Do organisms exist that are able to live indefinitely if they are not killed by external factors?
Under external factors I would consider things like predators and natural disaster but not illness by bacteria for example.
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I now found this Wikipedia article on biological immortality. It's pretty much what I was looking for.
Wikipedia describes the phenomenon as follows:
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 thing can still die from means other than senescence, such as through injury or disease.
It has a list of such organisms as well, consisting of
Addendum: This blog post takes a good look into the myth of lobster immortality. It seems to be as much a myth as the result of any substantial observation.
Yes. The Bristlecone Pine, Pinus longaeva, is one example. This species boasts the oldest individual living organisms, and also has been convincingly argued by Lanner and Connor (2001) to show no evidence of senescence.
While the Wikipedia page on Biological Immortality (as of June 2013) unfortunately ignores plants, the pages on Negligible Senescence and Longest-lived Organisms list many plant seeds, clonal groups, and individuals.
The immortal jellyfish (Turritopsis dohrnii) is capable of biological immortality.
It's one of few known species capable of reverting completely to a sexually immature, colonial polyp stage after having reached sexual maturity as a solitary (free-floating) individual (called a medusa).
Theoretically, this process can go on indefinitely, effectively rendering the jellyfish biologically immortal
Image source: Piraino et. al. 1996
Piraino, Stefano, et al. 1996. "Reversing the life cycle: medusae transforming into polyps and cell transdifferentiation in Turritopsis nutricula (Cnidaria, Hydrozoa)." The Biological Bulletin 190(3): 302-312.
Examples found in plant world also- especially those with high vegetative reproduction and regeneration property.
Yes, all life forms can become immortal in principle. As article https://www.karger.com/Article/Abstract/477209 states "The Emerging View of Aging as a Reversible Epigenetic Process". There are Yamanaka factors whose expression do almost miraculous things in rejuvenating and reprogramming cells, this is very hot topic. If one reviews many illnesses then most of them emerge from the aging phenonmena. Those phenomena are classified and acted upon by many medical scientists and especially by SENS research foundation http://www.sens.org/research/introduction-to-sens-research/extracellular-junk Rejuvenation is the only cure for many illnesses and scientists are pursuing it. Some in more moderate fashion at https://home.liebertpub.com/publications/rejuvenation-research/127 and some in more bold steps in SENS research foundation.
http://transhumanist-party.org/ is political organisation whose one aim is indefinite extension of life. So - only political will and social transformation is necessary to make human (and their beloved pets, like tortoises and turtles) indefinitely long living.
And rejuvenation is not miracle per se. Every act or reproduction is instance of this miracle - every new organism is created from two quite old cells.