Although it’s long been suspected that some turtles, fish, trees, and marine invertebrates do not age, it was always assumed that they are the proverbial exceptions confirming the rule. Now it is known that “forever young” species are as common as those that do age.
Imagine someone that never gets old. The first person that comes to mind might be Connor MacLeod from the Highlander movie series, or the vampire Lestat from Anne Rice’s Interview with the Vampire. And yet it turns out that exceptionally long-lived organisms do really exist. They live on Earth, alongside humans, and often closer than one would expect.
They join us, for example, in the lakes we swim in during summer vacation—and can even be seen with the naked eye, without the need for a lab microscope. Hydra vulgaris, or the fresh-water polyp, can grow up to 2.7 inches in length. Although it’s an animal, the hydra resembles a plant—it looks a bit like a tree trunk, and its tentacles resemble branches. The roots seem to be missing, but the hydra is able to cling to a given base object and stay there for years, which is also more typical of plants than animals. Still, its appearance and behavior shouldn’t deceive anyone—it is actually a voracious and effective predator that feeds on both zooplankton and small fish.
The most remarkable feature of Hydra vulgaris isn’t its appearance or diet, but the fact that it does not age. What is more, if it loses part of its body, it can rapidly heal and regenerate: like a phoenix from the ashes, it can grow its body back from the smallest segment. Its death is never the result of aging—it dies as a result of falling prey to another predator, due to illness, an unfortunate accident, or a harmful change of environment. In theory, if put in an aquarium and provided with optimal living conditions—i.e., the right temperature, food, and a safe environment—it could live forever, if the following generations of staff were told how to look after it. Such a pet would be able to outlive entire human families—unless someone forgot to feed it.
Hydra vulgaris isn’t the only organism that doesn’t age. Some other creatures are able to go back in time and become their younger version again. One example is the “immortal jellyfish,” Turritopsis dohrnii, which lives off the coast of Spain and Italy. It can repeatedly dial back its developmental cycle and revert to a sexually immature stage after having reached sexual maturity. In humans, such a process would look like this: children would mature into adults, reproduce, live their life to the full, and at some point return to being children, on and on, over and over again. Moreover, as children they could—just like Hydra vulgaris—create their own clones. Pure sci-fi.
What Is Old Age?
For most humans, old age—in terms of biology—is a period of certain changes occurring in the body. Getting winded going up the stairs, falling ill. Wrinkled, flaccid skin covered in age spots; wounds and fractures taking longer to heal. Thinning, graying hair, impaired hearing and vision, reduced mental capacity. These are just some of the features of aging. There is no consensus among scientists as to what exactly this process is and how it works. It is difficult to define old age, because life expectancy doesn’t have to be related to the aging of the body—as in the case of Hydra vulgaris. There are, of course, many more examples.
Biology distinguishes between two forms of aging: functional aging resulting from age-related decline in body efficiency; and demographic aging, when a decline in fertility is associated with an increased risk of death. More is known about the latter than the former. This is because it is easier to observe organisms and note when they start reproducing, how many offspring they have, and when they stop reproducing and die. It is much more difficult to observe and study changes taking place at a cellular level.
Physiological aging includes, among others, structural deterioration of certain parts of the body. Such changes can affect the teeth of mammals, leading to an inability to eat. When it comes to insects, the structural wear and tear of wings means that these creatures cannot perform mating flights or escape from predators. Both instances lead to imminent death. Other aging processes include telomere shortening, mitochondrial dysfunction, epigenetic changes, genomic instability, loss of proteostasis, dysregulated nutrient sensing, cell senescence, stem cell depletion, and altered intercellular communication. Perhaps then, “old age” is the accumulation of everything that once occurred and somehow left a permanent mark on the body—starting with cells, and ending with entire organs.
Coincidence or Programmed?
For humans, old age is something natural, inevitable. We may think that the same principle applies to all living organisms on the planet, but nothing could be further from the truth. Recent research indicates that all mammals and birds are subject to the process of aging. However, this isn’t so clear-cut when it comes to other groups. Some reptiles and amphibians age, others don’t. The same is true of some fish, arthropods, fungi, and the aforementioned Hydra vulgaris. This means that the existing paradigm of there being a single universal way of aging is now a thing of the past.
Is aging really inevitable? Are we just programmed to grow old and eventually die? Most scientists studying the evolution of aging believe that it may be a side effect of other biological and evolutionary processes, rather than an end in itself. This side effect might be accidental, a whim of nature, the right solution at the right moment—for humans, but also many other species. Most likely, non-aging was something natural for our ancestors from the very distant past.
In some organisms, the aging process seems to be programmed. These include annual plants, but perhaps the most spectacular examples are mayflies and Pacific salmon. The mating dance of mayflies is rather spectacular—millions of males swarm together, creating a huge cloud into which the females fly. Following copulation, the males die before falling to the ground, their dead bodies resembling a macabre carpet, while the females can continue living for several days.
The same is true, and equally quick, for Pacific salmon, who spend most of their lives in the ocean, but as bi-environmental fish, return to the river in which they were born in order to mate. They find their way home by smell and swim against the current, so by the end they are extremely exhausted. Once they arrive, after a short act of procreation, they die.
There are plenty of theories explaining the causes of aging. One of the more widely recognized ideas is that aging is the result of reproduction. We die to make room for our children and grandchildren. Childbirth is also extremely taxing. Resources can go towards either repairing the body or to the offspring. It’s an evolutionary compromise.
This is, at least, how it used to be. Today, humans are starting to resist evolution. Modern medicine and improving living standards mean that a woman who chooses to have more than one child will live longer than in the past. On one condition—researchers at the University of California have found that she should have her first child after the age of twenty-five. These are, however, just general conclusions from studies conducted on a large group of US women, and late motherhood cannot be seen as a universal recipe for longevity for every female—human or animal. In other species, early and intensive procreation doesn’t always equal a shorter lifespan. Blackbirds that breed early do not live shorter than those that start later. Still, female blackbirds who have their offspring earlier lose the ability to lay more eggs more quickly in later life, which suggests that they age faster.
Another interesting theory indicates that oxidative stress is responsible for the aging process. Everyone has probably heard of the infamous free radicals. They’re undesirable, because they lead to cell damage. Of course, the body is equipped with mechanisms to protect itself from them, but these weaken over time. Free radicals are formed in the breathing process—and the more we move, the more we breathe. The more often one runs from predators or in marathons, the faster the body deteriorates, because more oxygen is used. Sport is healthy, as long as it isn’t high impact.
A Recipe for a Longer Life
Silicon Valley billionaires are investing heavily in start-ups striving to find ways to prolong human life. Some of the richest backers predict that they will live up to the age of five hundred and become the longest-living vertebrates on Earth. We don’t yet know whether they’ll succeed, but they should remember that they’ll have to compete with a real record holder—the Greenland shark that populates the dark, cold waters of the Arctic Ocean. These fish can live for up to five hundred years. It might be equally difficult to outlive the invertebrate with the longest known lifespan, a marine bivalve mollusk, the ocean quahog (Arctica islandica). The oldest known individual of this species was determined to have lived to 507 years. This is hardly surprising, given that these creatures are remarkably resistant to oxidative stress. Humans don’t seem to have that ability.
At any rate, no living creature can dethrone the North American aspen, Populus tremuloide, which is considered to be the oldest and largest organism on the planet. A colony of these trees, named Pando, covers an area of over forty hectares, and its age is estimated at about 16,000 years. Although it looks like a forest, all the trees—around 47,000—are connected to each other by their roots, forming a single, giant organism. They are all genetically identical.
The Institution of the Grandmother
Old age has its advantages. Although many animals reproduce throughout their entire lives, there are also those for whom this process ends earlier. This stage is known as menopause. It occurs not only in humans, but also among orcas, narwhals, beluga whales, and short-finned pilot whales.
Most likely, the human lifespan has been prolonged thanks to the institution of the grandmother. Grandmothers took care of children weaned from the mother’s breast, allowing the latter to become pregnant again. From an evolutionary point of view, this was a win-win situation—both females passed on their genes in a larger number of offspring. Children in the care of their grandmother for a longer period matured later and for this to happen, needed a longer life. In addition, over the years grandmothers accumulated vast knowledge that was helpful in raising offspring. Older, more experienced women gave their daughters information on where to find food, how to breastfeed their first newborn baby, how to care for them, and how to help them in case of trouble. They also mediated in resolving conflicts, because they knew all members of their family and the relationships between them. It was all priceless knowledge.
But not only humans have grandmothers. This role is particularly apparent in groups of resident orcas living in the North Pacific. Matrilineal groups are led by the oldest female, who throughout her life (which can last up to ninety years) surrounds herself with sons, daughters, and their offspring. They never leave the group. Thanks to this, the oldest female doesn’t have to reproduce until the end of her days. Instead, she can look after the less experienced members of the group and leave reproduction to the younger, stronger females. She becomes a grandmother, as well as the leader and defender of the group. Calves raised with the help of older orcas have a better chance of survival—without their grannies, they are 4.5 times more likely to die. But if an older orca has not yet gone through menopause and looks after her own offspring, her grandchildren’s risk of death is 1.5 times higher than that of calves whose grandmothers have gone through menopause. Orca grandmothers are better hunters and share food, especially with their grandchildren. They also teach other individuals how and where to hunt, most likely are familiar with other groups of orcas and know how to avoid confrontation. Should conflict be unavoidable, they support their sons—throughout their entire lives.
Evolution doesn’t offer one simple recipe for death. It might be preceded by old age, but it doesn’t have to be. Some organisms are meant to grow old, while others can enjoy youth throughout their entire lives. Is it really possible to talk about youth when there is perhaps no old age? By studying species that do not age, we might learn more about this process and perhaps even extend human life. Yet if this were to happen, the psychological consequences could prove more difficult to endure and overcome than the physiological ones. I love my human life and everything that comes with it. I wouldn’t want to change it too much.
Translated from the Polish by Joanna Figiel
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