What Opossums Can Teach Us About Why We Age
The connection between natural selection, healthspan, and lifespan.
Posted Jun 10, 2019
Everyone knows the story of Charles Darwin’s finches, and how observing the various species among the Galapagos Islands helped confirm the famous 19th-century scientist’s basic theory of evolution. But fewer people know about Steven Austad’s opossums—and what they can teach us about why we age.
Dr. Austad, who is now a professor of biology at the University of Alabama, first became interested in opossums in the 1980s in Venezuela where he teamed up with another scientist studying the animals’ reproductive behavior. He was shocked by the very short lifespan of these Venezuelan opossums: just two years. And soon, the mystery of these fast-aging marsupials took him to Sapelo Island off the Georgia coast, where he was able to study an isolated group of Virginia opossums that had lived separately from their mainland counterparts for roughly 4,000 years.
After observing the two populations over a few years, Dr. Austad discovered that the group of animals living on the island lived 25 to 50 percent longer than their mainland cousins. They also reproduced later, birthed smaller litters, and showed fewer physiological signs of aging, meaning they also stayed healthier longer.
Why? The answer, just like the answer to the mystery of Darwin’s finches, is evolution by natural selection.
Natural selection, if you’ll recall from your high school science class, is the long process of weeding out traits that aren’t beneficial to a species’ survival in a given environment. So, in the Georgia opossums’ case, while the mainland group lived among multiple predators, the island opossums faced far fewer threats. The result: Over the years, the island opossums evolved to reproduce on a more lax schedule. Meanwhile, the mainland opossums had to reproduce earlier, or they might not get a chance to reproduce at all. Larger litter sizes also helped ensure their young would survive.
This phenomenon is similar to what happens in other species’ lifespans as well: As humans, for example, we are at the top of the totem pole in terms of predator-prey relationships, and we also spend 20 years or so raising our children, much longer than many other species. All of this translates to our having evolved to have a much longer lifespan than say, opossums.
For years, this presented a curious paradox for scientists: If evolution by natural selection is able to optimally design organisms for the survival of the species, why doesn’t evolution also prevent the terrible aging and decay we face later on?
After reproduction, the influence of natural selection wanes, ultimately leading to a haphazard process (what we know as aging) in which many life processes and functions begin to fail simultaneously.
Not only does natural selection stop working its magic as we get past reproductive age, but it’s also even possible that some genes, selected specifically because of the way they help keep you healthy in earlier years, could begin to backfire later in life. For example, there’s a protein called p53, which is known to help prevent cancer; p53’s basic function is to recognize cancer cells and eliminate them by helping to regulate a variety of cellular processes, such as DNA repair and cellular senescence, which is what happens when cells stop dividing. That sounds like a gene you’d want to have, right? It’s one that can help get you from birth to reproductive age because it prevents a deadly cancer.
But ultimately, over the course of a lifespan, senescent cells created by p53’s activity build up in many tissues. While senescent cells were once thought of as simply inert cells, they are now known to release inflammatory chemicals that play a key role in aging. Despite the fact that eventually, p53 will wear you down later, this gene will still be selected for its protective effects early in life, illustrating another way evolution influences aging.
Today, humans are living longer than ever before, and so the toll of aging is also greater than ever. In 2030, more than 20 percent of Americans will be 65 or older, compared to less than 10 percent in 1970, according to estimates from the U.S. Census Bureau. Given the complexity of aging and the myriad changes that occur, including late-onset diseases, the challenge is to design strategies to mitigate this debilitating predicament. Recent research in the field is leading to the hope that we can rise to it.
In the end, these new discoveries, which we will discuss in future columns, may recapitulate the work of natural selection in slowing the aging of the Sapelo Island opossums. But instead of this work taking 4,000 years over 2,000 generations, these interventions—be they drugs, supplements, or other remedies—may slow or even reverse aging, leading to an immediate impact on our species as we move through the 21st century and beyond.
Instagram images by Nick Pecker, Becky Sheridan, and Geoffrey Kuchera via Shutterstock.