Intraterrestrials
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Imagine that you are a single-celled organism. You live on the ocean floor, perched on the mud there. And far, far above you, there might be plankton living and dying or animals eating that plankton. But deep, deep, deep down where you are in the ocean mud, the pace of things slows. As Karen Lloyd, a microbiologist, puts it, you're basically just hanging out as traces of life drift down around you.

It's the slow and steady rain of the leftovers of what everything else in the ocean has eaten and pooped and died, settling very, very slowly over your head. You have a head, which you don't because you're a microbe. You're essentially being buried alive under layer after layer of dust and poop and tiny dead things sinking deeper and deeper into the earth as more mud builds up above you. Nothing changes. You're just...

getting pushed down and very little happens to you. It's like suspended animation for a very, very long time. Karen calls microbes like these "intra-terrestrials," which is an appropriate name because it means "inside the Earth."

but also because it sounds so much like extraterrestrials. And these microbes are kind of life form from outer space levels of weird. They are alien to us in the like real sense of the word. Alien as in different. Like as researchers have started pulling up samples of these cells from deep mud, they've discovered that one, there are a lot of them. Like one estimate suggests there are more of these ocean mud microbes than there are stars in the sky. But two,

These microbes don't look like other life that we've encountered before. Karen says these lifeforms started evolving away from the life that we're more familiar with a long time ago, like maybe even billions of years ago. And as a result, some of these microbes are so genetically different from all the rest of life on Earth that scientists have had to create new branches on the tree of life to put them on.

And maybe most alien of all, as researchers have poked and prodded these guys and taken measurements, they keep coming across things that don't seem to make sense. Like things that break some fundamental assumptions researchers have had about what life might need in order to stay alive. It's like, oh my goodness, life can be anything. So, this is Unexplainable. I'm Braid Pinkerton.

And today on the show, these alien cells are like riddles that Karen and her colleagues are trying to solve. And to solve them, they kind of have to stretch their imaginations to the edges of life as we know it. So what makes these cells so puzzling to scientists like Karen? Well,

Life is notoriously hard to pin down. Scientists and philosophers have debated where life's boundaries lie, sort of what does or does not count as alive. And life just does not come with a clear and simple rulebook. Over time, scientists have developed some assumptions about what they can reasonably expect when they look at a life form. For example, there is an assumption that organisms need energy in order to survive.

And specifically, Karen says, biologists have some rough ideas about how much energy, either from food or some other source, an organism might need. But when researchers studied these cells from deep in the ocean mud and looked at their energy consumption...

They were confused. Because remember, these cells have been buried alive. Nothing around them changes very much after that. Which means, as Karen puts it, that a microbe like this probably only has whatever food fell around it when it was first buried. You know, it's kind of like if somebody gave you a pizza to eat for lunch and then they were like, oh, wait, this is all the food you're ever going to get for the rest of your life. Make it last.

Researchers can see that these microbes do make it last, right? They're still alive. But they've also done the math to basically figure out how much energy these microbes seem to be consuming. And the numbers come out to be things that are very unreasonable for biology. Karen says these microbes are living on thousands of times less energy than even the most super low energy organisms that scientists have studied in labs before.

It'd be like if you learned that your neighbor was living on two sticks of celery a week and nothing else. How does something live like that? It seems like they live like that by essentially hibernating. It's like they're little single-celled sleeping beauties slumbering away in the mud in a kind of stasis. And it looks like they use the small amount of energy that they do have to

More for fixing parts of themselves as they break down than for growing or moving or really doing much of anything, including reproducing. But that runs up against another pretty fundamental assumption that scientists have about life, that it, in fact, reproduces to make more of itself in some way.

In the case of a single-celled organism, that's often asexual reproduction, reproduction without sex. So a parent divides itself into, say, two daughter cells. But as you might imagine, it takes a fair amount of energy to split yourself in two. And that's energy that these deep ocean cells do not have. The energy that it would take to make a whole new daughter cell, you know, to do that process, they...

Just, they can't. I mean, it's barely enough even to survive. Not reproducing is profoundly weird. So researchers have kind of tried to check their work here, and they have found evidence that really does seem to suggest that these cells are not reproducing very much in this mud.

But if this is true, some of the mud that researchers like Karen pull up, it can go back hundreds of thousands or even millions of years. And there are still microbes, even in that million-year-old mud, living. So if these microbes are basically just eking out an existence in this mud, kind of hibernating, not reproducing, not making new microbes, then if Karen is looking at a

The only conclusion we can make from that is that that is the same cell that was laid down at the surface like a million years ago, which is kind of crazy. So the cell is a million years old? Yes! And the crazy thing about—I know, it almost like—it's crazy to say, but that is the safe conclusion. That's the conservative conclusion from our data. That would make these microbes some of the oldest living things we know about.

But that kind of lifespan feels like it should belong in geology, not biology. It's life at the speed of rocks. When Karen looks at a cell like this in the lab, she could be looking at a microbe that began its life a million years ago, before the evolution of modern humans. She might even be looking at a cell from 100 million years ago, because some of the oldest cells in deep-sea mud might be that old. 100 million years ago, dinosaurs still walked the Earth.

It feels absurd. But to Karen, it's the best way to explain what she's seeing when she studies these weird microbes. Except this explanation gets us into a little bit of a weird place. If these microbes are going for millions of years without reproducing, then there's another big biological assumption that they run up against. A big problem.

Biological theory, you might even say. Yeah, it sort of seems like it gets in a little bit of trouble with Charles Darwin. Because if evolution requires reproduction, and these things don't reproduce, then how did they evolve?

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All the time I need and all the time I want. Let's talk through this last big puzzle that these microbes create for researchers, right? It comes down to a fundamental idea in biology. If you have a single-celled organism, right, and it's reproducing, splitting into new cells, copying itself, sometimes that copying goes wrong. A mutation crops up.

If that mutation helps the cell survive and thrive, if it's an adaptive mutation, then the cell with that mutation is more likely to reproduce again, and then it'll pass its adaptive mutation on to another generation.

That's natural selection, right? Mutations get selected for through reproduction. And us sexual reproducers, we add some fun twists to natural selection. But as far as we know, it is the basic driver of evolution. This is very, very well known. We see no exceptions from it in all of biology. So when Karen and her colleagues found lots and lots of these microbes that seem to live for millions of years not doing much at all...

They figure that they must have evolved to be the way they are. But how do you evolve to do nothing for a million years? It seems like a terrible strategy. You know, the rest of us are trying to make babies as fast as we can to, like, get our genes in the next generation. And these guys are like, nah, I'm good for, like, an ice age or two. It feels like an impossible riddle, almost.

But Karen does think that it is possible to imagine a solution here, a way for these microbes to evolve and also a reason for them to be the way that they are. To solve this riddle, she says we kind of have to step outside of our limited human box and really try to picture what it means to live for millions of years. Think of what if a human lifespan was only a day?

You know, if you just like are born at midnight and you like rebel from your parents at like 5 a.m. and like get a job and have kids and like maybe you retire at like 6 p.m. And by midnight, you're, you know, giving your sweet send off, having lived a long, beautiful 24 hour life. I mean, it's crazy to think about, but just do it.

Now imagine that you're born in upstate New York on December 1st. That means your children and your children's children and your children's children's children's children's children's children's children's children's children will also all be born in December. You're going to have hundreds of generations in winter. And every book that's ever been written in human history would happen within winter.

And as these humans, who only live for 24 hours, as they're going about their lives, maybe they notice deciduous trees, right? The trees that lose their leaves in the winter. People would be talking about trees like they were dead and doing nothing. And what even is the point of trees? Why are they even alive? How can you evolve to do nothing for literally ever?

You and I, people whose lifespans are more than 24 hours long, we know what trees are waiting for, right? We know that spring will eventually come. But for these people with their short lifespans, it's a mystery.

Karen says that these microbes might be to us what trees are to the 24-hour people. They're waiting for something that we're not aware of because our lifespans are too short to really see it. They might be waiting for a spring that we will never live to see. We can, though, try to imagine what that spring might look like. And Karen has. Let's go back to our microbe living at the bottom of the ocean.

To my eyes, that microbe is utterly still, right? It is slowly being buried under layers of ocean mud and going nowhere. But technically, Karen says, it is going somewhere. Because the mud that it's sitting in, that mud is on top of a tectonic plate, one of the many massive rock plates that cover the globe. They're slowly, slowly moving against each other, moving into each other, away from each other, basically at the

rate of growth of a fingernail, say. Some of the tectonic plates under the ocean, some of those are moving very, very slowly over towards the tectonic plates that hold continents. And when they hit those continental plates, the oceanic plates basically get shoved down under them. As that oceanic plate, remember it's covered in marine sediments, as that gets shoved underneath the continent, sometimes those sediments sort of bunch up and mix around.

So if you're a microbe at the bottom of the ocean sitting on an ocean plate, in my lifetime, you're not going to go very far. In my children's children's children's lifetime, you're not going to go all that far. But in your lifetime, if it lasts millions of years, you could conceivably creep along with that plate, waiting and waiting and hibernating like a tree in winter until the moment when the plate finally goes through this bunching up sediment upheaval situation.

That could be spring. Spring could be getting bunched up at the edge of a continent. Karen thinks that this would be a spring-like situation because if the sediments do get bunched up, it could potentially give these microbes new sources of energy. So there might be new food from the continental plate that gets mixed in with the old mud or...

Somehow the bunching up process might create new food that these microbes could eat, or maybe just the old mud gets mixed in with fresh mud and they have access to new food that way.

It still might not even be a ton of food overall, but these microbes don't need much. Their heaven, their absolute dancing in the streets, I have so much energy, is still not a lot. Karen thinks this springtime bonanza could maybe shock these microbes out of hibernation. It could be enough of a boost of new energy to make it possible for a microbe to finally reproduce.

Maybe it makes enough babies that some of them get sort of picked up in the water and get thrown out to sea and they settle and another cycle goes again. If this is right, then this is where the natural selection comes in. Because not every microbe is going to survive this long winter wait, right? In the beginning, when they first fall to the ocean mud, there might be lots of microbes. But as more and more time passes and food runs low, some of the microbes, the microbes that can't hack it,

Maybe those microbes die off. So only the hardiest microbes, the ones that are best at hibernating, only they make it through the winter millennia and emerge in spring to bloom and reproduce. And so there's an evolutionary advantage to being able to make do with very little and being able to wait for a long time for it to get slightly better.

That evolutionary advantage could explain how we got so many of these weird interterrestrial ocean mud microbes. Again, potentially more of them than there are stars in the sky. If something like this is happening, then slowly, over millions and millions and millions of years, these would be the microbes that won out evolutionarily. They'd still be going through natural selection, just at a much slower pace than we are used to.

So this is Karen's solution to the riddle of these interterrestrials. And she's the first to admit that it could be wrong. But she says there does have to be an answer out there. We know for a fact that these organisms are there.

And we know that nothing has happened to them for this long, and we know how much energy they have, and we know that they are on these weird deep branches on the tree of life that we never knew existed before we started taking samples like this. And so this scenario sounds crazy, but it is reasonable. Karen's still working on ways to test her hypothesis, but ultimately, I'm less interested in whether or not she's come up with the exact correct solution that explains these microbes, right?

I'm more excited by just the riddle of these microbes itself. Because when I think about why it would be exciting to find alien life out somewhere in the universe, to me, it'd be because that alien life would probably look so radically different from us, right? Maybe the aliens would speak in light or, I don't know, breathe microwaves. Stuff that pushes our understanding of what life could look like.

But these tiny microbes living in ocean mud, they do the same thing. Like they force microbiologists to reconsider the very basics of what we think we know to say, wait, is that true? Like, should we reevaluate this?

I think that, like, everything I had always learned about biology was functioning within this narrow confines. And now I discover that that's not true. Like, life was not confined to this narrow subset of possibilities. That's just deeply exciting to me. Like, if microbiologists can poke around in some mud and discover that there are trillions upon trillions of little weirdos in there that we've just sort of failed to notice...

What else might be hiding in the microscopic world? Living such quiet and slow lives that we just haven't spotted it yet. You could have, you could imagine that there's a shadow world of organisms that are living right next to fast-growing organisms and they just don't interact much.

If you have the tools to look, we could be living on a planet that's simply crawling with tiny aliens, each with their own riddles to solve. It's almost like, it's like I get a whole new Earth to play with and to think about.

Karen has actually already encountered a bunch of different weird microbes in her career besides just these odd hibernators. So if you want to read more about these hibernators or about the other microbes that Karen and her fellow microbiologists have studied, please check out Karen's upcoming book. It's called Intraterrestrials and it's available for pre-order.

This episode was produced by me, Bird Pinkerton. It was edited by Meredith Hodnot, who also runs the show. Noam Hassenfeld made the music for this episode, and Christian Ayala did the mixing and the sound design. Melissa Hirsch checked the facts. Julia Longoria is the fact that some frogs can use their eyes to help them swallow. And we are always, always, always grateful to Brian Resnick for co-creating the show.

Do you have thoughts about microbes or ideas for the show? Please tell us. You can write in to unexplainable at vox.com. I really love reading your emails and they make my day. It also makes my day when people support the show and help us keep making it. So if you'd like to do that, please join our membership program. That's at vox.com slash members.

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