HANNA ROSIN, HOST:

Hey there. It's Hanna, and we want to hear from you. Please help us out by telling us what you like about the show and how we can improve by completing a short, anonymous survey at npr.org/invisibiliasurvey. It should take less than 10 minutes, and it is so helpful to us here at INVISIBILIA. That's npr.org/invisibiliasurvey - one word. Thank you.

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LULU MILLER, BYLINE: I was trying to tell a simple science story, one of those stories that shifts just a little bit how you think the world works. I thought we could all use a little distraction right now - something free of complication, of darkness. I failed.

Hello.

DANIEL MARTINEZ: Hello.

MILLER: How are you?

MARTINEZ: Hi. Good. How are you?

MILLER: I'm good.

This is Daniel Martinez, the scientist whose work led me down the rabbit hole in the first place. And the proverbial rabbit that led him down the hole was a tiny aquatic creature called the hydra.

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MARTINEZ: They live under rocks or leaves and in ponds and rivers, and they are beautiful. They have a cylindrical body with a head on one side with long tentacles, so they look like a sea anemone.

MILLER: Some of them are bright green, some of them are kind of clear, and they're about a quarter of an inch in length. And back in the 1980s, there was this really weird rumor going around about them.

STEVE AUSTAD: So there was a kind of a rumor that they might be immortal.

MILLER: This is Steve Austad, one of Daniel's colleagues in the biology world. Around that time, he heard the rumor, too. And he said the reason people thought it was the case was because hydra had...

AUSTAD: The incredible ability to regenerate.

MARTINEZ: You can grab a hydra. You can cut it in three pieces, and you get three hydra.

MILLER: Cut it into four pieces, four new hydra - 20 pieces, 20 new hydra.

AUSTAD: You can even treat them with chemicals that basically dissolve all the things that make their cells stick together. So you just make a pile of cells, and they will eventually reassemble into a hydra. So...

MILLER: What?

AUSTAD: It's a Terminator. Yeah. Isn't that bizarre?

MILLER: Yes.

And because of this ability, one of the first people to study hydra back in the 1700s...

AUSTAD: A Swiss biologist named Trembley - well, he assumed, then, that they were probably plants.

MILLER: But the closer he studied them in his microscope, he realized they did something very un-plantlike.

ROB STEELE: It could move. Hydra somersaults.

MILLER: This is Rob Steele, another hydra dude - last one, I promise - who says that, yes, hydra usually stay suctioned to leaves or rocks to stay safe. But...

STEELE: If things get really bad - it's been there for a week, hasn't gotten any food - it just detaches, and whoosh.

MILLER: And Trembley...

AUSTAD: Well, he saw that, and he said, aha. They must be an animal.

MILLER: A new animal that eventually got the name hydra after the monster of Greek mythology, a serpent with regenerating heads that was so hard to kill, Hercules himself could not slay it alone. And as more and more scientists observed this thing, the rumors began to build that the real animal was so good at regenerating, its cells so freakishly good at repairing, that it really might be...

STEELE: Immortal.

AUSTAD: Potentially immortal.

MILLER: And when that theory hit the ears of Daniel Martinez, he did what you are likely doing right now. He called bull.

MARTINEZ: Everybody eventually dies.

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ALIX SPIEGEL, HOST:

This is INVISIBILIA. I'm Alix Spiegel.

ROSIN: And I'm Hanna Rosin. And that was Lulu Miller, co-founder of this here operation. And today, we're handing the mic over to Lulu to tell the story of Daniel Martinez, the man who would spend years trying to slay the rumor of the hydra's immortality.

SPIEGEL: As Lulu warned, though it starts out as a science story, it veers into some thorny emotional territory and asks real questions about how to live and how to live well and how to protect yourself from the pain that usually comes with living. Stay with us.

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ROSIN: This is INVISIBILIA. Today, we have a story about a man who sets out to debunk what he thinks is an impossible claim. But along the way, he gets entangled with the question of how to live a good life. Here's Lulu.

MILLER: Yeah. So Daniel's whole battle with the hydra starts back in the 1980s, right as he lands in the U.S. to start a grad program at Stony Brook University.

MARTINEZ: I'm learning English. And I'm freezing my butt in New York. And I'm dealing with snow.

MILLER: He'd grown up in Argentina, and he left behind a life of warm weather, amazing food, long motorcycle rides all over the countryside in the name of finally starting his scientific career in earnest. And so when he heard this fantastical-sounding claim about the hydra, it bothered him.

MARTINEZ: So I set out to prove that hydra could not be immortal because I believed that that was not possible and that any animal - you know, like a good animal, hydra should die.

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MILLER: Now, to be clear, it is possible to kill a hydra. Though cutting it in half won't work, you could burn one or freeze one to death. You could asphyxiate it or starve it. It could get killed by a fish or a bacteria. But what the scientists were claiming was that if left to its own devices, a hydra would never die. Its body would never wear out. It would just keep going on and on and on forever.

MARTINEZ: No.

MILLER: And that was what Daniel wanted to debunk.

MARTINEZ: Animals should die.

MILLER: No one had ever tried the experiment before, partly because of how hard it is to keep hydra in the lab. Firstly, they're carnivores, but they will only eat live animals such as...

MARTINEZ: Brine shrimp.

MILLER: ...That Daniel would have to hatch in his lab and then hand feed to each animal. And secondly, they are barfivores (ph), which is my technical term for the fact that they regurgitate all their undigested food. So approximately five hours after every feeding, you have to change their water.

MARTINEZ: Because otherwise, the water is spoiled.

MILLER: And finally, they have the ability to reproduce asexually, which means they're always shooting out these little babies, these little clones that you also have to remove from the water.

AUSTAD: So following individual hydra is laborious.

MILLER: Steve Austad again.

AUSTAD: Because if - you can't just put them in a dish and come back and check every so often because you'll find gazillion hydra, and you won't know which one...

MILLER: Which is which.

AUSTAD: ...Was the original parent. So it's a lot of work. I certainly wouldn't have the patience.

MILLER: But Daniel? Well, a man needs a thesis, right?

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MARTINEZ: So anyway, so I start the experiment.

MILLER: He heads out to a pond on Long Island, rummages through the muck, unsuctioning (ph) hydra after hydra, brings them all back to his lab and then slowly begins the grueling process of feeding each one, changing its own little dish of barf water, throwing away its babies.

MARTINEZ: Every other day.

MILLER: By this point, Daniel's in his early 30s. He's swapped the bell bottoms of his 1970s youth for Gap jeans and these big sweaters (laughter). He gets an American girlfriend.

MARTINEZ: Maggie.

MAGGIE PARKINS: We met in a tango class, actually.

MILLER: They hit it off while dancing.

MARTINEZ: That was very romantic (laughter).

MILLER: Yeah.

MARTINEZ: She is a cellist. She's a cellist.

MILLER: Decided to go out for coffee, which turned into dinner, which turned into...

PARKINS: Hydra. It was, like, 11:00 or something. And he's, like, you know, this is weird, but I've got to go to the lab.

MILLER: So she goes with him, watches as he feeds and changes the water for over a hundred hydra. And he explains to her that, soon, this experiment will be over.

MARTINEZ: Everybody eventually dies.

MILLER: And then they return to their date - and more dates...

MARTINEZ: Concerts...

MILLER: ...Camping. They'd spend mornings reading poetry.

MARTINEZ: I'm trying to bribe you with uncertainty, with danger, with defeat.

MILLER: Aw.

Punctuated with sounds of insight...

MARTINEZ: So good.

MILLER: ...And laughter...

(LAUGHTER)

MILLER: ...And...

PARKINS: I've got to go to the lab (laughter). So that was a refrain because it had to be done.

MARTINEZ: Every other day.

MILLER: Months turned to years.

MARTINEZ: Feed them, wait, clean them.

MILLER: They get married.

MARTINEZ: In Martha's Vineyard.

PARKINS: It was under a tree.

MARTINEZ: Buying a house overlooking the ocean.

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MILLER: But they rush home pretty quickly because...

PARKINS: Got to go to the lab (laughter).

MILLER: Daniel graduates from Stony Brook. He gets a job at UC Irvine, drives a U-Haul all the way across the country - bringing, of course, the hydra.

MARTINEZ: In a cooler in the seat of a truck.

PARKINS: You know, they were like babies. Like, he had to really take care of those guys.

MILLER: Maggie gets pregnant.

PARKINS: Easy pregnancy.

MILLER: And all the while, nearly four years after he had started the whole thing, the hydra are still there - hungry, swaying their tentacles, alive.

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MILLER: At some point, Daniel looked at Maggie, puzzled, and said, hey...

PARKINS: This is really lasting a long time (laughter).

MILLER: Because while four years might not sound that long, there is this pattern in nature.

AUSTAD: It's true of mammals. It's true of birds. It's true of almost every group of animals we know.

MILLER: This is Steve Austad again, who explains that simply...

AUSTAD: Smaller ones are shorter lived, and bigger ones are longer lived.

MILLER: So little worms and bugs...

AUSTAD: They would live a matter of a few months.

MILLER: Badgers and turkeys...

AUSTAD: Fifteen, 20 years.

MILLER: With some whales living centuries.

AUSTAD: Yeah, right.

MILLER: And it's so robust, this pattern, that you can actually see it on a graph - this hard diagonal line that's almost like a guillotine that shows, based on your size, when you should die. And while there are a few factors that can budge that line, there's only so far that an animal can push off aging and dying - except, it seemed, the hydra. If you were to plot its lifespan on that line, you'd be way off the charts, like a human living to over a thousand years old without a single wrinkle.

AUSTAD: That's exactly right.

MILLER: Because, see, Daniel was reporting that the hydra were not aging at all. They weren't withering or growing weak. They weren't getting any worse at catching shrimp. There was no sign of any physical deterioration whatsoever in four long years.

AUSTAD: For something that's a quarter of an inch in length, it's absolutely shocking.

MILLER: As Steve explains, he is interested in the outliers. He studies creatures that live an ultra-long time, like clams, whose shells help them gain tons of extra years. But he says that even those creatures...

AUSTAD: They just age slowly. So we can make - we could make a heart that beat for 500 years. My 500-year-old clams have a beating heart, by the way. So there's a heart out there that's been beating since before Shakespeare was born. But...

MILLER: That heart is growing ever weaker. Its cells are slowly wearing out.

AUSTAD: Even the things that we used to think did not age, like bacteria - it turns out even bacteria age.

MILLER: And yet, there was Daniel saying that the hydra...

AUSTAD: They don't age at all.

MARTINEZ: So I was beginning to, you know, be convinced that there's something very unusual about this animal. And I was ready to share my findings with the world.

MILLER: And in the spring of 1998, shortly after their child was born, Daniel publishes a paper in experimental gerontology called "Mortality Patterns Suggest Lack Of Senescence In Hydra," which - well, let's just translate that for a sec. Mortality patterns - aka the ways in which it dies - suggests a lack of senescence - aka a lack of aging - aka the way it dies suggests a lack of aging-caused death. In no uncertain terms, Daniel was telling his colleagues that, when left to their own devices...

MARTINEZ: Hydra are immortal.

MILLER: And were you laughed out of science? I mean, how did this - how was this received? I mean, OK, because just coming to this as an outsider, like, I thought that things couldn't be immortal. I mean, I - this sounds, like, so insane to me. How did your colleagues receive it?

MARTINEZ: Well...

AUSTAD: I think few people believed him. I didn't...

MILLER: You didn't?

AUSTAD: ...For one.

MILLER: You didn't?

AUSTAD: I didn't.

MILLER: Steve Austad again.

AUSTAD: Because I didn't think - I couldn't understand any kind of evolutionary logic.

STEELE: Aging is inevitable.

MILLER: And that's Rob Steele again, hydra expert.

STEELE: I didn't really appreciate the impact initially.

MILLER: The paper just sort of disappeared into the din. And Daniel took those creatures that very few people believed were immortal and...

MARTINEZ: So I put them in alcohol.

MILLER: So you killed them? You killed them?

MARTINEZ: Yeah.

MILLER: You didn't wait to see if they would keep going?

MARTINEZ: Because, you know, after feeding an animal three times a week in their mouths with brine shrimp, you know, every other day for four years, you're ready to kill yourself or the animal. And I prefer...

MILLER: (Laughter).

MARTINEZ: ...To kill them. I just go and kill myself - I was not ready for that. So I killed them.

MILLER: And maybe it all would've stayed there, the truth of biological immortality hidden in a subterranean lab in Southern California. Except about seven years later, this prestigious institute in Germany, the Max Planck Society, reached out to Daniel and convinced him to run the study again in tandem with them. And as those hydra kept living under the watch of even more scientists past four years, past five years, six years, seven years, slowly, other scientists began saying...

AUSTAD: Whoa. This is real. This is real and really, really interesting.

STEELE: This is a game changer.

AUSTAD: This is a biologically very unique situation.

STEELE: Shows that it's possible there is an evolutionary outcome that can give you an animal that's immortal. It hasn't happened very often, but it's not - it doesn't disobey the laws of physics.

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MILLER: And it was right around then - 2011, 2012 - that the hydra started getting a lot of attention. The New York Times wrote about it. The BBC said it might contain a, quote, "universal anti-aging mechanism." And all kinds of scientists started trying to figure out how the hydra did what it did.

THOMAS BOSCH: Lulu.

MILLER: Hi. Hi, is this Dr. Bosch?

BOSCH: Yeah, that's me.

MILLER: This is Thomas Bosch, a zoologist at the University of Kiel in Germany, who figured that maybe the hydra was able to stay so plucky and fresh because its body is basically just a fountain of stem cells.

BOSCH: Self-renewing stem cells which continuously proliferate, and so they never stop.

MILLER: Until, that is, he and his grad student, Anna-Marie Bohm, messed with one of its genes, and suddenly, they were able to slow down that stem cell production.

BOSCH: So in essence, what we produced is aging hydra.

MILLER: And that gene, it's called the FOXO gene. And humans have it, too. And it turned out that very old humans - people who make it to 100 years or older - a lot of them have a mutation on their FOXO gene.

BOSCH: That was - wow.

MILLER: Because it meant that maybe this ancient gene that helps keep the hydra's body ever young could have real relevance for us.

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JAMES PEYER: It really is one of the rock stars in the field.

MILLER: Really?

PEYER: Yeah.

MILLER: This is James Peyer, the owner of a biopharma company.

PEYER: I like to say that we're partnering with scientists to build new medicines with the potential to ultimately be used to promote healthy longevity.

MILLER: James is just one in a shockingly vast sea of biotech and pharma companies trying to extend the human lifespan, and he says hydra really does loom over the industry as a reminder that a tweak in our biology could have profound consequences for a lot of the diseases that come with age.

PEYER: Hey; look. Would you take a pill that would give you or your loved ones a lower chance of getting Alzheimer's and a lower chance of having a heart attack or cancer as you approached 80 or 90? The answer there is almost always yes.

MILLER: So I called Daniel, high on the thought that a creature is not condemned to wear out, to ask him how his latest batch of hydra - now about 2,000 years old in human terms - were doing. Had they shown any signs of aging - a wrinkle, some withering? And he told me...

MARTINEZ: We have more than eight years of data with basically no mortality, and the hydras seem to be fine.

MILLER: You sound so unenthused about the result (laughter).

MARTINEZ: Well, it's just - OK. I guess I'm - I don't know. This probably shouldn't go on the air. I just - you know, I do the best I can. I do good science. I - but I, you know - this is just one finding. I mean, the world is a little more interesting than just science. So if we were talking about, you know, motorcycling or poetry, I would probably be a little more enthusiastic. But this is just...

MILLER: Are you having a career crisis?

MARTINEZ: No. It's just I'm not married to the hydra being immortal. If they are, fantastic. I was the one that say that in a paper, so that's great.

MILLER: But he told me he had no interest in studying their immortality any further.

MARTINEZ: Sorry.

MILLER: I had asked him to bring a hydra with him to the studio. And to conclude the interview, I just had him describe it for me.

MARTINEZ: I'm looking at this beautiful ball of cells (laughter) with little tentacles. And it's resting on its side, actually, in the bottom of this jar.

MILLER: So how long do you think this one will live?

MARTINEZ: I don't know - maybe hundreds of years.

MILLER: You think it could?

MARTINEZ: Yeah.

MILLER: Yeah. Do you envy it at all?

MARTINEZ: Not at all. No.

MILLER: And then...

OK.

MARTINEZ: OK.

MILLER: We hung up.

Bye.

MARTINEZ: Bye.

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MILLER: This conversation was recorded some years back. And as the time went by, I often wondered about Daniel Martinez, this guy who had glimpsed biological immortality and turned away. As my hair got more and more gray strands, as a friend got cancer and died, I wondered why Daniel had turned away from so much promise and what he was doing instead.

SPIEGEL: When we come back, Lulu heads out to California to figure it out.

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SPIEGEL: This is INVISIBILIA, and we are telling a story we're calling The Reluctant Immortalist. We pick up with Lulu at the wheel of a rental car about to descend on Daniel to try to figure out why he turned away from hydra's eternal youth and what he turned towards.

MILLER: Yeah.

MARTINEZ: You made it.

MILLER: Wow. I underestimated...

MARTINEZ: Hi. How are you?

MILLER: ...All this traffic. Oh, it's so great to meet you for real.

MARTINEZ: Nice to meet you, too.

MILLER: So it took me seven years to finally get out to Claremont, Calif., where Daniel Martinez lives. He is a biology professor at Pomona College. And the very first thing we did was to leave his office and head out to a reedy little pond...

So should I just go barefoot?

...To search for wild hydra.

MARTINEZ: And I got this leaf.

MILLER: Daniel told me that they live almost everywhere. There's likely one near you right now - immortality under our noses this whole time.

MARTINEZ: They're normally the underside of things.

MILLER: He started pulling up all these sunken reeds and leaves, running his finger along them...

OK. That's not...

MARTINEZ: No, that's a snail.

MILLER: ...Until...

MARTINEZ: OK. So here's a green hydra.

MILLER: ...He found one.

MARTINEZ: Oh, this is a beautiful hydra. Look at that.

MILLER: So, like, what's the daily life of a hydra out here? Like, what's their life like?

MARTINEZ: Well, they're probably - they are sitting on their leaves and just being immortal, waiting for nothing to happen forever (laughter).

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MILLER: I have talked to you a lot about science. But I think for right now, I want to just get to know you more, talk about life. (Laughter) You just rolled your eyes.

MARTINEZ: I need some alcohol.

MILLER: Over the next four days, I would grill Daniel about his life, both personal and professional. And there had been some pain, as there tends to be with most creatures that cannot somehow sidestep the damage that usually comes with living. His sister had passed away too young.

MARTINEZ: She just got lung cancer. And in a few months, she just die. It was very, very fast. Yeah.

MILLER: And after about ten years of marriage, he and Maggie had split.

MARTINEZ: Making the decision was very hard.

MILLER: They had two small kids by then, a son and a daughter.

MARTINEZ: It was more clear for me than for Maggie, but I think that we both knew.

MILLER: And it was around that time - 2004, the time of the breakup - that Daniel had first tried to turn away from immortality. But the Germans had pulled him back in with that second study. And when he was finally free of that experiment eight long years later - not because the hydra aged, but because they decided to publish - he decided to do something that one could argue showed just how little he cared for the pursuit of life extension.

What's this little...

MARTINEZ: So this is an off-road KTM 350.

MILLER: He started riding motorcycles again pretty seriously. He's now got four of them in his garage.

MARTINEZ: This is a BMW F800GS. This is a Kawasaki KLR650.

MILLER: While he had refused to ride at all when his kids were young, by that point...

MARTINEZ: Maggie had already remarried, so my kids will be fine if I die.

MILLER: Was that really the thought?

MARTINEZ: Yeah.

MILLER: You're just like, OK. They're up and running.

MARTINEZ: Yeah.

MILLER: If I die, that's OK.

MARTINEZ: Yeah. They'll be fine.

MILLER: He started doing rides out to Joshua Tree, through Death Valley.

MARTINEZ: It's just an amazing feeling of freedom.

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MILLER: And he couldn't get enough. He set off on a two-month-long ride with his brother down Argentina, up through the Andes and down these windy mountain roads with incredible vistas and sharp drop-offs.

MARTINEZ: And sand. And all of a sudden, you have a patch of sand. And the motorcycle starts to wave like crazy, so you really need to stand up.

MILLER: And on stretches like that, he said he was reminded that what he's doing...

MARTINEZ: Is super dangerous. Motorcycles are dangerous.

MILLER: Is the danger part of why you like it?

MARTINEZ: No.

MILLER: So why do you like it?

MARTINEZ: I just love landscapes and sunsets and the smells and the noises. It's just that thrill of discovering what's next.

MILLER: He said that's why he loves riding through mountains in particular.

MARTINEZ: You go through passes. And in every corner, you're wondering, what's beyond that? What's beyond that?

MILLER: And beyond that was shrines with red flags and hundreds of cacti. And...

(SOUNDBITE OF MUSIC)

MILLER: He had hit a rock, fallen from the motorcycle. And when he tried to stand up...

MARTINEZ: I tried to lift my leg like that. I immediately feel that my foot basically just hangs from my leg.

MILLER: Eventually, he makes it to a hospital, where he discovers he's broken both bones in his leg and he's going to need surgery and that he will not be able to board a plane home anytime soon because of risk of clotting.

MARTINEZ: I was so, so mad.

MILLER: He's sent home to his sister's house in the village where they grew up and ordered to remain on bed rest.

MARTINEZ: With my leg up.

MILLER: One month turns to two...

MARTINEZ: You know, in bed all day...

MILLER: ...Two to three. There are complications with the surgery, and he still can't leave his sister's house.

MARTINEZ: And I decided that I could not be old in that condition. I will kill myself if I had to live like that.

MILLER: And what was so bad? Like, can you just - what...

MARTINEZ: I cannot be not moving. I just - you kill me if you don't let me move.

(SOUNDBITE OF MUSIC)

MILLER: And did that incident make him suddenly curious again about how the hydra heals, how it repairs and stays young?

MARTINEZ: Not at all.

MILLER: Instead, as he slowly recovered and made it home, he just wanted to keep riding his motorcycle. And on one hand, he knew he probably shouldn't.

MARTINEZ: I love riding, but I know it is dangerous - just fear.

MILLER: But as he limped through his day...

MARTINEZ: (Non-English language spoken).

MILLER: ...This line of poetry kept getting louder in his head.

MARTINEZ: From the poem "Ewigkeit" by Borges. At the end, everything belongs to the worms.

MILLER: And that tipped his scales.

MARTINEZ: Dude, you're going to die. You'd better leave.

MILLER: He got back on the motorcycle. And in the years since, he's done two tours of Canada, another tour of Argentina. He's crossed the Andes on horseback, hiked nearly 2,000 miles of the Pacific rim trail solo.

MARTINEZ: I don't want to take my life for granted because maybe in a year, I won't be here. So I don't want to wait too long. I want to do it now.

MILLER: And when you ask him about maybe dialing back the risk for the sake of his kids, he is clear that he doesn't take unnecessary risk. He doesn't ride his motorcycles fast. But he said stopping going on adventures altogether...

MARTINEZ: That would not be something I want my kids to see - that I'm going to just stay and not take risks because, I mean, life is so amazing. And not to live it is just - I think that's insane.

MILLER: And with that, I figured he had just handed me the key - the true key to why Daniel stopped studying immortality. It seemed exposed in that line of poetry that had convinced him to keep writing.

MARTINEZ: At the end, everything belongs to the worms.

MILLER: Bear with me here. If he uses the thought of death to energize himself, to pack in as much as he can before it's over to make everything feel more urgent and sacred and dear, then wouldn't the thought of immortality have an equal and opposite effect? Wouldn't it flatten life? Wouldn't staring at a hydra all day, contemplating the possibility of a life with no end, weirdly drain the sweetness from it?

So just to think about it...

I tested my theory on Daniel, and he said flatly...

MARTINEZ: No.

MILLER: He said, sure, the thought of immortality is tiring.

MARTINEZ: It would be such a drag to live forever.

MILLER: But he said that wasn't why he had tired of studying it in the hydra at all.

MARTINEZ: It's just the thrill of discovering what's next.

MILLER: After figuring out that the hydra didn't age, he just wanted to move on to something new. Shortly after that trip, he was awarded a bunch of money from a foundation interested in immortality. But he says he used it to study other things, like how the genomes of different species of hydra differ. And these days, he is wildly passionate about collecting hydra from all over the globe, a pursuit that itself keeps him in motion. He said his favorite part of science is that he gets to just keep chasing the unknown.

MARTINEZ: There's probably something very, very primitive about that. As humans, we like to move. I mean, if you think of the Americas, I mean, generations walked from Asia all the way to Patagonia. And they were people walking.

MILLER: He says that's how he extends his life - by constantly moving on to the next thing by packing more into each day - by extending it not in length, but depth.

MARTINEZ: Why worry so much about living longer instead of living better? Ten more years of crap - it's not worth it.

MILLER: Which was a really interesting way to think about how to sneak more life into life, though I did start to wonder about its side effects, about those things left in the wake of his movement.

OK, I have two questions, and then I'll stop.

MARTINEZ: OK.

MILLER: OK. One is...

MARTINEZ: Can I suggest something?

MILLER: Yeah.

MARTINEZ: Can we go for lunch? This is already 12:30.

MILLER: Oh, yeah.

MARTINEZ: And then we can continue.

MILLER: Yeah.

Lunch - Nuno's, an upscale Portuguese place. Ramon (ph), the manager, greets us with a hug. We meet the owner and the chef, and Daniel whispers as an old man walks by that he used to be a priest. Oh, I say, that reminds me - I have read studies that say that priests and nuns and monks live longer, possibly because they don't have sex.

There's this theory that because reproduction takes a toll on the body - the production of gametes, et cetera - when a creature foregoes it, it may be a path toward longevity. So maybe that's why the hydra don't age - because they don't have sex. They do have sex, Daniel says and rolls his eyes. He explained that the hydra can switch to reproducing sexually, and when they do, they don't necessarily die.

We split a margherita pizza with extra Parmesan. Ramon made us an award-winning cocktail from fermented blackberries, which I absolutely swear I did not expense to NPR. I was gearing up to ask him my question, but I was also feeling a little woozy and thinking about how, in two days' time, I had to return to Chicago, where it was snowing. So when Daniel suggested a hike instead of an interview, I found myself unable to say no.

We drive up to Mount Baldy, by a lodge where I dreamed of taking my wife and wondered who would provide day care for our baby. And then we passed the sweetest mountain school, and I imagined taking the baby with us and just moving there, leaving it all and becoming an art teacher or slide builder. Are there people whose job it is to build slides? Yes, there are. Of course, there are. Capitalism really is fantastic.

As we drive further and further up the mountain, though, I slowly start to remember who I am and why I'm here. And that distant pulling of guilt, of responsibility grows stronger and stronger until about 20 minutes into the hike, I finally pull out my recording gear.

Can we pause for one sec? OK. I guess so then the last - I think the last question that I have with the movement thing is, like, that's what makes you feel alive. That's what, like, thrills you. Do you think about, like, can it hurt the people you're moving away from? And has that been a cost?

MARTINEZ: No.

MILLER: And when I asked him more explicitly...

Have you hurt people?

...If his movement had ever hurt people, he said, of course.

MARTINEZ: Oh, yeah. I'm sure I have.

MILLER: He'd actually been married before Maggie to an Argentinean woman who he'd left when he'd moved to the States.

MARTINEZ: It was clear after a very short time that we were not meant to be.

MILLER: Well, why?

MARTINEZ: I have no idea why. I don't remember exactly. But it was conflict, OK.

MILLER: He does remember that making the decision to leave...

MARTINEZ: It was hard.

MILLER: And it had been even harder with Maggie. But he ultimately decided he would cause less pain by leaving than by staying in a thing that wasn't working for him.

MARTINEZ: I think it's a mistake. Like, many couples stay together for the kids, right? I mean, the kids suffer. Their kids are not going to learn what love is because there's not going to be - if there is not a loving relationship in front of them, why will they learn? Where will they learn? So I think that you need to move on.

MILLER: He pointed to the rushing stream alongside the trail.

MARTINEZ: The theme of life is movement. I mean, rocks don't move, but animals and plants move. Bacteria move. I mean, one way of defining - if you think of animals, one way of defining an animal is just that - movement.

MILLER: He pointed out that movement, of course, had been the very quality that helped convince scientists hydra was one of us.

MARTINEZ: So I don't know. Movement, to me, is the essence of life.

MILLER: And, like, is it hard to be the partner of a person like that? Like, was that any part of what was hard in the relationship?

PARKINS: Yeah. We're getting in there now, aren't we? Yeah, I mean, you know, this is where it gets a little tricky. But, you know, that's - he's intense. You know, he's intense that way. And it's both beautiful and intense.

MILLER: Maggie and Daniel are still friends. They text about logistics with the kids all the time. They crack jokes. And she's wary of saying anything that could topple that. But she said the movement thing was definitely real.

PARKINS: Listen. I should have known that very first date. That's still that same first date from when we're going to the lab at 11:00. Remember, we went - decided, OK, well, let's make some dinner or something. And I remember at the supermarket, he just, like, charged forward. And I remember following him, thinking, why aren't we walking - you know, like, I remember chasing after him in the grocery store. You know, like, he's just one of those people - boom. That should have been a warning bell.

MILLER: She said she spent a lot of time in the relationship looking at his back.

PARKINS: I remember kind of trying to catch up.

MILLER: And not just physically.

PARKINS: Like, his brain just moves probably faster than my brain, too. So I felt like I think I followed - I followed Daniel.

MILLER: And she said that when he told her he thought the marriage was over, in the moment...

PARKINS: It was devastating.

MILLER: But over time, she's come to think about his movement differently.

PARKINS: Now, I mean, I'm a Jew from Detroit. You know, I mean, we didn't do - I mean, our vacations were reading on the beach, OK. And I'm - here I am with this guy. I've been to Argentina 10 times. I've backpacked in the Sierras.

MILLER: His movement has taken her to so many places.

PARKINS: I mean, I've seen so many amazing things.

MILLER: His movement was what got her to California, where she now has a world of friends, new musical collaborations and a new husband. He's a violinist, and she says life with him...

PARKINS: It's a completely different kind of tempo.

MILLER: A lot slower.

PARKINS: You know, it's more kind of a duet.

MILLER: In fact, the two of them recently formed a duo - him on violin, her on cello - and these whirls and chortles that Maggie explains come from birds.

(SOUNDBITE OF THE SMUDGES' "SONG AND CALL")

PARKINS: You can slow them down and just hear these - how beautiful the melodies are...

(SOUNDBITE OF THE SMUDGES' "SONG AND CALL")

PARKINS: ...Which you can't really tell when they go really fast.

MILLER: As Daniel ages, he only wants to move faster. Just a few blocks away from Maggie's house, he's busy making repairs on his motorcycle, doing training hikes, readying himself to walk new stretches of the PCT, to motorcycle further.

MARTINEZ: Just - you kill me if you don't let me move.

PARKINS: Yeah. I think that's where we - I think that's where we're kind of different in that way.

MILLER: As she's grown older, she's discovered the pleasure of stillness.

PARKINS: Just being able to enjoy something and take it in and maybe not move and just keep enjoying it. You know, it's kind of nice to just stay with something a little bit longer, you know, and come back to it.

(SOUNDBITE OF MUSIC)

MILLER: Evolution is impartial. It doesn't care what adaptation you employ, be it song or slime, speed or stillness that helps you endure. As for the hydra, the best guess scientists have these days as to how it cheats death, as Rob Steele explains, isn't just its crazy stem cell production but its highly unusual ability to let go.

STEELE: Hydra's constantly just sort of pushing stuff out.

MILLER: Each and every cell that makes up the hydra's body, Rob estimates, is pushed out within a matter of a few weeks.

STEELE: Whereas we aren't so good at getting rid of cells. You know, our brain cells are pretty much with us for life, so if we get a brain tumor, we can't get lucky and have it sort of ooze out our ear and disappear. It's there, and eventually, the brain's going to have to deal with the consequences.

MILLER: So maybe Daniel, for all his professed disinterest in the hydra's immortality, is the one who had known, who had been practicing its secret all along. Time and time again, when I would ask him about hard memories - exes or break-ups or deaths - he would say...

MARTINEZ: I don't remember the details.

MILLER: I assumed that he was dodging.

Really?

MARTINEZ: Yeah. I don't.

MILLER: But maybe he wasn't. Maybe, like the hydra, he, too, is constantly shedding the things that could hurt him in the name of staying youthful and free from pain. He is 61 years old, and I cannot keep up with him on the mountain.

(SOUNDBITE OF MUSIC)

ROSIN: That's Lulu Miller. Lulu has just come out with a book. It's an excellent nonfiction adventure story about a mad scientist and a little about herself. It's called "Why Fish Don't Exist." It is a wonderful read. Go out and buy it.

SPIEGEL: Stick around for a sneak peek of next week's INVISIBILIA.

Hey there, everybody. It's Alix coming to you from the closet of my home. Because we want to hear from you, we have a survey that we would love people to fill out. It will help us to understand what you like and how we could improve. It will take less than 10 minutes. It's at npr.org/invisibiliasurvey. That's one word. That's npr.org/invisibiliasurvey. We'd love to hear from you. And stay safe. Thanks.

ROSIN: Next week on INVISIBILIA from NPR, the power of sound.

(SOUNDBITE OF MUSIC)

ROSIN: We've got the story of a man whose whole life was about making noise until one day, he heard a sound so beautiful that he decided to spend the rest of his life listening. And he heard something few people have ever heard before.

(SOUNDBITE OF MUSIC)

UNIDENTIFIED PERSON: It occurred to me in my half-sleep state that what I was hearing was kind of like an orchestration, like music.

ROSIN: Plus, we consider how to listen now in a world that's suddenly so strange and different. That's next week on NPR's INVISIBILIA. Don't forget your headphones.

(SOUNDBITE OF MUSIC)

SPIEGEL: INVISIBILIA is hosted by me, Alix Spiegel.

ROSIN: And me, Hanna Rosin.

SPIEGEL: Our senior supervising editor is Deborah George. This episode was produced by Lulu Miller, Abby Wendle and Kia Miakka Natisse. INVISIBILIA is also produced by Yowei Shaw. Our manager is Liana Simstrom.

ROSIN: We had help on this episode from Pranab Bhaskar (ph) and David Gutherz. Special thanks to Heidi Tissenbaum, Gerry Holmes, Micah Ratner, Emily Bogle, N'Jeri Eaton, Jon Hamilton, Johanna Weber, Sophie Ista (ph), Francesca Minerva, Celina Juliano and Matt Kaeberlein.

SPIEGEL: Fact-checking by William Brennan (ph) and Ayda Pourasad. Our technical director is Andy Huether, and our senior vice president of programming is Anya Grundmann.

ROSIN: Music for this episode provided by Timber Timbre.

SPIEGEL: Maggie's song with the birds is called "Call And Song" (ph) by her duo, The Smudges. Additional music from Blue Dot Sessions.

ROSIN: To see an original illustration for this episode by Leonardo Santamaria, visit npr.org/invisibilia. We'll be back next week with a new episode. In the meantime, stay safe out there. Stay connected, and do whatever it takes to keep yourself going day by day, you wonderful and weird living beings.

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