What is life? Scientists still can’t agree.

We know life when we see it. Flying birds are clearly alive, as are microscopic creatures like tardigrades that scurry around in a single drop of water.

But do we, humans, know what life fundamentally is? No.

“No one has been able to define life, and some people will tell you it’s not possible to,” says New York Times columnist and science reporter Carl Zimmer on Unexplainable — Vox’s podcast that explores big mysteries, unanswered questions, and all the things we learn by diving into the unknown.

It’s not for a lack of trying. “There are hundreds, hundreds of definitions of life that scientists themselves have published in the scientific literature,” says Zimmer, who wrote about them in his book Life’s Edge: The Search for What It Means to Be Alive. They include everything from simple definitions like “Life is a metabolic network within a boundary” to sentences that seem to require a PhD to decipher: “Life is a monophyletic clade that originated with a last common universal ancestor and includes all its descendants.”

There’s no consensus definition, but still the question teases us. It feels like it should be easy, something a fifth grader ought to be able to answer for science homework.

“It does feel like it should be easy because we feel it,” Zimmer says. “Our brains are actually tuned to recognizing things like biological motion. We’re sort of hardwired for recognizing life. But that doesn’t actually mean that we know what it is.”

But it still might be essential to answer. “Like imagine astronomers not agreeing on the definition of a star,” Zimmer says. “But this is even more fundamental. This is life.”

The problem is, for every definition of life, there’s a creature or perplexing life-like entity that just sends us right back to the drawing board.

I spoke to Zimmer about why it has been so damn hard to define life, and whether it might not be possible to define it at all.

(This conversation is pulled from the third episode in a series all about how life began on Earth. Check out the whole series here.)

This conversation has been edited for length and clarity.

So what’s wrong with the NASA definition of life, or any of the hundreds of others that exist?

There are lots of edge cases where things get really hard, so then people start arguing about who gets to be in the club.

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. We’re talking during a pandemic — there are who knows how many copies of SARS-CoV-2 that have been produced over the past few years through reproduction.

Not only that, but those viruses mutate. 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? It seems alive to me, right?

Yeah, that seems alive!

But you might say no because if what’s really important to you is 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. 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.

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, right.

So if viruses aren’t alive, what are they?

I don’t know. It would be weird to say that they’re dead because, by definition, they’d have to be alive first to be dead.

Would you say they’re inert or inanimate? Well, I don’t know.

Something that can go through such dramatic changes, but also be passing genes down through the generations — to say that that has nothing to do with life, just ... it feels weird.

What’s the problem if we expand the definition to include viruses? Why does that make people unhappy?

Well, you know, one issue is where do you stop?

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. These cells have lots of proteins inside of them that do lots of important jobs, particularly getting oxygen from your lungs and ferrying it around your body.

So here are these things, they have boundaries like living things do, they carry out complicated biochemical jobs.

People will talk about the lifespan of red blood cells. They basically are only around for a few months in your body. 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. Red blood cells have no genes. There’s no way for them to grow and divide and replicate, that’s it.

To sum up, what’s the case that red blood cells are alive distinct from us?

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 these cicadas. 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 are chemically incapable of surviving.

They have their own genes. So they can grow and replicate, but they’re not distinct. They actually have to be inside of cicada cells. So they are 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 all “it has to be distinct,” then I don’t think they meet that.

Those bacteria sound a little virus-like.

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.

We ourselves are resident to some former bacteria. Two billion years ago, our single-celled ancestors formed a union with these oxygen-consuming bacteria. 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 busting out of a cell and just crawling off by themselves. They can’t do it. They can’t. They don’t have the means to survive.

So are the bacteria in the cicadas alive and our mitochondria not alive?

Another way to talk about it is to say, well, they’re involved in the process of living.

Okay, so red blood cells and mitochondria might not be alive, they’re “involved in the process of living.” But are there also examples of things that definitely seem alive, no arguments, but still confuse definitions of life?

My favorite one is this fish called the Amazon molly.

This is a fish. It looks completely innocuous. You would not look twice as this tiny little fish 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 they produced a hybrid. And now that hybrid, the Amazon molly, only produces daughters. They’re all female, and they only produce daughters who are clones of themselves.

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.

So the Amazon molly needs a sexual partner to reproduce, but it doesn’t actually reproduce with them. It’s just reproducing with itself.

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 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 from a fish that swims around.

Right. Exactly. They are both sort of taunting us in the same way. It’s a fish. Of course 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.

I’m seeing why this simple question — what is life — has been so hard to answer. What are the words that puts you and me in the same box but keeps the red blood cells and viruses out and Amazon mollies in.

I can see the language problem of drawing that circle around all that.

We’re trying to draw these circles and maybe that’s part of the problem. This is more a philosophical problem than a scientific one. 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 instead of life, say like, well, what’s a game?

If you try to come up with some totally sharp circle definition of games, you’re gonna fail.

Are games really that hard?

Do games have to involve cards? Well, they can, but they can also involve tokens like in 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? Well, sometimes.

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.”

What [philosopher Ludwig] Wittgenstein said was that games are these things that have family resemblances, so they’re all connected in this sort of network of related meaning.

Yeah, that feels so wishy-washy, though. Like red blood cells all in the same family as wombats and giraffes. I don’t know.

Is this something we actually need to do as humans? Decide what life is?

Well, again, it really depends on who you talk to.

So there will be people who will say, 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 a definition of life for legal purposes. You know, because everyone’s shouting about quote-unquote when life begins.

There are all these situations where we really need clear-cut definitions of life.

But there are other people who say a definition of life is absurd and a waste of time. There’s a philosopher named Carol Cleland who has said this is like alchemists defining water in 1500. That’s a waste of time [without understanding molecules and atomic structure]. These molecules 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.

Leonardo da Vinci wasn’t there when it came to chemistry. 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.

Without having a solid theory of life ... does that complicate our search for it in space? Might we find something that looks totally unlike the life we have on Earth?

If we could find another form of life somewhere else, that would just change the game profoundly, and maybe we would have to step back and say, okay, what’s our theory to explain life both on Earth and off on Alpha Centauri or wherever.

[But] I would not be surprised at all if our first encounter with something that seems like life just leaves us completely baffled.