字幕表 動画を再生する 英語字幕をプリント To us humans, aging seems inevitable - probably because no human has ever not aged. But getting older isn’t as universal a fact of life as we might think. Take the naked mole rat. Unlike their less-naked brethren, they don’t appear to age after reaching adulthood - years pass, but the rats don’t get weaker, more susceptible to disease, or wrinkly...at least not any more wrinkly...and they keep on making as many babies as ever. Surprisingly, they aren’t any more likely to die in old age than when they’re young adults. It’s as if they’ve found the fountain of youth, though perhaps not the fountain of beauty. And naked mole rats aren’t alone in not aging: rockfish, lobsters, and bristlecone pines also seem to stay forever young...or at least forever middle-aged. We’re not exactly sure how these species do it, but their anti-aging secret may have to do with their ability to rebuild the DNA caps on their chromosomes. These caps, called telomeres, are one line of defense against aging in many species. That’s because cells need to divide to replace old or dysfunctional cells, but each time they replicate, they lose a little bit of DNA from the end of each chromosome. Normally that doesn’t matter, since these lost bits come from the telomere end caps that don’t encode important information. But after many replications, the telomeres get trimmed so short that the cells can’t afford to lose any more DNA, and they stop replicating. Age-defying species like naked mole rats, however, pump out high levels of a telomere-rebuilding enzyme that enables them to keep on replacing old and dysfunctional cells indefinitely. A few kinds of human cells make this enzyme, but the vast majority don’t. And even if we could trick the rest into producing it, then we’d have another problem: more replications means more chances for mutations that could turn a cell cancerous unlimited replication increases a cell’s chances of becoming cancerous. Naked mole rats don’t care because they seem to be immune to cancer; but we humans certainly aren’t. However, as amazing as naked mole rats are, they can only pause their aging – the tiny jellyfish Turritopsis dohrnii can age in reverse. Like butterflies, Turritopsis morph through multiple stages during their life cycle. But unlike butterflies, if Turritopsis get wounded or if times get tough, they can morph backwards, reverting to their immature polyp form until conditions improve. They’re like, real life phoenixes. However if humans could somehow imitate Turritopsis’ trick, it might not give us the kind of eternal youth we’d be looking for… for one thing, melting into an amorphous blob where our cells are reorganized and reprogrammed with new functions would not only be a mess; it would likely turn our brain cells into skin or muscle cells - and vice versa - erasing our memories and our sense of self. And no matter what, eternal youth wouldn’t make us invincible - in fact, the longer a creature lives, the more time it has to get chomped, starved, or smooshed. So eventually, every naked mole rat, pine tree, and jellyfish will ultimately meet its end - because it’s possible to be immune to aging, but not to death.