Why Do We Age? A 46-Species Comparison
Why we age is a tricky evolutionary question. A full set of DNA resides in each of our cells, after all, allowing most of them to replicate again and again and again. Why don’t all tissues regenerate forever? Wouldn’t that be evolutionary advantageous?
Since the early 1950s, evolutionary biologists have come up with a few explanations, all of which boil down to this: As we get older, our fertility declines and our probability of dying — by bus collision, sword fight, disease, whatever — increases. That combination means that the genetic underpinnings of aging, whatever they are, don’t reveal themselves until after we reproduce. To use the lingo of evolutionary biology, they’re not subject to selective pressure. And that means that senescence, as W.D. Hamilton wrote in 1966, “is an inevitable outcome of evolution.”
Except when it’s not.
Today in Nature, evolutionary biologist Owen Jones and his colleagues have published a first-of-its-kind comparison of the aging patterns of humans and 45 other species. For folks (myself included) who tend to have a people-centric view of biology, the paper is a crazy, fun ride. Sure, some species are like us, with fertility waning and mortality skyrocketing over time. But lots of species show different patterns — bizarrely different. Some organisms are the opposite of humans, becoming more likely to reproduce and less likely to die with each passing year. Others show a spike in both fertility and mortality in old age. Still others show no change in fertility or mortality over their entire lifespan.
That diversity will be surprising to most people who work on human demography. “We’re a bit myopic. We think everything must behave in the same way that we do,” says Jones, an assistant professor of biology at the University of Southern Denmark. “But if you go and speak to someone who works on fish or crocodiles, you’d find that they probably wouldn’t be that surprised.”
What’s most interesting to Jones is not only the great diversity across the tree of life, but the patterns hidden within it. His study found, for example, that most vertebrates show similar patterns, whereas plants are far more variable. “You have to then begin to ask yourself, why are these patterns like they are?” he says. “This article is probably asking more questions than it’s answering.”
This sweeping comparison didn’t require particularly high-tech equipment; it could probably have been done a decade ago, if not before. But nobody had done it. One challenge is that it required a deep dive into the published literature to a) find the raw data on all of these species, and to b) get in touch with the researchers who conducted the field work to see if they’d be willing to share it.
After rounding up all of that data there was then the problem of standardizing it. Mortality and fertility rates of various organisms can differ by orders of magnitude. What’s more, for some species — like the white mangrove, red-legged frog, and hermit crab — this data comes from defined stages of development rather than across the entire lifespan. Jones got around these obstacles by defining “relative mortality” and “relative fertility” numbers for each species, calculated by dividing fertility or mortality rate at a particular age by the average rate across the organism’s entire lifespan. This allows for easy comparison across species, just by looking at the shapes of the curves.
“That’s what’s so disarming about it,” says David Reznick, a distinguished professor of biology at the University of California, Riverside, who was not involved in the new study. “They’ve come up with a way of putting everything on the same scale, so you can perceive patterns that have never been looked at before.”
The study shows, for example, that most mammals and, importantly, the species that scientists tend to use in the laboratory, such as C. elegans andDrosophila, have shapes like ours. But others are weird, at least from a human-centric view.
“Some patterns have emerged in this paper that none of us knew were there,” says Reznick, who has studied aging patterns among different populations of guppies. “It’s crazy to think that we’ve been working on aging for so long and something as fundamental as this hasn’t been seen before.”
What the new study didn’t find, notably, is an association between lifespan and aging. It turns out that some species with pronounced aging (meaning those with mortality rates that increase sharply over time) live a long time, whereas others don’t. Same goes for the species that don’t age at all. Oarweed, for example, has a near-constant level of mortality over its life and lives about eight years. In contrast, Hydra, a microscopic freshwater animal, has constant mortality and lives a whopping 1,400 years.
This is a problem for the classical theories of aging that assume that mortality increases with age, notes Alan Cohen, an evolutionary biologist at the University of Sherbrooke in Quebec.
“The traditional idea is that this is what most things do, and that there were a few weird creatures out there that were exceptions,” he says. “But there are actually a lot of exceptions.”
The question that the classical theories try to answer — How could aging evolve? — is no longer the most interesting question, Cohen adds. “What we really need to explain is why some things age and some don’t.”
Cohen is currently collaborating with Jones’s team to formulate a new theory that answers that question. (Stay tuned for more on this! I’ll be digging into all of the various theories over the next couple of months, as I work on a feature story for Mosaic, a new digital publication.)
Given my obsession with people, I asked some of these researchers what the new findings might mean for our understanding of human aging, which most of us would like to avoid. Will studying species that age like we do — or those that live 1,400 years, for that matter — help us defy age-related decline? Would these studies lead to treatments that might, say, double our lifespan?
Cohen politely reminded me that we have already figured out how to extend our lives. The new study, in addition to comparing 46 species, compared trajectories of three groups of humans: hunter-gatherers, those who were born in Sweden in 1881, and modern Japanese women. The differences are stark.
“In industrial societies, we continue on average to add about a year of lifespan every five years,” Cohen says, thanks to advances in public health, nutrition, and medical care. That’s pretty impressive, and likely to continue if we all eat well, exercise, and avoid stress and smoking, he says. “That’s not going to get us living to 200, but it might eventually get us to 110.”