The human desire to live longer is hardly new, even though the language surrounding that desire has changed considerably over time. What previous generations might have imagined as a fountain of youth or an elixir capable of extending life is now more likely to be discussed through the language of biological aging, cellular repair, regenerative medicine, stem cells, sophisticated technologies, and an expanding collection of biological biomarkers.
Before becoming too absorbed in what humans might eventually accomplish, there may be value in walking backward from the human question and asking what biology has already laid bare before us. Humans naturally understand living and aging through experience. We know that a cut can heal but a lost arm does not grow back. Some injuries heal almost completely while others leave permanent scars or disability. We also know that children become adults and adults eventually become older, but we have no experience of an adult returning to childhood and beginning development again. Because these are the biological boundaries familiar to us, they can easily begin to feel like the boundaries of biology itself.
A comparatively wider look at the living world gives us insight into biology beyond our own experience. One of the most striking examples is a tiny animal called Turritopsis dohrnii, popularly known as the “immortal” jellyfish because its biology challenges our familiar understanding of the course of life. Human experience gives us a strong intuition of a one-way story from infancy through growth and maturity toward old age. However, under certain circumstances, the immortal jellyfish can reorganize its tissues from adulthood and return to an earlier stage of its life cycle, from which development can proceed again.
For an animal to possess something resembling a biological reset is sufficiently extraordinary that the word immortal becomes understandable. However, the word can also create an impression that the animal has somehow solved the larger problem of death. It has not. The jellyfish can still be eaten by predators, succumb to disease, suffer damage from which it cannot recover, or encounter environmental conditions incompatible with survival. Its remarkable biology therefore allows it to escape one familiar direction of aging without escaping the broader vulnerability that comes with being a living organism. Avoiding one pathway toward death is not the same thing as eliminating mortality itself.
Another remarkable example that approaches the problem from an entirely different direction comes from the axolotl. The axolotl is a salamander native to Mexico whose modest size and unusual appearance give little indication of the regenerative machinery hidden within it. If an axolotl loses a limb, it can regenerate the missing structure rather than simply closing a wound or covering the injured area with scar tissue. Its regenerative capacity also extends to portions of other tissues, including parts of the spinal cord and heart, which is one reason researchers interested in regeneration have studied this animal so closely.
Anyone familiar with serious human injury can appreciate how extraordinary this is. A severed human limb does not regrow, and major spinal cord injury can permanently alter movement, sensation, independence, and almost every aspect of daily life even when rehabilitation and modern technology restore substantial function. If humans possessed regenerative abilities approaching those of the axolotl, entire areas of trauma care, surgery, neurology, orthopedics, prosthetics, and rehabilitation would look very different. Strikingly, the axolotl is pointedly mortal.
Its ability to reconstruct body structures that humans cannot replace does not make the entire animal permanent, just as the jellyfish’s ability to return to an earlier developmental stage does not make it indestructible. One animal possesses something resembling a developmental reset, while another possesses a degree of structural regeneration that seems almost miraculous from the human perspective. That said, neither biological achievement by itself provides the thing we ordinarily mean when we imagine immortality.
It would be easy to regard these animals simply as fascinating biological curiosities whose unusual abilities demonstrate how different other forms of life can be from us. Human beings are regenerative organisms too, although our regenerative abilities are distributed differently throughout the body and are so familiar that we rarely place them in the same conceptual category as the regrowth of an axolotl limb.
Remarkably, much of the human body has been repairing, replacing, renewing, and remodeling itself throughout life without entering our awareness. The skin covering us today is not simply the same collection of cells that covered us decades ago. The lining of the gastrointestinal tract undergoes continual renewal while being exposed to food, digestive secretions, microorganisms, mechanical forces, and countless chemical substances. Red blood cells circulate for only a fraction of a human lifetime before being removed and replaced by newly produced cells, while bone, which appears so permanent when we look at a skeleton, remains living tissue undergoing continual remodeling.
These processes are easy to overlook precisely because they are normal. We tend to reserve our sense of biological wonder for things we cannot do, such as watching a salamander grow another limb, while the extraordinary processes occurring continuously inside our own bodies become ordinary through familiarity.
The liver, our own liver, makes the distinction between regeneration and indefinite preservation particularly difficult to ignore. The human liver possesses a remarkable capacity to regenerate after substantial tissue loss. If such an ability existed only in an exotic animal, we would probably describe it with the same sense of wonder that accompanies discussions of the axolotl.
However, a liver capable of substantial regeneration can still accumulate fat, experience repeated cellular injury, develop inflammation and fibrosis, progress toward cirrhosis, lose its normal architecture, and eventually fail. An organ can therefore possess extraordinary regenerative ability without possessing unlimited regenerative capacity, particularly when the conditions producing injury persist or when repeated repair no longer restores normal structure completely.
The liver within us therefore demonstrates both the remarkable capacity for regeneration and the limits of what regeneration alone can accomplish. By extension, the skin renews itself and nevertheless changes with age. Blood cells are continually replaced while the person producing them continues to grow older. Intestinal cells can turn over rapidly without making the entire organism permanently young. Bone remodels throughout life while age-related changes in bone structure can still occur. The presence of repair, replacement, and regeneration therefore does not eliminate aging, because maintaining individual components and maintaining the coordinated organism indefinitely are not the same biological task.
This distinction can still be misunderstood if we assume that a living organism is a machine whose worn components can simply be replaced. The stark difference is that machines can often be turned off, dismantled, repaired, and returned to service after the work is completed. A living organism has no comparable luxury. The heart must continue pumping blood while its own cells maintain themselves, the kidneys must continue filtering and regulating the internal environment while their tissues age, the liver must continue performing its enormous range of metabolic tasks while responding to injury. The brain must continue coordinating movement, sensation, memory, breathing, temperature regulation, behavior, and countless other functions while maintaining the cells and connections on which those functions depend. In short, the body performs its maintenance while remaining open for business and has been doing so continuously from before we became aware that there was a body to maintain.
Unquestionably, when we encounter an eighty-year-old person, our attention is naturally drawn toward the visible evidence of aging because those are the differences we can compare with youth. We notice gray hair, wrinkled skin, slower movement, reduced muscle mass, and a stooped posture or the use of a cane. Without intending to do so, we begin to see aging largely through what the body has lost. These observations are real because aging can bring disease, disability, pain, reduced reserve, dependence, and losses that profoundly affect quality of life.
However, beyond the future desire for longevity that is not promised, we would do well to appreciate the strength and resilience already demonstrated across a long life. This is possible if we stop looking at aging only from the direction of youth and reflect backward from an elderly person who has already traveled through most of a human lifetime.
In this case, the eighty-year-old person standing before us is not simply a body that has accumulated eighty years of deterioration. This is also a living organism that has survived eighty years of continuous biological demands, most of which occurred without ever entering conscious awareness. The heart began beating before birth and continued through childhood, sleep, physical exertion, infections, excitement, grief, fear, celebrations, quiet afternoons, and ordinary days that have long since disappeared from memory.
The lungs have exchanged gases through hundreds of millions of breaths, while generation after generation of red blood cells has carried oxygen through the circulation before being removed and replaced by those that follow. The intestinal lining has renewed itself repeatedly, skin has continually replaced cells exposed to the outside world, bones have remodeled in response to changing mechanical demands, wounds have healed, immune defenses have responded to innumerable encounters, and cells throughout the body have spent every second maintaining the chemical and electrical differences without which organized life would disappear.
This description captures only part of what this organism has experienced across eight decades, because the human body does not perform its maintenance in a protected laboratory where temperature, nutrition, physical demands, microorganisms, emotional experiences, and environmental conditions can be held constant. The body must remain alive in a world that is under no obligation to provide stable conditions. The external environment is free to fluctuate far more widely than the internal environment can tolerate, leaving the body with the continuing task of adjusting circulation, temperature, water balance, fuel availability, hormonal signals, ventilation, kidney function, and countless other processes so that the conditions surrounding its cells remain compatible with life even while the conditions surrounding the person repeatedly change.
Across eighty years there have been hot summers and cold winters, while the body’s internal temperature still had to remain within a relatively narrow range compatible with life. There were days when food arrived predictably and perhaps other periods when meals were delayed, appetite disappeared during illness, food was unusually abundant during celebrations, or circumstances changed what and how much could be eaten. Water intake varied while the internal concentration of water and electrolytes still had to be defended. Physical demands changed from the restless movement of childhood to work, recreation, caregiving, injury, recovery, periods of greater activity, and perhaps eventually the reduced movement that often accompanies older age.
Then there were the biological challenges arriving from outside. Viruses and bacteria did not wait for convenient moments to appear. Wounds occasionally opened the protective boundary of the skin. Food continually introduced foreign material into the gastrointestinal tract. Allergens, pollutants, sunlight, smoke, heat, cold, and innumerable microorganisms repeatedly encountered a body whose survival depended on maintaining boundaries while still remaining open enough to exchange matter and energy with its surroundings.
Some of those encounters produced illnesses that entered memory because symptoms became severe enough to be noticed, while countless others were probably handled so effectively that the person never knew there had been anything to handle.
Even this physical description of the external environment remains incomplete because human beings do not live only in climates and ecosystems. We also live in families, communities, economies, workplaces, cultures, and political environments, and events occurring in those environments eventually become biological experiences inside the person living through them.
An individual who reaches eighty may have experienced the deaths of parents, siblings, friends, colleagues, or perhaps children. There may have been periods of unemployment or financial hardship, years of demanding work, disrupted sleep while raising children, caregiving for sick relatives, marital difficulties, loneliness, fear, disappointment, or long periods when the future seemed uncertain. Some people have lived through war, political violence, forced migration, famine, natural disasters, racial or ethnic conflict, or the collapse of communities they once considered permanent. Even comparatively fortunate lives contain grief, unexpected illness, changing responsibilities, financial pressures, interpersonal conflicts, and events that the nervous, endocrine, cardiovascular, immune, and metabolic systems cannot simply dismiss because the original cause occurred outside the body. The body has to live through the life its person is living.
For someone who reaches eighty, the external world itself may also have changed almost beyond recognition during a single lifetime. A person who is eighty years old today entered the world in the mid-1940s, when transportation, communication, food systems, medical care, occupations, infectious disease exposure, housing, artificial lighting, heating and cooling, patterns of physical activity, and the technologies surrounding daily life were very different from those encountered in old age. The person did not receive a new human physiology every decade to match each changing environment. Instead, the physiology that had carried that person through the preceding years had to continue adapting as the world around them changed.
Those internal and external challenges undoubtedly left scars, with some contributing to chronic disease, reducing physiological reserve, or changing the trajectory of health permanently, while others were compensated for rather than completely corrected. Yet innumerable disturbances were handled well enough for the person to continue into another day, another month, another year, and eventually another decade.
Here is the story worth telling about the elderly body. The wrinkles, the slower gait, and whatever disease or disability may be present remain part of the story, but the noticeable deterioration should not tell the entire story. Behind what age has taken away lies the accumulated history of a living organism that continued through all those years of internal and external change.
We have spent so much time asking why life must eventually end that we have not spent enough time wondering how a living organism manages to continue for so long. This question does not make death less real and should not require anyone to stop hoping for additional healthy years. It does not establish an exact ceiling on human lifespan or suggest that medicine should stop trying to prevent disease, preserve function, reduce suffering, or extend healthy life. It simply places mortality within the much larger biological story that precedes it.
An eighty-year-old person is not evidence of eighty years without disturbance. That person is evidence of survival despite disturbance. The chemical reactions never stopped, cells continually required energy and repair, tissues were injured and renewed, and physiological systems had to remain coordinated while weather, microorganisms, injuries, changing food availability, physical demands, emotional experiences, social upheaval, and the unpredictable circumstances of a particular human life continued arriving from outside.
Back to the exotic immortal jellyfish and the axolotl. The immortal jellyfish initially captured our attention because it can do something that seems almost impossible from the perspective of human development, but its developmental reversal does not make it invulnerable. The axolotl captured our attention because it can rebuild structures that humans cannot regenerate, yet extraordinary structural repair does not make the entire animal immortal. The liver brings the lesson inside us by demonstrating that substantial regenerative capacity can coexist with progressive injury and eventual organ failure.
This is what the elderly human adds to the longevity question. There is no regrown limb to photograph, no dramatic reversal to an earlier developmental stage, and no single regenerative event spectacular enough to earn a name like immortality. Instead, despite the noticeable frailty, here is a person who has lived through decade after decade of change, while the body that carried them through those years continually repaired, adapted, and maintained the conditions necessary for life.
Longevity therefore cannot be understood merely as the discovery of a missing repair mechanism, because it also reflects the extraordinary duration for which countless repair, regulatory, adaptive, and compensatory mechanisms can continue preserving a living system. The presence of limits does not erase that achievement. The eventual biological boundary does not mean that the preceding decades represented failure any more than the end of a long journey means that the distance already traveled somehow disappears.
This can possibly bring a little proportion to the modern fascination with longevity without requiring us to choose sides in the debate. Wanting additional healthy years is understandable, and trying to understand aging is worthwhile, but neither requires us to regard the ordinary human lifespan merely as an inadequate starting point waiting for science to correct it.
Before asking why an eighty-year-old person cannot continue for another thirty or forty years, we should first wonder and stand in awe at the more upstream question of how their body managed to maintain them through the first eighty. That does not ask us to celebrate death, surrender to disease, or stop pursuing better health. It simply allows mortality and biological achievement to occupy the same picture.
The jellyfish can reverse part of its life cycle and still die. The axolotl can rebuild a limb and still die. The human liver can regenerate substantial tissue and still fail. If anything deserves a doff to the resilience of life, it is the human body’s ability to spend decades replacing cells, repairing injuries, remodeling tissues, defending its internal environment, adapting to an unpredictable external world, and preserving enough coordination for consciousness, movement, memory, relationships, work, laughter, grief, curiosity, and everything else contained within a human life.
Biology has already been telling us something about longevity that becomes difficult to hear when our attention is directed only toward extending the endpoint. Life is not remarkable because it avoids disturbance, but exceptional because it persists through disturbance.
An elderly person standing before us is therefore not merely evidence that aging eventually happens. That person is evidence of a living system that remained adaptive, reparative, and sufficiently organized to continue for an entire lifetime. We have spent so much time looking for extraordinary longevity elsewhere that we have overlooked the extraordinary longevity already standing in front of us.
Mukaila Kareem is a doctor of physiotherapy and founder of metabolichealthliteracy.com
