A puzzlement.

Why, I wonder, are both these things true? There is an animal, a wee little thing, the size of a poppy seed, that lives in lakes and rivers and eats whatever flows through it; it's called a gastrotrich. It has an extremely short life.

Hello, Goodbye, I'm Dead

It hatches. Three days later, it's all grown up, with a fully adult body "complete with a mouth, a gut, sensory organs and a brain," says science writer Carl Zimmer. In 72 hours it's ready to make babies, and as soon as it does, it begins to shrivel, crumple ... and usually within a week, it's gone. Dead of old age.

Sad, no? A seven-day life. But now comes the weird part. There's another very small animal (a little bigger than a gastrotrich) that also lives in freshwater ponds and lakes, also matures very quickly, also reproduces within three or four days. But, oh, my God, this one has a totally different life span (and when I say totally, I mean it's radically, wildly, unfathomably different) from a gastrotrich.

It's a hydra. And what it does — or rather, what it doesn't do — is worthy of a motion picture.

So we made one. Well, a little one. With my NPR colleague, science reporter Adam Cole, we're going to show you what science has learned about the hydra. Adam drew it, animated it, scored it, edited it. My only contribution was writing it with him, but what you are about to see is as close as science gets to a miracle.

I have so many questions. Why the hydra? If nonsenescence, or biological immortality, is an option in nature, how come this particular mini-bit of pond scum got the big prize? Why not the gastrotrich? Why not (excuse me for asking) ... us? Evolution is such a random, casinolike affair; it's startling to learn that longevity varies without regard to size. I know Daniel Martinez's hydras were cleaned, fed, protected. They didn't live in the wild. But still, they have lasted and lasted and lasted. I expect sequoias, redwoods and whales to last longer than mayflies, daisies and clover. Big things go on, but when two little freshwater animals have such drastically different life expectancies, all I can think is ... whoa! Life is a puzzlement.

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