All animals eventually grow old and die. It's an inevitable fact of life - except when it isn't. Some animals, like tortoises and lobsters, never grow old, and learning their secrets could let humans live as long as they want.
For most animals, there are three basic ways they can die: disease, injury, or old age, which is also called senescence. But a select few species are seemingly immune from aging itself, a phenomenon known as negligible senescence. The gradual accumulation of cellular damage and degradation that will eventually kill other animals (including us) slows to a virtual standstill, prolonging the life - and, in fact, the youth - of any animal lucky enough to be negligibly senescent.
Tortoises are the most famous negligibly senescent animals. An Aldabra giant tortoise named Adwaita was thought to be 255 years old when he died in 2006, and carbon dating of his shell confirmed that he really had been born around 1750. And it wasn't old age that did him in - he died of liver failure complicated by a wound brought on by a crack in his shell. If his handlers at the Alipore Zoological Gardens in Kolkata, India had had the resources and inclination to arrange for a liver transplant and surgery to rebuild his shell, it's possible he might still be crawling around today.
The notion of figuring out how negligible senescence works and applying its secrets to humans is at the forefront of current life extension research. Indeed, famous (his critics would say infamous) artificial life extension advocate Aubrey de Grey is the co-founder of the SENS Foundation, which stands for Strategies for Engineered Negligible Senescence. So could artificially recreating negligible senescence be the key to human immortality?
Before we even begin to answer that, we need a better handle on what negligible senescence actually is, how it works, and what its limits are. So we spoke with Caleb Finch, a world-renowned expert on the science of human aging, or gerontology. It's not much of an exaggeration to say he wrote the book on human aging - his 2007 text The Biology of Human Aging is considered one of the definitive works of the field.
I first asked him just how negligible this negligible senescence really is. I outlined a scenario in which I (and, in time, my descendants) swore to protect for the rest of time a tortoise from all dangers both internal and external, so that the only way it could die was of old age...under those circumstances, how long might it live? Because scientists are nothing if not patient, he humored my rather ridiculous scenario and explained what the ultimate life expectancies might be for negligibly senescent animals, both actual and (in the case of humans) hypothetical:
"In theory, if mortality rates did not increase as usual during aging, humans would live hundreds of years. I have calculated for humans (Finch 1990 book:Longevity, Senescence, and the Genome) that at mortality rates of 0.05% per yr, as found at age 15 in developed countries, the median lifespan would be about 1,200 years. In natural populations of long-lived animals, mortality rates are rarely less than 1% per yr. For very slowly aging turtles, rockfish, the number beyond 70 is 1-2%. However, there are long-lived trees, like the bristlecone pine at 5,000 years.
So if a human population maintained the same extremely tiny mortality rate it has upon first reaching sexual maturity, with only 1 in every 2,000 people dying every year, then the average person could expect to live for at least a dozen centuries. Even if we could only achieve artificial negligible senescence on the order of what animals experience in the wild, a perpetual 1% mortality rate would still allow people to live for multiple centuries, and the median age would likely be fairly close to the current maximum human life span. After all, as Finch explains, negligibly senescent animals enjoy a 1%-2% death rate once they get past their 70th year, and that rate has allowed multiple documented cases of turtles and tortoises living past 150.
As for where negligible senescence comes from, he explained that we simply don't know the answer to that yet. From an evolutionary perspective, there are two possibilities. Either negligible senescence is something certain animals developed because it gave them a reproductive advantage - obviously, the ability to keep reproducing indefinitely is a huge boon to a species's survival chances - or it was more or less an accident.
To better understand what that second possibility means, let's look at why humans aren't negligibly senescent. In humans, some of the same crucial genes that ensure our initial survival to reproductive age also seem exacerbate the aging process in later life. In other words, evolution essentially traded our long-term health for the best chance at short-term survival. Since natural selection is driven by who breeds the most and not who lives the best, that exchange will always make evolutionary sense.
But if turtles and other negligibly senescent animals weren't forced into that same type of genetic trade-off, then they could essentially have their cake and eat it too - maximizing their short-term survival odds without damaging their long-term health. So how and why did this happen in turtles, lobsters, and rockfish, and not in other animals? Well, that's the mystery, and we're likely still a long, long way away from a clear answer.
Still, there's a slowly emerging consensus among gerontologists that negligible senescence in humans is, if not definitely possible, at least something worth shooting for. As Dr. Finch so eloquently puts it:
One piece at a step, future medicine will learn to repair and replace age-damaged cells and tissues. Negligible senescence is a very long-term goal for humans, and means eradication of cancer, heart disease, and Alzheimer's disease.
Even if we never quite achieve human immortality, pursuing negligible senescence might solve a lot of humanity's worst diseases along the way. Seems like a worthy goal to me.