closest relative, the chimpanzee, lives to 37 years. But still, it doesn’t square with observations. Captive chimps with good healthcare don’t ever live to 80 or beyond, while people routinely do — and did so long before there was anything like modern medicine.

Socrates was 70 when he died, and was still so vigorous that his enemies had to do him in with poison. Benjamin Franklin, who had pointed out that 18th-century medicine did more harm than good, managed to live to 84. Historical records show that the pace of human ageing has been the same for 2,000 years.

People sometimes confuse death from old age with life expectancy, which was quite low for humans for a very long time. Remember, life expectancy is an average, and for centuries it included a disproportionate number of infants. Before modern medicine, infants and children often did die from infections, malnutrition, or birth defects, and newborns, as well as young women, died from childbirth complications. Life expectancy was just 46 in 1907, but that was because it was routine for kids to die – not because anyone died of old age at 46. People who survived the dangers of childhood were pretty likely to live to a ripe old age.

And modern life seems to be as much a killer as a saviour. The diseases that kill most people over 55 are heart disease and cancer, both of which, we are told, would be less rampant if people got more exercise, avoided cigarettes and ate less processed food, sugar and high fructose corn syrup. Even agriculture seems to have been a mixed blessing: Medical archaeologists have found that compared to their foraging ancestors, ancient people from farming societies were shorter and more likely to be diseased or undernourished.

Dr Mayne, who works at the Commonwealth Scientific and Industrial Research Organization in Australia, says that his system relies on a well-known marker of ageing called DNA methylation — a change in the material that gloms onto your DNA. It doesn’t affect the actual code, though it affects how the code is read and expressed.

Over time, our cells accumulate methylation much the way our arteries pick up plaque or our teeth start to decay. DNA of different species varies in the number of sites prone to methylation — the more sites, the faster the ageing clock ticks. Moreover, Mayne explains, the ageing clocks of animals are tied to their reproductive clocks. It tends to be the long-lived, slow-to-reproduce animals that are more vulnerable to extinction.

I ran this by S Jay Olshansky, an expert on ageing from the University of Chicago. He says it’s important to understand that we’re not programmed to die at a certain age. Our selfish genes are under evolutionary pressure to maximize their own propagation — which means you’re programmed to live long enough to care for your offspring until they are reasonably self-sufficient. After that, nature becomes indifferent and you are left to the capricious ravages of entropy. If you’re lucky and take good care of yourself, you can coast for a few decades after that.

The slower an animal’s reproductive cycle, the longer it takes before this coasting stage begins. If you’re a mouse or a forest shrew, surrounded by predators, natural selection favours those who reproduce early — within about a month of being born. A tough animal like the Greenland shark can afford to put off childbearing till around 150.

Humans are all coasting by 50 — that’s when CDC data start showing degenerative diseases going from rare to common — but when this phase begins for us isn’t clear. Olshansky doesn’t think it’s crazy to conclude that the coasting phase could start around age 38.

Healthcare is a way to help those who lose in that coasting lottery. It’s also a way to alleviate suffering — which can linger for years in people with degenerative diseases.

Since age is a risk factor for so many terrible diseases, and we can’t quit ageing the way we can quit soda drinking or smoking, scientists should keep trying to understand how ageing works and whether there’s any way to slow it down. And that’s where those long-lived, slow-reproducing animals clearly have something to teach us.