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Biological immortality refers to a stable or decreasing rate of mortality from cellular senescence as a function of chronological age. Various unicellular and multicellular species may achieve this state either throughout their existence or after living long enough. A biologically immortal living being can still die from means other than senescence, such as through injury or disease.
This definition of immortality has been challenged in the new Handbook of the Biology of Aging, because the increase in rate of mortality as a function of chronological age may be negligible at extremely old ages, an idea referred to as the late-life mortality plateau. The rate of mortality may cease to increase in old age, but in most cases that rate is typically very high. As a hypothetical example, there is only a 50% chance of a human surviving another year at age 110 or greater.
The term is also used by biologists to describe cells that are not subject to the Hayflick limit.
Biologists chose the word "immortal" to designate cells that are not subject to the Hayflick limit, the point at which cells can no longer divide due to DNA damage or shortened telomeres. Prior to Leonard Hayflick's theory, Alexis Carrel hypothesized that all normal somatic cells were immortal.
The term "immortalization" was first applied to cancer cells that expressed the telomere-lengthening enzyme telomerase, and thereby avoided apoptosis—i.e. cell death caused by intracellular mechanisms. Among the most commonly used cell lines are HeLa and Jurkat, both of which are immortalized cancer cell lines. HeLa cells originated from a sample of cervical cancer taken from Henrietta Lacks in 1951. These cells have been and still are widely used in biological research such as creation of the polio vaccine, sex hormone steroid research, and cell metabolism. Normal stem cells and germ cells can also be said to be immortal (when humans refer to the cell line).
Immortal cell lines of cancer cells can be created by induction of oncogenes or loss of tumor suppressor genes. One way to induce immortality is through viral-mediated induction of the large T‑antigen, commonly introduced through simian virus 40 (SV-40).
Tardigrades, also known as "water bears", are highly resilient microscopic animals. They are capable of surviving extremes such as heat, radiation, and drought by going into suspended animation, where their metabolism slows to near zero and they simply wait out the harsh conditions until the environment is more favorable.
Bacteria are said to be biologically immortal, but only at the level of the colony. An individual bacterium can easily die. The two daughter bacteria resulting from cell division of a parent bacterium can be regarded as unique individuals or as members of a biologically "immortal" colony. The two daughter cells can be regarded as "rejuvenated" copies of the parent cell because damaged macromolecules have been split between the two cells and diluted. In the same way stem cells and gametes can be regarded as "immortal".
Hydras are a genus of simple, freshwater animals possessing radial symmetry and no post-mitotic cells. The fact that all cells continually divide allows defects and toxins to be diluted. It has been suggested that hydras do not undergo senescence, and, as such, are biologically immortal. However, this does not explain how hydras are consequently able to maintain telomere lengths.
Turritopsis nutricula is a small (5 millimeters (0.20 in)) species of jellyfish that uses transdifferentiation to replenish cells after sexual reproduction. This cycle can repeat indefinitely, potentially rendering it biologically immortal. It originated from the Caribbean sea, but has now spread around the world.
Older lobsters are more fertile than younger lobsters. Lobsters, like many other decapod crustaceans, grow throughout life, and are able to add new muscle cells at each molt. Lobster express telomerase as adults through most tissue, which has been suggested to be related to their longevity.  Large lobsters are estimated to be perhaps 50 years old, although determining age is difficult.
Planarian flatworms (both sexual and asexual) appear to exhibit an ability to live indefinitely and have an "apparently limitless [telomere] regenerative capacity fueled by a population of highly proliferative adult stem cells".
Attempts to engineer biological immortality in humans
Although the premise that biological aging can be halted or reversed by foreseeable technology remains controversial, nascent research into developing possible therapeutic interventions is already underway. Among the principle drivers of international collaboration in such research is the SENS Research Foundation, a non-profit organization that advocates a number of what it claims are plausible research pathways that might lead to engineered negligible senescence in humans.
For several decades, researchers have also pursued various forms of suspended animation as a means by which to indefinitely extend mammalian lifespan. Some scientists have voiced support for the feasibility of the cryopreservation of humans, known as cryonics, a practice largely regarded as pseudoscience. Cryonics is predicated on the concept that some people considered clinically dead by today's medicolegal standards are not actually dead according to information-theoretic death and can, in principal, be resuscitated given sufficient technological advances. The goal of current cryonics procedures is tissue vitrification, a technique first used to reversibly cryopreserve a viable whole organ in 2005.
Similar proposals involving suspended animation include chemical brain preservation. The non-profit Brain Preservation Foundation offers a cash prize valued at over $100,000 for demonstrations of techniques that would allow for high-fidelity, long-term storage of a mammalian brain.
Immortalism and immortality as a political cause
In 2012 in Russia, and then in the United States, Israel, and the Netherlands, the pro-immortality "longevity" political parties were launched. They aim to provide political support to anti-aging and radical life extension research and technologies and want to ensure the fastest possible—and at the same time, as least disruptive as possible—societal transition to radical life extension, life without aging, and ultimately, immortality. They aim to make it possible to provide access to such technologies to the majority of people alive today.
Other life extensionists
Biogerontologist Marios Kyriazis suggested that biological immortality in humans is an inevitable consequence of natural evolution. His theory of extreme lifespans through perpetual-equalising interventions (ELPIs) proposes that the ability to attain indefinite lifespans is inherent in human biology, and that there will come a time when humans will continue to develop their intelligence by living indefinitely, rather than through evolution by natural selection.
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- More on human biological immortality