Scientists to this day can’t define what life is. For every definition, there’s an animal, a plant — something that breaks the rules. So how does science answer that? Writer Carl Zimmer joins us to explore the borderlands of life.
What does it mean to be alive?
“Scientists have been grappling with it for centuries, that question of what is life? The earliest that I can find that someone actually asked that question scientifically was in 1708. And there was a physician named Georg Ernst Stahl. He said, Above all else, consequently, it comes down to this to know what is life. And Stahl was basically thinking, Well, the scientific revolution is showing us how matter is acted on by forces, and is just almost mechanical. So what differentiates living things from this mechanical universe?
"And he believed that we were more than just mere matter, that there was an interdependence between our parts, that he considered what made life, life. But that was his definition. … There are hundreds of others of definitions. And there are new ones that come up all the time. And in a recent essay, some scientists joked that there are about as many definitions of life as there are people trying to define life. Because people put different emphasis on different things. Different people think that different things are important for life.”
Let’s talk about one of my favorite microorganisms, tardigrades. What do they tell us about life?
“They are barely visible to the eye sometimes. But they're animals, they're incredibly small animals with eight legs. They're sometimes called water bears because they kind of lope along with kind of a bear-like gait. They're everywhere, they're in forests, and oceans and ponds and so on. And it just so happens that if you try to kill them, it's quite hard.
"So, for example, if you let them dehydrate, they start losing water. And that would kill us. But it doesn't kill a tardigrade, because they do something kind of amazing. They start replacing the water with certain sugar molecules that act like water but don't evaporate. And then they start building special proteins that essentially encase their DNA and everything else in their cells in a kind of a glass-like material. So they essentially freeze into glass, as it were, and they can stay like this for decades."
Are tardigrades alive?
“They’re not alive. Again, if we say, Oh, well, you have to be metabolizing, you have to be carrying out chemical reactions to be alive. Well, these tardigrades are totally not alive then. But they're not dead either. Scientists call this state cryptobiosis."
How long can they be in cryptobiosis?
“Well, again, it all comes down to what you mean by your words. And if we say that in order to be alive, you have to be carrying out chemical reactions, then no, they're not alive. But they're certainly not dead in the way we think of death. So scientists use another word. They call it cryptobiosis, a third state of existence, some scientists call it.
"We really don't know how long they can go like this. Because you need to put away some tardigrades for decades and then bring them back. And, you know, scientists have done that for 30, 40 years or more. But who knows, maybe in 100 years or 200 years, we discover they can stay this way for centuries, maybe.”
We talked about what life is, but also what it does. And you describe a slime mold, which shows sign of intelligence. Why is intelligence a hallmark of life? And what is a slime mold?
“I argue in the book that intelligence, very broadly defined, is another hallmark of life. In other words, all living things have to make sense of their environment, and make a choice about what to do. And you don't need a brain to do that. Slime molds are these weird little cellular inhabitants of the forest floor, they have lovely names like dog vomit.
"So if you ever see something when you're hiking that looks like a dog threw up, it's probably a slime mold. And these sort of pulsing networks of protoplasm can reach out and figure out how to find food, and make all sorts of clever decisions about the best way to eat. So they can make their way through mazes when scientists build a little slime on the mazes for them and all sorts of other things.”
In your book, you visit scientists who are creating some kind of mini brain out of differentiating neurons. And that raises a whole bunch of questions for you. What research are these scientists doing?
“So I got interested in life in a personal way, I mean, the first thing that we really know is that we ourselves are alive. And well, how do we know that? Well, we're aware that we're alive. In other words, like our brain is getting signals from our bodies and we somehow interpret that as meaning that we are alive. And the brain for us is centrally important to our lives. Brain death is the legal definition of death. So there is also this question of, Well, when does life begin? I mean, this is a huge political flashpoint. There are experiments that scientists are doing with human cells that challenge a lot of our assumptions, a lot of the things that we take for granted about human life.
"So I went to visit some scientists in San Diego. They can take just a little skin sample from you, put it in a dish, break up the skin cells, hit them with some chemicals. That basically reprograms the skin cells to become like cells in an embryo that can become any tissue, they hit him with some more chemicals and turn them into neurons. These are like neurons in your brain. In fact, they're neurons like in a developing embryonic brain, and they start to divide, grow, divide, grow and divide.
"And they form structures much like in a human brain, and they start giving off what look, in many ways, like brain waves. So scientists refer to these as brain organoids. And these things, they get to be a little bit smaller than a pea. They can, quote-unquote, live for years, it seems. Giving off these signals, feeding on food that the scientists give them. What are these? Are these alive? Do we consider them a human life? What are they? And I have talked to philosophers and bioethicists and they're like, We don't really have the language for this yet.”
We as humans have been compelled for so long to try and find answers to life’s questions, and why do you think that is? What drives humans to question what it means to be alive?
“I think we just have this deep-seated awareness of our own lives. And we look around us and we have brains that can perceive life and other things. And so we're aware of life, even if we can't explain it. And so scientists have been probing living things in exquisite atomic detail to try to understand how each of them works. And maybe even be able to try to like figure out to step back and ask, Well, what is all that research [adding] up to in terms of understanding life? But it's OK that we're not there yet. This takes time.”
Excerpt from "Life’s Edge" By Carl Zimmer
An excerpt from "Life’s Edge" By Carl Zimmer. All rights reserved. No part of this excerpt may be republished without permission from the publisher, Penguin Random House.
From The Reading List
New York Times: "What Does It Mean to Be a Living Thing?" — "Carl Zimmer’s book begins with a bang. Not a Big Bang, but a small one. In the fall of 1904, a 31-year-old physicist, John Butler Burke, working at the Cavendish Laboratory in Cambridge University, made a 'bouillon' of chunks of boiled beef in water."
Science Magazine: "How we think about what it means to be alive will always depend on what questions we ask" — "Carl Zimmer’s Life’s Edge is a departure from his previous work in that it is a book that is as much about what scientists have so far failed to understand as what they have come to understand."
Washington Post: "The surprisingly elusive definition of ‘life’" — "A few months into the pandemic, my girlfriend and I made a horrifying discovery in our bedroom. A mossy, bone-white substance was — and we could imagine no other word for it — growing along the inner edge of one window."
This program aired on April 1, 2021.