Photo: Sean Gallup (Getty Images)
Wiki WormholeWe explore some of Wikipedia’s oddities in our 5,664,405-week series, Wiki Wormhole.  

This week’s entry: Negligible senescence

What it’s about: Everyone knows that aging and death are inevitable for all living things. What this article presupposes is… maybe it isn’t? A handful of organisms display negligible senescence, meaning they don’t show the symptoms of aging—neither slowing, nor losing fertility, nor becoming more likely to die as the years pass.

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Biggest controversy: It’s very hard to prove a negative. There may be cases where we think a creature doesn’t age, simply because we haven’t seen it happen yet. Turtles were thought to be negligibly senescent, but biologists eventually observed “decreasing fitness with age.” So it’s possible some forms of life mentioned here have aged without showing any outward signs, like Sigourney Weaver.

Strangest fact: Some organisms actually age less as time passes. The phenomenon is called negative senescence, or “late-life mortality deceleration.” This doesn’t actually mean anything ages backwards, but that the likelihood of death decreases with time instead of increases as usual. One cause for skepticism: The phenomenon is discussed in depth, but no type of living thing is mentioned specifically.

Thing we were happiest to learn: Some things take not-aging one step further. A very few organisms are considered biologically immortal. This doesn’t mean they can’t be killed through injury, starvation, etc., but simply that they do not die of old age. Hydras—small freshwater animals about a centimeter long—reach maturity in only five to 10 days, reproduce asexually, and do not seem to age whatsoever. Their stem cells “have a capacity for infinite self-renewal,” and as such, they can regenerate and stay young indefinitely.

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One type of jellyfish, Turritopsis dohrnii, replenishes its cells after mating, rendering it biologically immortal, as do planarian flatworms, whether they reproduce sexually or asexually.

Turritopsis medusa
Photo: Stefano Piraino/Barcroft Media (Getty Images)

Thing we were unhappiest to learn: A few animals have to go through that awkward age more than once. When the Turritopsis dohrnii, or “immortal jellyfish,” reaches sexual maturity, it reverts to larval stage, going back and forth between these two stages indefinitely.

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Also noteworthy: Back to biological immortality for a minute. Lobsters are very nearly biologically immortal—they don’t seem to get slower or weaker with age, and may actually become more fertile. But as they age, they molt, growing ever-larger shells. This is an exhausting process, and eventually the expenditure of energy kills the animal.

But how they stay young in the meantime is especially interesting. Scientists believe lobsters produce an enzyme called telomerase, which is present in most embryonic vertebrates, but almost never in adult animals. When cells divide, each strand of DNA splits in two and recombines in the new cell. Every time this happens, the ends of each chromosome—called telomeres—fray. Eventually, this fraying reaches the part of the chromosome that carries genetic information. Essentially, every time a cell divides, the DNA held within is a copy of a copy of a copy, and eventually it becomes unreadable. The result is the body gradually breaking down, in a pattern we call aging.

Except lobsters don’t experience this. It’s theorized that, if we could use telomerase or some other method to stop human telomeres from fraying, we too could become biologically immortal. But rest assured, that probably won’t happen until long after you’ve grown old and died, so you’ve still got that to look forward to.

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Best link to elsewhere on Wikipedia: One of earth’s most fascinating creatures is the tardigrade, a tiny and extremely hardy animal that’s able to survive everywhere from Antarctica to the depths of the ocean to volcanic mud. It can also survive 1,000 times more radiation than humans and pressure extremes ranging from 1,200 atmospheres to a vacuum, and is the only known animal to have survived in outer space. While the tardigrade is not considered to have negligible senescence—it tends to have a lifespan of between three months and two years, depending on which species—and age under normal circumstances, it has the rare ability to enter a state of suspended animation. The animals can almost completely dehydrate, and then rehydrate years later, springing back to life.

Further down the Wormhole: While there are animals that don’t age and live for remarkably long times, in the end no one can escape the grasp of death. The Onion’s Man Of The Millennium comes for us all, but sometimes rumors of people’s deaths are greatly exaggerated. We’ll look at a list of premature obituaries next week.