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

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

“No 1 has been capable to define life, and some persons will inform you it is not doable to,” says New York Occasions columnist and science reporter Carl Zimmer on Unexplainable — Vox’s podcast that explores significant mysteries, unanswered concerns, and all the points we discover by diving into the unknown.

It is not for a lack of attempting. “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 Implies to Be Alive. They contain every little thing from very simple definitions like “Life is a metabolic network inside a boundary” to sentences that look to need a PhD to decipher: “Life is a monophyletic clade that originated with a final prevalent universal ancestor and contains all its descendants.”

There’s no consensus definition, but nonetheless the query teases us. It feels like it need to be simple, a thing a fifth grader ought to be capable to answer for science homework.

“It does really feel like it need to be simple simply because we really feel it,” Zimmer says. “Our brains are really tuned to recognizing points like biological motion. We’re sort of hardwired for recognizing life. But that does not really imply that we know what it is.”

But it nonetheless may be crucial to answer. “Like envision astronomers not agreeing on the definition of a star,” Zimmer says. “But this is even a lot more basic. This is life.”

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

I spoke to Zimmer about why it has been so damn tough to define life, and whether or not it may not be doable to define it at all.

(This conversation is pulled from the third episode in a series all about how life started on Earth. Verify out the entire series right here.)

This conversation has been edited for length and clarity.

Brian Resnick

So what’s incorrect with the NASA definition of life, or any of the hundreds of other folks that exist?

Carl Zimmer

There are lots of edge situations exactly where points get truly tough, so then persons start out arguing about who gets to be in the club.

Brian Resnick

I know 1 of the most well-known ones are viruses. Can you clarify why viruses have been just so confounding? Are they alive? Are they not alive?

Carl Zimmer

So in some strategies, viruses just look extremely alive. We’re speaking throughout a pandemic — there are who knows how quite a few copies of SARS-CoV-two that have been created more than the previous couple of years by way of reproduction.

Not only that, but these viruses mutate. Some of these mutations make them greater at specific jobs. It is produced of genes. It is produced of protein.

I imply, what a lot more do you want? It appears alive to me, appropriate?

Brian Resnick

Yeah, that appears alive!

Carl Zimmer

But you may say no simply because if what’s truly vital to you is metabolism, you know, consuming stuff, effectively, viruses do not do it. Viruses do not have any way of taking in molecules and fashioning these molecules by themselves into new molecules. They do not have a mouth, they do not have a stomach, they do not have enzymes, they do not have any of that.

All they have are fundamentally guidelines that reprogram a cell. And that cell, not the virus, tends to make new viruses.

Brian Resnick

You talked about that NASA definition, “life is a self-sustaining chemical method capable of Darwinian evolution.” So viruses verify Darwinian evolution, they’re a chemical method, but they’re not self-sustaining.

Carl Zimmer

Suitable, appropriate.

Brian Resnick

So if viruses are not alive, what are they?

Carl Zimmer

I do not know. It would be weird to say that they’re dead simply because, by definition, they’d have to be alive initially to be dead.

Would you say they’re inert or inanimate? Properly, I do not know.

Anything that can go by way of such dramatic alterations, but also be passing genes down by way of the generations — to say that that has absolutely nothing to do with life, just … it feels weird.

Brian Resnick

What’s the difficulty if we expand the definition to contain viruses? Why does that make persons unhappy?

Carl Zimmer

Properly, you know, 1 challenge is exactly where do you quit?

Brian Resnick

If you have a a lot more expansive definition of life, what else could be viewed as alive?

Carl Zimmer

Red blood cells are an exciting instance.

If I took all your red blood cells out of you, you’d be dead. Carried out. These cells have lots of proteins inside of them that do lots of vital jobs, specifically acquiring oxygen from your lungs and ferrying it about your physique.

So right here are these points, they have boundaries like living points do, they carry out complex biochemical jobs.

Individuals will speak about the lifespan of red blood cells. They fundamentally are only about for a couple of months in your physique. So you’d feel that a thing that has a lifespan is alive.

What are these points? Are they alive or not? They have some of the traits of life, some truly vital ones, but they’re completely missing 1 of these truly central ones.

Brian Resnick

The central 1 becoming?

Carl Zimmer

Genes. Red blood cells have no genes. There’s no way for them to develop and divide and replicate, that is it.

A seal, blissfully unaware of the head-spinny reality of life becoming incredibly tough to define.

Spencer Platt/Getty Pictures

Brian Resnick

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

Carl Zimmer

That is exciting that you would say that — “distinct from us.” Do points have to be distinct from you to be alive?

Brian Resnick

Oh, I have no notion.

Carl Zimmer

Properly, feel about this. So there are some sorts of insects — like cicadas, for instance — that develop specific organs inside their bodies exactly where specific sorts of bacteria reside inside the cells.

These bacteria are very important to these cicadas. They will make specific sorts of amino acids for the insects that the insects cannot get from consuming plants.

These bacteria in turn get lots and lots of meals from the cicadas. And they can not reside outdoors of the cicadas. They are chemically incapable of surviving.

They have their personal genes. So they can develop and replicate, but they’re not distinct. They really have to be inside of cicada cells. So they are as merged with them as you can envision.

Are they alive? Properly, you know, I feel you can make the case, but you cannot. If 1 of your guidelines is all “it has to be distinct,” then I do not feel they meet that.

Brian Resnick

These bacteria sound a tiny virus-like.

Carl Zimmer

Viruses are a lot a lot more alive in a way than these bacteria. These bacteria get passed down from mothers to their offspring. They’re not floating about.

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 tiny squishy points inside of our cells known as mitochondria, which produce our fuel. We take out our mitochondria, we’re dead.

They nonetheless have a couple of genes left inside them. But you will under no circumstances see mitochondria busting out of a cell and just crawling off by themselves. They cannot do it. They cannot. They do not have the indicates to survive.

Brian Resnick

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

Carl Zimmer

One more way to speak about it is to say, effectively, they’re involved in the course of action of living.

Brian Resnick

Okay, so red blood cells and mitochondria may not be alive, they’re “involved in the course of action of living.” But are there also examples of points that surely look alive, no arguments, but nonetheless confuse definitions of life?

Carl Zimmer

My preferred 1 is this fish known as the Amazon molly.

This is a fish. It appears absolutely innocuous. You would not appear twice as this tiny tiny fish darts about in streams in Mexico and the Southern United States. It evolved quite a few hundred thousand years ago when two other species of molly interbred and they created a hybrid. And now that hybrid, the Amazon molly, only produces daughters. They’re all female, and they only make daughters who are clones of themselves.

On the other hand, if you just preserve an Amazon molly by itself, or a entire tankful of Amazon mollies by themselves, they will not reproduce. The explanation becoming that they really nonetheless have to mate with a male from 1 of these ancestral species.

Brian Resnick

So the Amazon molly demands a sexual companion to reproduce, but it does not really reproduce with them. It is just reproducing with itself.

Carl Zimmer

This is a species that can not reproduce inside itself. It demands to go and locate a male of a further species of fish. The sperm triggers this course of action of its eggs beginning to create. But that female Amazon molly destroys the sperm and all of the genes inside of it. It is like, thank you incredibly substantially. I’m on my way. And then as soon as it is been capable to mate with a male fish from a further species, it then just tends to make a entire bunch of clones of itself. So biologists contact them sexual parasites.

Brian Resnick

There’s a funny head-spinny point right here simply because that also sounds like what the virus does. But the virus is not alive. It demands a further host to make a lot more copies of its precise self. But the virus appears so diverse from a fish that swims about.

Carl Zimmer

Suitable. Specifically. They are each sort of taunting us in the exact same way. It is a fish. Of course it is alive, of course. But when you really attempt to place into words what it indicates to be alive, the Amazon molly and points like it can get you all tangled up.

Brian Resnick

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

I can see the language difficulty of drawing that circle about all that.

Carl Zimmer

We’re attempting to draw these circles and possibly that is aspect of the difficulty. This is a lot more a philosophical difficulty than a scientific 1. Philosophers have been considering about these challenges for fairly a when. A incredibly very simple way of attempting to recognize this difficulty and probably 1 remedy is rather of life, say like, effectively, what’s a game?

If you attempt to come up with some completely sharp circle definition of games, you are gonna fail.

Snow monkeys: surely alive.

Spencer Platt/Getty Pictures

Brian Resnick

Are games truly that tough?

Carl Zimmer

Do games have to involve cards? Properly, they can, but they can also involve tokens like in Monopoly. Do you make cash playing games? Properly, specific games, yes. And other folks you have to spend to play them. Do you have to win in a game? Properly, at times.

But you under no circumstances have a kid go to a toy shop and go to the game section and be like, “What is this? I do not recognize.”

What [philosopher Ludwig] Wittgenstein stated was that games are these points that have household resemblances, so they’re all connected in this sort of network of connected which means.

Brian Resnick

Yeah, that feels so wishy-washy, even though. Like red blood cells all in the exact same household as wombats and giraffes. I do not know.

Is this a thing we really need to have to do as humans? Make a decision what life is?

Carl Zimmer

Properly, once more, it truly depends on who you speak to.

So there will be persons who will say, we truly do need to have a definition of life for scientific purposes. So NASA can have some notion of what they’re carrying out, for instance.

We need to have a definition of life for legal purposes. You know, simply because everyone’s shouting about quote-unquote when life starts.

There are all these scenarios exactly where we truly need to have clear-reduce definitions of life.

But there are other persons who say a definition of life is absurd and a waste of time. There’s a philosopher named Carol Cleland who has stated this is like alchemists defining water in 1500. That is 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 diverse behaviors that we know of for water.

Leonardo da Vinci wasn’t there when it came to chemistry. He would pull his hair out attempting to recognize what water is. He would create in his journals like, “I do not know, like, you know, it is diverse colors. It has diverse tastes. It is like, what is this point?” He was banging his head against the wall. We come about to reside at a time exactly where a theory of chemistry is quite effectively worked out so we can recognize water, whereas we’re not there however for life.

Brian Resnick

Devoid of possessing a strong theory of life … does that complicate our search for it in space? May possibly we locate a thing that appears completely in contrast to the life we have on Earth?

Carl Zimmer

If we could locate a further type of life someplace else, that would just adjust the game profoundly, and possibly we would have to step back and say, okay, what’s our theory to clarify life each on Earth and off on Alpha Centauri or wherever.

[But] I would not be shocked at all if our initially encounter with a thing that appears like life just leaves us absolutely baffled.