It's the one macabre truth of being human: one day, all of us will die.

Naturally, this inevitability has led many of us to dream of a life of immortality, being able to stick around for hundreds more years.

While we humans are yet to shed the shackles of our limited lifespans, there are many animals out in the natural world that are seemingly breaking the laws of nature, defying the inevitability of death.

These animals—which include axolotls, hydra, jellyfish and more—and the mechanisms they use to lengthen their lives may even one day help us to elongate our lives further.

"There are many organisms that either don't age or age so slowly that we haven't detected it: rockfish, lobsters, sturgeons, the microscopic hydra, and some sharks, among others," John K. Davis, a professor of philosophy and bioethics at California State University, Fullerton, told Newsweek. "There are colonies of coral that are over 20,000 years old. Bacteria don't age; they just divide and their offspring divide again, indefinitely."

Humans age due to a combination of factors, including the slow degradation of our chromosomes and the body running out of stem cells, both of which contribute to the lagging of cell repair and the aging of the body and its systems—a process called senescence. Therefore, in order to supersede our short human lifespans, we would need to find a way of stopping these issues from arising.

"There is currently no consensus among scientists who study aging as to its causes," David Gems, a professor of biogerontology and aging at University College London in the U.K., told Newsweek. "One idea that has been around for a long time is that aging is caused by accumulation of damage, and that non-aging organisms are very good at repairing themselves."

Greenland Shark

Some animals have a profound ability to live exceptionally long, such as the Greenland shark. These gentle giants grow to between 8 and 23 feet long and live in the deep, cold waters of the North Atlantic and Arctic Oceans. They have the longest lifespan of all vertebrate species and can live for many hundreds of years, with the oldest shark that has been found so far being thought to have been born somewhere between 1504 and 1744.

"We studied their metabolic rates for the first time a couple of years ago and discovered that they have very slow metabolisms, unsurprising given they're large cold-blooded fish living in the freezing waters of the Arctic (metabolism slows with decreasing body temperature and increasing body mass)," Eric Ste-Marie, a marine ecologist and Greenland shark researcher at the University of Windsor in Canada, told Newsweek. "Their slow metabolism likely contributes to their longevity, but more research is needed."

However, the unique environment of Greenland sharks means that their long-lived tactics are unlikely to be of much use to humans.

"Since we're warm-blooded mammals and Greenland sharks are cold-blooded fish, it's unlikely that figuring out the mechanisms leading to their longevity will help us live longer," Ste-Marie said.

Immortal Jellyfish

Another animal that appears to be able to live indefinitely is the so-called "immortal jellyfish", or Turritopsis dohrnii.

This tiny, 0.18 inch-jelly can seemingly live forever: if it's injured physically, the jellyfish is able to turn back into a polyp, the first stage in its development process, that is genetically identical to the adult version. This is akin to frogs being able to suddenly revert to being a tadpole.

The jellyfish is also able to do this in cases of starvation, the American Museum of Natural History explains, meaning that unless the jellyfish is eaten, it can live for an extremely long time. The process behind this Lazarus-like transformation is called transdifferentiation, which is when a specialized body cell changes into another type of cell, an act that is usually impossible.

This process is of particular interest to scientists, as it could be applied to humans, allowing us to replace cells damaged by disease.

Tardigrades

One organism, called tardigrades, also displays a strange form of near-immortality. These tiny 0.02-inch creatures, also called water bears, are capable of withstanding a vast range of extreme conditions. They appear unperturbed by extreme temperatures, extreme pressures, high doses of radiation, dehydration, and starvation, and have even been found to survive the void of space.

Hydra

"There isn't really such a thing as immortality in the natural world. What there is, though, are organisms that lack an aging process, such as the freshwater polyp Hydra vulgaris. But they still die, just not from aging," Gems said

According to a 2015 study published in the journal Proceedings of the National Academy of Sciences, hydra contain a huge number of stem cells, and over time, show very little signs of cell degradation or senescence, seemingly forever.

"Genes [may] cause aging to happen, for reasons relating to the way that evolution works," Gems said. "In particular, evolution plays nasty buy now, pay later games to improve performance (e.g. in terms of reproductive success)—where payback comes in the form of diseases of aging. By this view, non-aging organisms simply lack a genetically-determined aging process—perhaps because the conditions for those nasty games to be played are not there. This would mean that activating mechanisms from non-aging organisms in humans to make us non-aging is not feasible."

Hydra are also capable of regrowing their heads if they're lopped off, just like their mythological counterparts, simply regrowing the missing appendage and carrying on.

Regenerative Abilities

Regeneration of body parts is also seen in a number of other species, including some salamanders and axolotls, which can regrow missing limbs after an injury. Some planarian flatworms can be cut in half and simply regrow their entire body from both sections, creating two new worms, NIH reported.

"The ability to regenerate is actually very common in animals and widely distributed across the roughly 35 phyla (groups) of animals on earth. Many of these groups are marine invertebrates that can have incredible regenerative abilities. Some of the more famous examples include sea anemones and flatworms, but there are many others (sea stars, acorn worms, mollusks, etc)," Heather Marlow, an assistant professor of organismal biology and anatomy at the University of Chicago, told Newsweek.

They do this by generating new tissues, growing new cells like a fetus in the womb.

"They can do this by dedifferentiating existing cells and signaling to them to divide and differentiate into new tissues," Marlow said. "They can also do it by activating an existing resident population of stem cells that respond to signals of injury and divide to form the lost tissues. While it's unlikely that sea anemone or flatworm stem cells could be compared one on one to a human stem cell, the basic principles of how these stem cell populations are maintained, how they are activated and the genes they use to develop into the regenerated structures are important to study."

Salamanders

Why and how salamanders can regrow limbs is a matter of great interest to the regenerative biology community.

"Some think they may carry unique—salamander-specific genes, (i.e: only present in the genomes of salamanders), which help promote regeneration," Igor Schneider, an assistant professor of limb regeneration at Louisiana State University, told Newsweek. "Others, such as myself, think that they use genes that we all have, but whereas they orchestrate a proper gene expression response that results in regeneration, we fail to do so."

He continued: "Whereas there could be a correlation between longevity and regenerative capacity, especially for organisms capable of whole body regeneration, longevity and body part regeneration are two independent phenomena [that] can also evolve separately. It means that some species can live long but fail to regenerate their body appendages (such as some sharks that can live for centuries but cannot regenerate their fins if bitten). Conversely, others can readily regrow fins but are very short-lived."

There is a large amount of research going into applying limb-regrowing abilities to humans, as this could provide amputees with life-changing options in the future.

"Scientists are working on many fronts to discover how to regrow a human limb," Schneider said. "One promising avenue of research is the identification of genes that code for proteins important to kickstart regeneration in salamanders, and then the production of creams or wearable biomaterials that contain these proteins or drugs that activate signaling pathways of regeneration. This approach was used to some success for activating some regenerative growth in severed legs of frogs," according to research published in Science Advances' January 2022 edition.

"I believe the field will eventually succeed. Whether future therapies for human limb loss involve sophisticated brain-machine integrated prosthetics or medicine that promote regrowth of limbs (or both), is anyone's guess," he said.

With such a range of tactics used across animals to live much longer than we do, it may be one day possible to apply them to our own lifespans. However, due to the huge differences between our bodies and the bodies of these immortal animals, this is likely to be very difficult, and not yield quite the same degree of "immortality", rather, just helps to treat some conditions.

"My own view is that the longest we could ever get using non-genetic therapies (e.g. drugs) over the next century or so is not more than 5-10 years more than we currently live (which would still be a very thing... a greater benefit than one would get from completely eradicating cancer, for instance)," Schneider said. "To live much longer would require very extensive and very complex germline genetic modification. This might one day be possible (politically, ethically as well as scientifically)—many centuries in the future (if not millennia) I would guess, in a much better world than our present one."

https://www.newsweek.com/animals-immortal-aging-human-death-1799699?amp=1