Three immortal beings inhabit Earth: some can reverse time, while others simply do not age.

No one likes the thought of aging, but despite numerous attempts to slow down this process, nothing has worked so far.

The issue lies in senescence or cellular aging. This means that over time, cells stop dividing and eventually die. Senescence can refer to the organism as a whole, when a living being can no longer adequately respond to external stress factors, as well as to individual organs or tissues.

There are methods that help to slightly delay the onset of aging, but none can completely "cancel" it. However, some species have managed to completely evade the aging process.

The Immortal Jellyfish Turritopsis dohrnii

To date, there is only one species that can be called "biologically immortal": the jellyfish Turritopsis dohrnii. These small, transparent creatures inhabit oceans around the world and can reverse time by returning to an earlier stage of their life cycle.

The new life of the jellyfish begins with a fertilized egg that develops into a larval stage called a planula. After a brief swim, the planula attaches to a surface (such as a rock, the ocean floor, or the hull of a boat), where it transforms into a polyp—a tubular structure with a mouth at one end and a unique "foot" at the other. For a while, it remains in place, growing into a small colony of polyps that share the nutrient tubes.

Eventually, depending on the species of jellyfish, one of these polyps forms a bud, or separate segments stacked on top of each other, which can then detach from the rest of the colony. This process is responsible for the subsequent stages of the jellyfish's life cycle.

For most other jellyfish, this stage is the final one. But Turritopsis dohrnii (and possibly some other jellyfish species) has a clever trick: when it encounters stress, such as hunger or injury, it can revert back to a tiny mass of tissue, which then transitions back into the mature polyp stage. This is akin to how a butterfly turns back into a caterpillar or a frog becomes a tadpole again.

Of course, Turritopsis dohrnii is not truly "immortal." They can still be eaten by predators or killed in other ways. However, their ability to switch between life stages in response to stress means that theoretically, they could live forever.

Hydra

Hydras (Hydra – Latin) resemble jellyfish polyps with a tubular body and a tentacle-like mouth at one end and a sticky foot at the other.

These animals are designed to be as simple as possible and spend most of their lives remaining in one place in freshwater ponds or rivers. They use their stinging tentacles to catch prey that swims by.

A surprising revelation is that hydras seem to be entirely immune to aging.

Instead of gradually deteriorating over time, the stem cells of hydras possess the ability for infinite self-renewal. This appears to be due to a special set of genes known as FoxO genes, which are found in animals ranging from worms to humans and play a role in regulating cellular lifespan.

In the case of hydra stem cells, the expression of FoxO genes seems to be excessive. When researchers interfered with the functioning of the FoxO genes, they found that the hydra's cells began to show signs of aging and no longer regenerated as they had before.

Researchers still do not fully understand how this all works, but they are confident that these genes clearly play an important role in maintaining the hydra's endless youth.

Almost Immortal Lobsters

Lobsters are also not subject to aging. However, unlike hydras, which depend on specific genes, their longevity is attributed to their ability to endlessly repair their DNA.

Typically, during the process of DNA replication and cell division, the protective end caps of chromosomes, called telomeres, gradually become shorter. When they get too short, the cell enters a state of aging and can no longer continue to divide.

Lobsters do not face this problem thanks to an endless supply of the enzyme telomerase, which aids in the regeneration of telomeres. This enzyme is produced in all their cells throughout their adult life, allowing them to maintain youthful DNA indefinitely.

Telomerase is not unique to lobsters. It is present in most other animals, including humans, but after passing the embryonic stage of life, the level of telomerase in most other cells decreases and becomes insufficient for continuous telomere repair.

Unfortunately for lobsters, there is one catch: they literally grow too large for their shells. Lobsters keep getting bigger, but their shells cannot change size. These unfortunate creatures spend their lives shedding too-small shells and growing new ones. This process consumes a considerable amount of energy.

Eventually, the energy required for molting and growing a new shell becomes too great. The lobster dies from exhaustion, disease, predation, or shell damage.