Origins: The meaning of “life”
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So far, we've been talking about how life on Earth started, that first somehow came water. And then in that water, the right ingredients mixed and formed some very basic living thing. From that point on, life has just been thriving. You know, there are swoopy flyers, microscopic squigglies, giant earth-shaking elephants. Life is everywhere. But it turns out there's a question we've kind of taken for granted.

We've talked about what it might take to make a very basic living cell, but that doesn't mean we know what life fundamentally is. No one has been able to define life, and some people will tell you it's not possible to. This is science writer Carl Zimmer. It may simply be that we are...

trying to put life in a box when it's not the sort of thing that you can fit into boxes. This feels like it should be easy. Like a homework question for a little kid. Dogs. Alive.

Creepy crawlies. Alive. Rocks. Not alive. It does feel like it should be easy because we feel it. You know, our brains are actually tuned to recognizing things like biological motion. We're hardwired for recognizing life, but that doesn't actually mean that we know what it is. Carl really wanted to know why.

Why can't we define life? Like, imagine, like, astronomers not agreeing on the definition of a star. But this is even more fundamental. It's his life. And it kept sort of gnawing on me. And then I realized that before I knew it, I got pulled into this vortex. And it's been fascinating. That vortex, it became a book. It's called Life's Edge, The Search for What It Means to Be Alive. And that's what we're going to talk about this week. What is life?

Why has this simple question been so hard to answer? What's so cool about this question is that it really forces us to take a look at the world around us. Because for every definition of life, there's a creature that just sends us right back to the drawing board. Every marvelous living creature on our Earth is built of complex living cells. Life is made up of atoms and molecules and chemical reactions. But what makes them alive?

So are there some examples of definitions of life that are close or useful or, you know, anything? Well, there are hundreds, hundreds of definitions of life that scientists themselves have published. And I have a list here. So, um,

just to give you a few. Life is an expected collectively self-organized property of catalytic polymers. Life is a metabolic network. Life is a new quality brought upon in organic chemistry. Life is the process of existence of open. Life is a monophyletic clade that originated with a last common universal ancestor and includes all its descendants. I feel like each of those definitions could probably use about 10 minutes of unpacking. Do we have anything kind of simpler? So...

Perhaps the best known is sometimes called the NASA definition of life. In 1992, NASA brought together some scientists to make some plans about how the agency should go look for life on other planets. It kind of occurred to them, well, maybe we should actually have a definition of what this is that we're going to help NASA go find. Yeah, it can't just be a gut feeling when you're NASA. Right. They said that life is a self-sustaining process.

chemical system capable of Darwinian evolution. That's it. All right. That, you know, that sounds fine to me. What's, what's wrong with it? I think part of the problem is there are lots of kind of edge cases where things get really hard. And so then people start arguing about who gets to be in the club.

Okay, I want to hear about a few of these edge cases. I know one of the most famous ones are viruses. Can you explain why viruses have been just so confounding? Are they alive? Are they not alive? So in some ways, viruses just seem incredibly alive. I mean, we're talking during a pandemic. There are who knows how many copies of SARS-CoV-2 that have been produced online

over the past few years through this process of reproduction. Not only that, but those viruses mutate, and some of those mutations make them better at certain jobs. It's made of genes. It's made of protein. I mean, what more do you want? I mean, it seems alive to me, right? Yeah, it seems alive. But you might say no, because...

If what's really important to you is, for example, metabolism, you know, eating stuff, well, viruses don't do it. Viruses don't have any way of taking in molecules and fashioning those molecules by themselves into new molecules. You know, they don't have a mouth. They don't have a stomach. They don't have enzymes. They don't have any of that.

All they have are basically instructions that reprogram a cell, and that cell, not the virus, makes new viruses. That feels pretty alive-y. Are they missing something else other than not being able to eat stuff?

Well, I mean, homeostasis, that's another really important thing about life that some people really think is important, that it's stable, that it can withstand all the chaos outside of itself. That's what we do, you know, keep our body temperatures very steady. You know, viruses can't do that either because, again, they don't have all of that molecular equipment for keeping themselves stable. So somebody actually asked one of the people who...

came up with the NASA definition of life. Well, what about viruses? And he said, no, not according to this definition, they're not. So really, depending on who you talk to in the scientific community,

you'll get a different answer to that question. You know, if you're out with a bunch of, like, virologists or other molecular biologists, and you really want to rile them up and get a good argument going... Always looking to do this. There you go. Our virus is alive. And then just step back and let the fireworks begin. You know, you mentioned that NASA definition, life is a self-sustaining chemical system capable of Darwinian evolution. So...

Viruses check Darwinian evolution. They're a chemical system, but they're not self-sustaining. Right. So if viruses aren't alive, what are they? I don't know, because it would be weird to say that they're dead.

because, you know, you kind of, by definition, have to be alive first to be dead. Would you say they're inert or inanimate? Well, I don't know. Like, I mean, something that can go through such dramatic changes but also be passing genes down through the generations, through this process of heredity, I don't, you know, to say that that has nothing to do with life, just, you know, it feels weird. Then what problem happens if you were to expand that definition further

using the right language to include viruses, why does that make people unhappy? Well, you know, one issue is, where do you stop? So, like, if you have a more expansive definition of life, what else could be considered alive?

Red blood cells are an interesting example. If I took all your red blood cells out of you, you'd be dead, done. And the reason being that these cells have lots of proteins inside of them that do lots of important jobs, particularly getting oxygen from your lungs and ferrying them around your body. Mm-hmm.

So here are these things. They have these boundaries like living things do. They carry out complicated biochemical jobs like with oxygen. And people will talk about the lifespan of red blood cells. You know, they basically are only around for a few months in your body. And then they die. Then they get destroyed.

So you'd think that something that has a lifespan is alive. What are these things? Are they alive or not? They have some of the characteristics of life, some really important ones, but they're totally missing one of these really central ones. The central one being? Genes. Genes. Red blood cells have no genes. There's no way for them to grow and divide and replicate. That's it.

Just to sum up, the case that red blood cells are alive, distinct from us, what is that? Well, that's interesting that you would say that, distinct from us. Do things have to be distinct from you to be alive? Oh, I have no idea. Well, think about this. So there are some kinds of insects, like cicadas, for example, that grow special organs inside their bodies where certain kinds of bacteria live inside the cells.

These bacteria are vital to the cicadas because they will make certain kinds of amino acids for the insects that the insects can't get from eating plants. These bacteria, in turn, get lots and lots of food from the cicadas. And they cannot live outside of the cicadas. They will die. They are chemically incapable of surviving.

They have their own genes. They can grow and replicate, but they're not distinct. They actually have to be inside of cicada cells. So they're as merged with them as you can imagine. Are they alive? Well, you know, I think you can make the case, but you can't. If one of your rules is, well, it has to be distinct, then I don't think they meet that. Yeah, when you're describing that, it's like, oh, that kind of sounds like a virus. Yeah.

I mean, viruses are a lot more alive in a way than these bacteria. These bacteria get passed down from mothers to their offspring. They're not floating around. Viruses, you know, there are some viruses, they can float around in the air for miles before finding a new host and then infecting them.

Whereas there are a lot of bacteria that have become fundamentally trapped inside host cells and have totally changed. They've given up everything that they would need to be able to survive outside. I mean, we ourselves are resident to some former bacteria. Two billion years ago, our single-celled ancestors kind of formed a union with these oxygen-consuming bacteria. Mm-hmm.

And they became these little squishy things inside of our cells called mitochondria, which generate our fuel. We take out our mitochondria, we're dead. They still have a few genes left inside them, but you will never see mitochondria just busting out of a cell and just crawling off by themselves. They can't do it. They don't have the means to survive.

So is the, and I imagine the answer is we don't know. So is the bacteria in the cicadas alive and our mitochondria not alive? Well, you know, another way to talk about it is to say, well, they're involved in the process of living.

Is there also examples of the opposite thing? So something that on the surface seems very much like you put it in the life category, but when you look at it, it still confuses our definitions of life. Yeah, I think that nature does a great job of throwing up exceptions to the rule. And my favorite one is this fish called the Amazon molly.

This is a fish. It looks completely innocuous. You would not look twice at it. It's this tiny little fish, and it darts around in streams in Mexico and the southern United States. It evolved several hundred thousand years ago when two other species of molly interbred. And now that hybrid, the Amazon molly,

It only produces daughters. They're all female and they only produce daughters who are clones of themselves. Okay. However, if you just keep an Amazon molly by itself or a whole tankful of Amazon mollies by themselves, they will not reproduce. The reason being that they actually still have to mate with a male from one of those ancestral species. This is a species that cannot reproduce within itself.

It needs to go and find a male of another species of fish. The sperm triggers this process of its eggs starting to develop, but that female Amazon molly destroys the sperm and all of the genes inside of it. It's like, thank you very much, I'm on my way. And then once it's been able to mate with a male fish from another species, it's

it then just makes a whole bunch of clones of itself. So biologists call them sexual parasites. There's a funny head-spinny thing here because that also sounds like what the virus does, but the virus isn't alive. It needs another host to create more copies of its exact self. But the virus...

seems so different than a fish that swims around? Right, exactly, exactly. They're both sort of, you know, taunting us in the same way. It's a fish. It's alive, of course. But when you actually try to put into words what it means to be alive, the Amazon molly and things like it can get you all tangled up. Up next, something kind of surprising. The case that maybe life is just undefinable. Support for Unexplainable comes from Greenlight.

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And there's no perfect circle around what's alive and what's merely lifelike. Right. And again, we're trying to draw these circles, and maybe that's part of the problem. And in a way, this is more a philosophical problem than a scientific one. And philosophers have been thinking about these issues for quite a while. A very simple way of trying to understand this problem, and perhaps one solution, is to

instead of life say like, well, what's a game, you know, what are games? And, you know, like,

If you try to come up with some totally sharp circle definition of games, you're going to fail. Are games really that hard? Well, I mean, is it games have to involve cards? They can, but they can also involve tokens like a monopoly. Do you make money playing games? Well, certain games, yes. And others, you have to pay to play them. Do you have to win in a game sometimes? Yeah.

But you never have a child go to a toy store and go to the game section and be like, what is this? I don't understand. Like the kids are like, this is great. I want that, I want that, I want that. Like they get what games are. And what Wittgenstein said was that, you know, games are these things that have family resemblances, you know? So they're all connected in this sort of network of related meaning, right?

So, you know, yeah, red blood cells are not exactly like us and they're not like us in some very profound ways. The Amazon mollies are not quite like us. They're probably more like us than a virus. So maybe we can be thinking about living things as these things that are connected by family resemblances. Yeah, that feels so wishy-washy though. Like red blood cells,

Like, all in the same family as, you know, wombats and giraffes. I don't know. But they're like the relatives you only see, like, at Thanksgiving, you know? Like, they're not close relatives. So is this something we actually need to do as, you know, humans decide what life is? Is this, you know, in your research and reporting on this, are we coming to a stop here? This is a dead end. Like, let's not try. Or do we still need to try to find...

that perfect definition? Well, again, it really depends on who you talk to. So there will be people who will say, no, we really do need a definition of life for scientific purposes. So NASA can have some idea of what they're doing. For example, we need definition of life for legal purposes, you know, because everyone's shouting about quote unquote, when life begins and

There are all these situations where we really need clear-cut definitions of life. But, you know, there are other people who say a definition of life is absurd and a waste of time. There's a philosopher named Carol Cleveland who has really argued very strongly about this and said, this is like alchemists defining water in 1500. Yeah.

Just coming up with like a list of characteristics of water that you think somehow encompasses it, that's a waste of time. You know, what you need to be doing is be doing experiments and leading to a theory, you know? Like alchemists could not appreciate that if you look inside of water, there are these molecules that are composed of hydrogen and oxygen. And the way that they bond leads to all sorts of

different behaviors that we know of for water. That's what they need to focus on. And the more time you spend just yelling about definitions, the less time you're spending actually finding a new theory of life. And it sounds like we're not there yet.

or that atomic theory of what life is. No, we're not there. But, you know, I mean, Leonardo da Vinci wasn't there, you know, for when it came to chemistry, you know, like he was, he would pull his hair out trying to understand what water is. He would write in his journals, like, I don't know, like,

You know, it's different colors. It has different tastes. It's like, what is this thing? He was banging his head against the wall. We happen to live at a time where a theory of chemistry is pretty well worked out so we can understand water, whereas we're not there yet for life. I like knowing there's something I know that Leonardo da Vinci didn't, but I'm still hungry for more here.

Carl says, you know, maybe in our lifetimes we can get better answers, that scientists are really starting to think deeply on this sort of atomic level view of life. And maybe they'll come up with something. But if they do get a definition of life, the biggest test is still to come. So if we could find another form of life somewhere else, that would just, it would just change the game profoundly. And, you know, maybe we would have to step back and say like, okay, what's our theory of

to explain life both on Earth and off on Alpha Centauri or wherever. That would be saying, how do we explain water as a liquid and as ice?

If we found life on another world, that would help us answer a lot of the questions we've asked in this series. It could teach us whether water is really necessary for life. You know, maybe some sort of liquid methane or ethane or some bizarre gasoline-like substance that maybe something could be living in.

It could teach us different ways life could be built. You know, it might not use DNA. Maybe it has some other molecule that is replicated. Maybe we wouldn't even recognize it as life. I would not be surprised at all if our first encounter with something that seems like life just leaves us completely baffled.

Or there's a final option here. It could be terrifyingly similar to us. So similar that maybe we'll learn that life didn't start on Earth at all. Maybe it came from somewhere else. Whatever the answers are, what's just so exciting about the science of life is we're really just still getting started. ♪

This episode was reported by me, Brian Resnick, and produced by Noam Hassenfeld. Edits from Meredith Hoddenot and Catherine Wells. Christian Ayala is our engineer who creates all the funky, fresh sounds. Zoe Mullock checked the facts. Mandy Nguyen is working on some jokes. And Bird Pinkerton...

Well, she was staring at the tiny doctor balancing on the edge of a soda can. This is the last hospital of its kind, he said. Bird, you're our only hope. Thanks again to Carl Zimmer. His book is called Life's Edge, The Search for What It Means to Be Alive. Great read.

This is the end of our Origin series. If you have ideas for future series on our show, our email is unexplainable at vox.com. Let us know. And if you feel like leaving us a review or a rating, that would be very much appreciated. Unexplainable is part of the Vox Media Podcast Network, and we'll be back next week.