IRA FLATOW: This is Science Friday, I’m Ira Flatow. I’ve got a really simple question for you to mull over. What is life? Oh, yeah. I’m sure you’ve never thought about that before, right? Sounds like something out of your college philosophy class.

But what makes this question different at this time is that the definition of life has left the pages of philosophy. It’s gone way past Frankenstein, has become an important legal and biological discussion. I mean, we’re searching for life on other planets in other galaxies.

How will we know when we found it? Does it have to meet certain physiological criteria? What about the ability to reproduce or self-sustain? And scientists still have not come to an agreement over what is alive and what isn’t. If you really want to get your head around this, check out a new book by an author you are very familiar with. He joins me now. Carl Zimmer, science columnist for The New York Times and author of the book Life’s Edge– The Search for What It Means to Be Alive. Welcome back to Science Friday, Carl.

CARL ZIMMER: Great to be here.

IRA FLATOW: Tiny little topic you chose to bite off here.

CARL ZIMMER: Well, you know. Life is short, forgive the pun.

IRA FLATOW: There you go. Thank you for joining us. You stick very closely to the history of this concept in science for most of the book. You got to tell us what drew you to this topic from a science perspective.

CARL ZIMMER: Well, I suppose a lot of us wondered what life is as soon as we started using the word as kids. And maybe we asked our parents, and maybe our parents gave us an answer that wasn’t terribly satisfying, and then we thought maybe science had the definitive answer. And I went on to become a science writer and got to write about all sorts of different kinds of life, jellyfish and trees, and humans, and all sorts of other things.

And yet if you stop and ask somebody who studies, say, snakes, well, what is life? It gets to be a little awkward. They may not have a definition at the ready, or they might say, well, you know, that’s not really my thing. I’m a snake person. And then if you dig deeper, you discover that there are hundreds of definitions of life out there and they’re not converging on one. It just seems like there are more and more new definitions that come out every year.

So I just find that fascinating. I mean, what could be a bigger question for biology, and yet one that really doesn’t have an answer yet?

IRA FLATOW: And there are so many historical theories of what life is, when it begins, et cetera that you talk about in your book that we know now are not correct. For example, how fermentation was seen as an act of life, or how a concoction of radium and beef– this was a fairly interesting story that you talked about– was thought to create the most primeval form of life. Do you have any favorite theories that you came across during your research that aren’t quite right? Some of the ones that you really thought were fascinating.

CARL ZIMMER: Well, in a way, I find the stumbles and the mistakes in science just as interesting as the grand successes. We tend to just learn about the big victories and we forget that it’s part of a much bigger process.

So one example is that there was a great biologist in the 19th century, Thomas Huxley, who convinced himself that the entire ocean floor was carpeted by what some people called a living paste, the most primordial form of life. It wasn’t even a cell, it was just the mass of protoplasm. And he even gave it a name, a species name, he called it Bathybius, and he made all sorts of grand pronouncements about how this is the wellspring of all life on Earth.

And it turned out he was totally wrong. I mean, this Bathybius– and in the 1970s, you could open up a textbook on about biology and on page one, there was Bathybius, this weird, blob-like thing and it did not exist. It was just the result of a chemical fluke. And to Huxley’s credit, when other scientists pointed out to him that Bathybius was not real, he said, hey, if anyone made a mistake, it’s me, and he totally copped to it. So I think it just shows you just how hard it is to really figure out where is the line between the living and the non-living.

IRA FLATOW: Yeah. And as you said before, and I’m going to quote from your book, asking biologists what it means for something to be alive makes for an awkward conversation. They demure, they stammer, they offer a flimsy notion that crumbles under scrutiny. Is this something they just don’t want to talk about or is it something they are embarrassed that we don’t know?

CARL ZIMMER: I honestly just think it’s something that biologists are not trained to really think about that much. As a biologist, you’re trained to get answers about life. And so you might be very interested in maple trees, and so you want to understand how do maple trees reproduce, and you drill down and down and down into the molecular details of that, and you published paper after paper after paper.

There is much less of an incentive in biology to step back and be like, OK, like what does all this have in common? What does it mean for all these things that we call alive to be alive? So there are people who dare to tread on that very dangerous territory, but not a lot.

IRA FLATOW: Along with the history you go through, this book traces your adventures through a lot of different labs, environments where you look at creatures, and experiments that complicate our understanding of what is alive and what isn’t. And it struck me that throughout the book, you get to do science yourself. I was reading some of these things saying, I wish I could do that stuff, putting together experiments and lab with researchers, getting your hands dirty with kelp on the beach. Do you have a favorite experience of any of those that you’ve written about in the book?

CARL ZIMMER: It was a really wonderful experience to be able to visit with scientists, travel with scientists in order to explore the territory of life. And there were different hallmarks of life that everyone generally agrees on, and I wanted to really get to know those hallmarks by looking at certain species that were really just the exemplars of it. So for example, life has a certain kind of intelligence.

A lot of scientists would argue, in other words, living things make decisions about what to do based on their environment. So I mean, we make decisions every day. What are we going to have for lunch? But I was really fascinated by scientists who study these things called slime molds, which are these bizarre, science fiction-like creatures that live on the forest floor. They’re everywhere. Go out in the summer and you’ll see them if you know what to look for.

And they send out their tentacles across the forest floor looking for bacteria to eat, and they have to decide where to go and basically how to lay out their network. And it’s amazing that they are like little mathematicians making calculations on the most efficient way to cover a certain area, and to find certain kinds of food. They can solve mazes. So I have to say that was a particularly fun experience.

IRA FLATOW: Let’s go into some more modern theories about life because these are the kinds of things we cover all the time, and it’s amazing how quickly the definitions change. I mean, you have a line in your book that I absolutely love. Let me quote it. “We don’t know when a theory of life may arrive, but we can hope that our own lives last long enough to let us see it.” As long-time science journalists, we’ve both seen research on biology really accelerate. Do you think we’ll get more insight into this in our lifetimes?

CARL ZIMMER: I do, actually. I think that there are a number of scientists, not just biologists but physicists, for example, who are really working hard on developing a theory of matter that could explain life. And this would be in the same way that chemists developed a theory of matter that explained molecules and water and then all those inanimate substances around us.

And so you need theories to really get a deep understanding and be able to understand even more. And those theories are in the works right now. And there are several different theories that are being developed, which are very exciting. I don’t think anyone really pretends that they’ve got the definitive theory all worked out.

But when you talk to people in this field, they feel like, I think we’re going to sort this out. They’re pretty optimistic, which is nice. But it is a little frustrating that imagine living right before the theory of chemistry started to make molecules and atoms and elements make a lot more sense. That’s where we are now.

IRA FLATOW: You mean agreeing, all of us, on what life is?

CARL ZIMMER: Yes, to have it–

IRA FLATOW: So is it criteria? I mean, we’re looking for it? We’re going to other planets? Do we need a set of specs to understand, hey, this is life?

CARL ZIMMER: Well, on the one hand, it would be nice to have a really clear theory of life before we go searching for it, but on the other hand, we don’t want to wait forever. So NASA is using working definitions of life to get some clues. And so honestly, like they’re looking on Mars for rocks and other patterns that kind of look like signs of life on Earth.

Which is fine, but what if life on Mars or on the moon or around Saturn or in another solar system doesn’t play by our rules? Isn’t based on DNA? Doesn’t even use carbon? We need a deeper understanding of life to really be open to all the possibilities.

IRA FLATOW: Let’s move on to talk about cutting-edge research on synthetic organisms because it really has moved very, very quickly. Just last month, there was news that scientists created a synthetic cell that grows and divides as normal. Other cells have been created with completely synthetic genomes. What do you think, Carl? Do these synthetics ride life’s edge?

CARL ZIMMER: Yes, I think they do. Although in some cases, what we’re looking at are examples where you’re taking life as we know it and pulling pieces out of it and figuring out, what’s the bare minimum that you can get by and this thing is still operates in the same way that it did originally before you started pulling all the genes out?

So there are other scientists who say, like, well, wait a minute. Life didn’t start with a complicated cell and got stuff pulled out. What probably would happen is that there were protocells that didn’t even have DNA in them. They probably had a much simpler molecule, RNA, and we have RNA in our cells. But sometimes, just think that inside of these protocells, the first cells on Earth, you just have RNA that was carrying genetic information and carrying out chemical reactions.

So they’re trying to build these protocells as well. And that’s still got a ways to go before you have a really full-blown RNA protocell that can really just thrive and grow and do all the things on its own, but they are getting there. And so again, I wouldn’t be surprised if in 10 or 20 years where we’re looking at these things, it could be a very good model for how life began.

But there are other people who are just saying like, well, let’s not fixate on RNA. What if we just look at different combinations of simple chemicals and see if they form droplets that show lifelike behavior? And that sounds crazy, but the University of Glasgow, they’re actually making some strange droplets that skitter around in Petri dishes, that split into– looks like they’re reproducing. There’s just some weird stuff happening there and they’re using robots to keep changing these recipes to see if they might just stumble across droplets that we might call alive.

IRA FLATOW: Just a quick note that I’m Ira Flatow and this is Science Friday from WNYC Studios. We’re also making, as you point out in your book, little mini organs, body organs, that are not fully-formed brains or lungs, but they are enough to work with.

CARL ZIMMER: Yeah, when we think about life, we often think about when life begins. We think of that boundary in terms of the start of life, and that’s very controversial when we think about human life in particular. And scientists are really complicating our efforts to draw that line by being able to grow things from cells.

So for example, I went to a lab where they can take one of your skin cells and put it in a dish it with some chemicals and it basically turns into a neuron, a neuron much like in a human embryo, which then will start to grow and develop and grow and divide into hundreds of thousands, millions of neurons that organize themselves on their own into a brain-like structure.

And if you just keep giving it the right chemicals for food, you can keep it alive. I think we’d agree it’s alive for years. So here’s this thing that even seems to be producing brain waves, but it’s not inside a human body. It started out as a skin cell, so is that alive? And how do we deal ethically with this, this life? We have no answers to these questions.

IRA FLATOW: Well, what about the question about artificial intelligence? Do you think it would ever meet the criteria, if we come up with some definition of life?

CARL ZIMMER: Well, there’s artificial intelligence which is able to say this image looks like a cat to me, but that might tick off some of the boxes that we have for what it means to be alive, but I suspect it wouldn’t tick off other ones. It’s not as if Google has artificial intelligence programs that are reproducing themselves.

So if that’s your standard for life, then I don’t think they would consider it alive. But it’s interesting that we might be able to just skip all that and all those criteria and have something we might agree is intelligent or even conscious without all the other things that go with being alive.

IRA FLATOW: Interesting stuff. There’s so much to talk about, Carl, in this book, so much to contemplate. Thank you for taking time to be with us today.

CARL ZIMMER: Thank you. It’s been a pleasure.

IRA FLATOW: Carl Zimmer, science columnist for The New York Times and author of the book Life’s Edge– The Search for What It Means to Be Alive.

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