Contemplating what it means to be alive in the new book 'Life's Edge'

About 4 billion years ago, give or take a couple of hundred million years, the whole trouble started.

In a warm puddle, or perhaps around a geothermal vent in the deep ocean, or perhaps somewhere we haven't identified yet, a mixture of chemicals started to do something new — something more complicated than they'd done before. It was the dawn of life.

Fast forward a few billion years, and our planet is teeming with life. Plant, animal, and particularly microbial life can be found pretty much anywhere — no matter how remote and extreme the environment. As far as we know, life is Earth's distinguishing and unique feature.

Life is a fascinating subject, worthy of the attention of Carl Zimmer, author and science columnist for the New York Times, one of science writing's brightest lights. In his new book, Life's Edge: The Search for What it Means to be Alive, Zimmer explores the big questions of life, including what it is, where it comes from and the troublesome dividing line between life and non-life.

Carl Zimmer spoke to Bob McDonald about his new book. Here's part of their conversation.

The interview below has been edited and condensed.

Let's start with the question that much of the book addresses: 'What is life?' Why is it such an interesting question to ask in the first place? 

Well, one thing I find really interesting is that people have been asking this question for a long, long time. You know, scientists have learned a lot about life, but it's still really, really hard to answer that basic question. In fact, I would argue it's actually kind of impossible. 

But I think if you ask almost anyone, they can say, 'Well, I know what's alive and what isn't. I can look at a rose garden and a rock garden and tell you which one is alive just by looking at them.' Why is it so difficult from a scientific point of view? 

That's the thing. We know it when we see it. We don't need to define life to have that instinct that we are alive. And if we look at something, an animal that's dead on the road, well, that's dead. 

But, you know, you mentioned a rose garden. The roses are growing out of the ground. They're photosynthesizing. They're doing all these things that sometimes are considered hallmarks of life. 

But you know what happens if you clip one of those roses and you're holding in your hand? Is it now still alive? It's no longer connected to its roots in time. Its petals are going to wilt. The cells are still metabolizing, but its days are numbered. And when are you going to decide when it's alive and when it's dead? 

You know, that's just one example of many, many that you can explore once you start to really ask yourself, 'Well, if I really think I know what life is, let me look around and really see if I can come up with clearer answers for all the things that surround me.' It'll be harder than you think. 

Can you give me another example or two of where it's hard to define what's alive and what's not? 

There are these hallmarks of life that come up again and again when people talk about life and certainly when scientists study living things. One of them is metabolism. You know, we eat, we turn our food into fuel. The rose uses sunlight and carbon dioxide. 

Well, OK, what about a virus? So viruses can do all sorts of other things that we think of as hallmarks of life — they're amazing at evolving, they're amazing at replicating. They don't metabolize. They have nothing inside themselves except for just a package of genes. They're just a vehicle almost to deliver those genes into cells. And then those cells do the work of making new viruses.

So are viruses alive? You can get scientists into a really spirited debate over that one. I've had lots of fun pitting scientists against each other because that is not settled. 

It sounds like it's a bit like art. You know, we have trouble defining it, but we know when we see it. 

I think in a way that some of our trouble is that we have so many intuitions about life. For one thing, we are sure that we are alive as individuals. But there's actually a psychiatric disorder called Cotard's syndrome, where people are very much convinced that they're dead. They'll explain to you why they're able to talk to you despite being a corpse. 

I think that gives you a clue that our sense that it's obvious that we're alive is actually maybe more of an intuition that comes about because we have certain brain circuits that may sort of monitor our own experiences and integrate them into this sense that we call "being alive." 

And we also have biological sensors in our brains. We have circuitry that's very sensitive to biological motion, for example. And so we distinguish between living things moving and inanimate things moving. And this is something that goes back millions and millions of years to our primate ancestors. 

There are some fascinating scientific questions about what life is, but there are  also striking practical ones that have a long history, like how to tell if someone is alive or dead.

For quite a long time doctors really struggled to be able to give a sort of a definitive announcement that their patients were indeed dead. A doctor would say their patient was dead and the patient would then be put in a coffin and buried and then would actually wake up buried alive. And so there were a lot of elaborate measures that were put in place, sometimes coffins would have a bell connected with a string so that if someone did wake up in their coffin, they could ring the bell to just let people know there was a mistake. 

Technology sort of helped doctors to draw that line a little better. The stethoscope in the early 1800s was a huge advance because doctors could say, 'All right, I'm going to say when that heart stops beating, that's it, I can declare this person dead.'

But technology actually ended up sort of creating more challenges in terms of understanding the nature of death, because with the invention of ventilators, somebody could, in some respects stay alive for days or even longer because they were getting a supply of air in their lungs, even though their brain might be incredibly damaged so that they couldn't breathe on their own. 

That led, in the 1960s, to the establishment of brain death as a standard for legal death, for what the law recognizes as death. And yet, you know, there have been some disputes sometimes where people have said about a loved one 'Well, they're still alive, they're still breathing, their heart is still beating,' and so it kind of depends on how you define life to decide when that life is over. 

The development of Darwin's theory of evolution really started people thinking about the origins of life.

Darwin sought to explain the diversity of species he saw around him through evolution. In other words, all the species he saw shared a common ancestry. What this meant was that if you went all the way down to the base of the tree of life, then you would find the common ancestor of all living things.

And maybe that common ancestor originally just arose through chemistry on the early Earth. And he speculated about a "warm little pond." The implication being maybe some chemicals in a warm little pond could somehow come together into primitive cells. 

There's one experiment that I talk about done by a scientist named David Deamer, who worked at the University of California at Santa Cruz for a number of decades. He has been exploring the possibility that maybe Darwin was right about these warm little ponds. Maybe a pond or a hot spring on a volcano on the early Earth could have filled up with water and then dried out, filled up with water again, dried out, and the chemicals in there through that are drying and wetting could build up into maybe something like a "protocell." 

He has essentially kind of an artificial primal Darwinian pond, and he and he can actually build very interesting molecules in there. 

Lee Cronin, at the University of Glasgow, has built robots that just do chemical experiments 24 hours a day looking for interesting reactions that produce droplets that act in lifelike ways. 

These aren't even cells. They're just droplets that are just mixes of different chemicals. And they're starting to do interesting things like they might race around a dish. They look kind of like tadpoles or something like that. And they swarm together in packs or they avoid each other. It's very strange and weirdly lifelike. 

Even though all those experiments have happened, nothing has crawled out of the test tube yet. 

Not yet. But I wouldn't be surprised if in my lifetime one of them does do something that makes scientists say, 'Wow, I think we've made something here.'  

Produced and written by Jim Lebans