Ah, immortality: the ever-elusive dream of both utopians and transhumanists. To us, death is a necessary evil, but to many organisms, plants and fungi that form clonal colonies, it’s but a technicality. Now it seems that some organisms managed to ditch it altogether.
Turritopsis nutricula is a tiny jellyfish from the Caribbean, a mere 5 mm in diameter, which has achieved the impressive feat of not dying. This trait is possibly shared with its less exotic cousin Hydra. T. nutricula is a hydrozoan, a class of simple aquatic animals called cnidarians. Like most of the members of this group, it has a hydroid, or polyp stage, as well as a free-swimming, sexually-reproducing medusoid, or jellyfish, stage.
What makes T. nutricula different from all other animals is that instead of dying some time after sexual reproduction, as most medusoids do, it reverts back to the polyp stage. It seems to be able to go through this process over and over again. It’s as if an old person could revert back to toddlerdom, and go through the kindergarten, college, and other milestones in order as many times as they pleased.
The process of such reversal is known as transdifferentiation. Generally, we start life as a ball of undifferentiated cells (stem cells are an example), and as we grow and develop, they differentiate into different types, choosing their cellular fates. In most organisms, this process is not reversible—despite many claims that lack of exercise makes your muscles turn into fat, humans simply don’t work that way. But some jellyfish do, and use the process to revert from a free swimming medusa to a tiny, asexual hydroid.
Of course, mere biological age is not the only thing that makes us grow old: Many of us remember our excitement over Dolly the sheep and quick disappointment at the realization that she wasn’t a very good sheep. Her cellular clock was that of an older animal, and she passed away after only 6 years, from a disease affecting older sheep.
Telomeres—repeating DNA sequences that protect the ends of eukaryotic chromosomes—shorten with each round of DNA replication, and the enzyme telomerase repairs them, increasing their length. Trouble is, telomerase is usually active early in life. In most adult cells telomerase is not active, and as we age, our cellular death clock is ticking along, the ends of our chromosomes fraying with each cell division. When the telomeres reach the critical length, the Hayflick limit, the cell will stop dividing and die. Each cell comes with built-in limit on a number of divisions it can undergo. The only antidote to this is an active telomerase, which can restore the telomeres to their original length.
The telomerase of the cnidarians, of which T. nutricula and Hydra are members, seems to be active throughout their lifespan. There are also some studies indicating that T. nutricula‘s neat trick of reverting back to its polyp stage might not only recapture its lost youth but also the telomerase activity, which is the real villain responsible for our senescence.
Ever since the role of telomerase was discovered, it seemed that restoring telomerase activity in adults would be an easy way to combating senescence (and even those of us who believe in benefits of a limited time span agree that biological aging itself is an unpleasant hassle, what with creaky joints and strange lumps). Unfortunately, telomerase activity in adults is also associated with cancer—after all, cancerous cells are truly immortal, since they have an active telomerase that enables them to divide without limits. Some of the oldest cell lines we have include human cancer cells that have outlived people they originated in. A good example of it is HeLa cell line, grown in culture since 1951 from biopsy cells obtained from Henrietta Lacks who died of cancer soon after. The descendants of her cells have been used to test Jonas Salk’s polio vaccine, as well and many cancer and AIDS drugs. They were also used in studies of human genome. The current estimate is that HeLa line has been used in 60,000 scientific studies.
It’s difficult not to envy the tiny T. nutricula then, with its ability to re-live its lost developmental stages and its active telomerase that doesn’t result in tumors. But cnidarians are not the only immortal organisms, or close to it. Lobsters and giant tortoises, while mortal, do not experience senescence as we know it. Many plants live staggeringly long—some bristlecone pines reach ages of 5,000 years—a good chunk of human civilization.
Additionally, many plants that reproduce asexually form clonal colonies (some fungi do something similar). While each individual member of the colony is mortal, the whole (in its singular genetic makeup) lives on, indefinitely. Since in many cases the members, called ramets, remain connected to each other by shared root system, even the boundaries of what constitutes an individual become elusive.
Could our future immortality lie in a shared consciousness then, rather than extending a life of a single individual? Whatever the case, I’m certain that no single organism, cnidarian or plant, holds a definitive answer. And if we ever manage to make ourselves immortal, I hope we’ll also manage to make ourselves smaller: even the little harmless T. nutricula is spreading all around the world. Imagine what immortal humans would do!
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