Many think that immortality is something that only mythical and supernatural creatures can have,
however nature has shown us that immortality is possible and many organisms living on Earth today are already immortal or at least do not age like we do.
Biological immortality (also called bio-indefinite mortality) is a state in which the rate of mortality due to age-related deterioration is stable or decreasing,
thus decoupling it from chronological age. Various unicellular and multicellular species, including some vertebrates,
achieve biological immortality either throughout their existence or after living long enough. However,
one must remember that biologically immortal living beings can still die from means other than old-age, such as through injury or disease .
Scientist believe that biological immortality might be related to the fact that the rate of mortality as a function of chronological age may be negligible at extremely old ages - a theory referred to as the late-life mortality plateau. For some organisms, the rate of mortality may cease to increase in old age, but for humans that rate is typically very high. As a hypothetical example to illustrate this, there is only a 50% chance of a human surviving another year at age 110 or greater .
In humans, one of the events associated with ageing cells is related to telomere length. A telomere is involved in protecting chromosomes from deterioration. In order to function normally, cells in our bodies must keep dividing to replace cells that are worn out or damaged. During this division process, our genetic material must be passed on to the next generation of cells. This genetic material inside cells is arranged in strands of DNA called chromosomes. At the end of these strands is a protective 'cap' called a telomere. Each time a cell divides the protective telomere 'cap' gets shorter. When they get too short, the cell loses its ability to renew and divide, which leads to deterioration and subsequently aging.
As opposed to humans, Planarian flatworms - one of the immortal organisms, continue to amaze scientists with their apparently limitless ability to regenerate cells. They are able to maintain telomere length indefinitely so that they can continuously replace and regenerate aged or damaged tissues .
Aside from flatworms, nature provides us with several other cases of biological immortality. Hydras, for example, are simple, freshwater animals that have the amazing ability to regenerate, allowing them to recover from injury and to reproduce asexually. All hydra cells continually divide. Researchers suggested that hydras do not undergo age-related deterioration, and therefore are biologically immortal. The underlying mechanism of their immortality is also linked to the ability to maintain telomere lengths. Biologists succeeded to keep a Hydra in the lab for more than four years - which is incredibly long time for an animal that only measures 15 mm. Moreover, at the end of the four-year experiment, Hydra looked as youthful as on day one .
Turritopsis dohrnii is a small (5 millimeters) species of jellyfish that uses trans-differentiation to regenerate cells after sexual reproduction. Scientists described how the species - at any stage of its development - can transform itself back to a polyp, the organism's earliest stage of life, thus escaping death and achieving potential immortality. This finding appeared to debunk the most fundamental law of the natural world - you are born, you mature, and then you die. The anthropomorphic analogy is that of an old man who grows younger and younger until he is again a fetus. For this reason, Turritopsis dohrnii is often called the "Benjamin Button jellyfish" .
Although lobsters, are not immortal in the traditional sense, studies suggest that they may not slow down, weaken, or lose fertility with age, and that older lobsters may be more fertile than younger lobsters. Nevertheless, they are listed between organisms with the longest lifespans, their longevity may be due to the expression of telomerase, an enzyme that regenerates telomeres. Telomerase is expressed by most vertebrates during embryonic stages but is generally absent from adult stages of life. However, unlike vertebrates, lobsters express telomerase as adults through most tissue, which has been related to their longevity .
Biological immortality is not an unfamiliar concept in nature. Therefore, scientists started to investigate into the premise that biological aging can be halted or reversed, which would have an enormous impact on life extension possibilities.
Several experiments confirmed that targeting senescent cells extends the life-span of lab mice, suggesting that treatments aimed at killing off these cells, or blocking their effects, might also help to combat age-related diseases in humans. Cells that are no longer able to divide - called senescent cells - were closely linked to old-age related diseases, such as kidney failure and type 2 diabetes. Senolytics, a new potential class of drugs that selectively remove senescent cells, could therefore be an attractive treatment strategy. 
In 2009, Elizabeth Blackburn at University of California, San Francisco won the Nobel Prize for Physiology or Medicine, for proving that telomeres could be maintained by the activity of an enzyme called telomerase. This discovery opened new doors for anti-aging research, and scientists soon began to investigate into the possibility of lengthening telomeres. In the following years, studies confirmed that telomere extension can turn back aging clock in cultured human cells . More recently, US based company, BioViva Sciences Inc. has taken research to a new level and announced that they have successfully reversed 20 years of normal telomere shortening, over a 7-month period. The experiment began in 2015 and the beneficiary was Elizabeth Parrish, the CEO of the company. This surprising result is based on the average T-lymphocyte (white blood cell) telomere length compared to the American population at the same age range. However, it remains to be further investigated whether the success in lymphocytes can be expanded to other tissues and organs, and repeated in future patients .
It's hard to answer with absolute certainty when we will be able to successfully apply these treatments in humans, however recent progress in medicine allowed us to identify important mechanisms involved in biological immortality and scientists are confident that noteworthy progress in extending human life is imminent.
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