I remember a patient I saw as an intern. She was 95 years old, and had come in with a kidney stone. When I pulled her medical chart, I was taken aback. It was only 5 pages long. She had never been admitted to the hospital, unlike the typical 65 year olds who had hundreds of pages of records.
A sage attending physician told me that was very common, that 90 year olds often had never been admitted to a hospital. “That’s how they get to be 95,” he said. At the time, I thought it was very odd that the 95 year old looked much healthier than most 65 year olds.
A recent paper in Science, reviewed in NY Times by Zimmer, may shed some light on this. It found that your risk of dying starts decreasing after age 80 and raises the possibility that there may be no limit to human lifespan. More importantly, it lends support to the hypothesis that aging is genetically programmed, and that it’s under evolutionary selection.
First, let’s start with our current thinking.
When you are young, your mortality rate is high. Infant mortality has been high through most of our evolution and even now it’s higher than in adults. As you grow, mortality rate drops until you get old, and then climbs again.
When you reach your 60s and 70s, your risk of dying goes up again. The assumption has been that your mortality goes higher and higher as your body wears out, and then eventually you die. This would fit with the theory of degenerative aging–that your body wears out like an old car.
This is apparently not what happens in real life.
What the Science paper presents is data that beyond age 80, your risk of dying decreases. This is an extraordinary finding, if true. That means that longer you live beyond age 80, lower the likelihood of dying is. And after age 105, the risk of dying plateaus. That’s as close to immortality as you can get.
Could this be true? Isn’t it crazy?
It is crazy…except it’s not. You see this exact same thing–that really old individuals are immortal (i.e., their mortality declines at very old age) in many other species, including medflies. You see it in wasps, yeast, and a host of other species. See here and here. There have even been previous evidence that this may be true in humans, though I don’t believe any study has shown an actual reversal of mortality in very old humans.
This means, first, that there may not be a hard limit to human lifespan. That’s pretty big.
It also means that current theories of aging, such as mutation accumulation theory and antagonistic pleiotropy theory (both of which posit that genes that are harmful later in life are under less evolutionary selection than ones that are harmful early in life) can not be correct. To an evolutionary biologist such as myself, that’s pretty big too. This data also support the heretical theory that aging is a result of evolutionary selection, because if aging was under selective pressure, there should be mutants that have extraordinarily long lives, or infinite lives. Otherwise there would be no natural variation for evolution to work on.
To be frank, I would not be surprised if there were a lot of additional data out there supporting this conclusion. It’s the kind of data that is so against current paradigm that people are likely to ignore the data when they run across it, thinking of it as a fluke. But if older you get, more likely you are to live longer, then this is quite a finding.