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Ep #21 Transcript | Carl Zimmer: "Life's Edge," Extreme Lifeforms & the Origins of Life

Full episode transcript (beware of typos!) below:

Nick Jikomes

Carl Zimmer, thank you for joining me. Thanks for having me. Where are you calling in from today? And can you give everyone like a short bio of who you are? Well, I write books about science. And I'm also a columnist for the New York Times. And I'm talking to you from lovely Connecticut. And how are things going over there right now in terms of COVID? Have you gotten a vaccine yet? I have gotten the vaccine. I'm talking to you a couple days out of my second dose. So I've gotten through that.


Carl Zimmer 2:49

That challenging time of just relaxing on the couch. Well, my body makes lots of Spike proteins. So I'm feeling pretty good. Cool. Yeah, I'm actually the same. I got it. My second dose two days ago, which which one? Did you get? Pfizer? Yeah, no, say My God. So you felt it a little bit after the second dose. Felt a little after this first, though. So it felt after the second dose. But you know, I just just stretched out on the couch and listened to podcast series for the afternoon. And next day, I was back in action. Awesome. Well, thank you for joining me, I hope to talk to you mostly about your latest book. So I've got it right here. It's called Life's edge. I believe it's, it is out now. Is that right?


Yes, it is out and available at all fine bookstores.


Nick Jikomes 3:39

So it's basically about this question. What is life that many, many people have asked. And I immediately start to think about the famous one liner from a Supreme Court case from a few decades ago, that was about obscenity, or one of the justices is asked to define pornography. And he can't actually come up with the definition. He just says, I know it when I see it. And I think life is kind of like that, right? We all know life when we see it. And yet, it's this concept. It's this thing that escapes a precise definition, or at least very difficult to come up with something like that, when you when you try and stop and think about it. So just to start out with what, you know, what was the reason you decided to write about this topic? And why do you think it's it's such a difficult question to answer?


Carl Zimmer 4:27

Well, you know, maybe all of us have asked this question when we were kids. Once we started using the word life or alive and you know, ask our parents will what is it and we probably didn't get a great answer from them. I was one of those kids and then grew up and became a writer and I ended up writing mostly about the science of biology, which is the science of life. So I write about all sorts of living things, not just human beings, but bacteria and maple trees and jellyfish, and just all sorts of things that we call alive. What kind of repeatedly surprised me was just how difficult it was for biologists themselves to agree on a definition of life. There are hundreds of definitions published in the scientific literature by scientists themselves, and new ones keep coming out. So overall, this time, scientists really aren't converging, I'd say on some very clear definition. And I just find that fascinating, because, you know, pornography is one thing. I mean, pornography is a human creation. So you know, I mean, how do we, how do we define games? Like, what's a game? Well, you know, like, that's, that's complicated, too, and for good reason, because it's a human activity. But, but life like, that's something that's one of the most important things that science can address. And if scientists are struggling to define life, if they're struggling to draw a line between the living and the nonliving, worlds, I just find that fascinating. And and what's really fascinating is that there are some scientists and philosophers who are, are actively grappling with this this problem. Why is it so hard? And does that actually tell us something deep about about life? how life began, how we'd recognize it, if it came in a very unfamiliar form on another planet? So yeah, so it's, it's something that sort of this is a book that has bubbled up within me over over many years.


Nick Jikomes 6:52

One of one of the things that strikes me is, it's, it's very intuitive for people to think about being alive, right? Where we all are alive, we all feel alive. And one of the interesting things that you brought up towards the beginning of the book is that's actually not universal. Because there are a small number of people that have a neurological problem that actually prevents them from feeling alive. So can you talk about what that syndrome was and how it originates?


Carl Zimmer 7:23

Yeah, so this is called Cotard's syndrome, and is named after a French physician named Cotard, who, in the 19th century, encountered a patient who explained to him that she was dead. You know, it's seems kind of paradoxical. How can someone be dead and be able to tell you that they're dead? Well, this person would had absolute conviction and had a very elaborate explanation about being dead. And over the years, from time to time, psychiatrists and neurologists have come across a few people who also will say they are dead, they will tell you how they died, they will explain to you how they're, you know, now just an empty husk, how they don't take a bath, because there, they know that they will just dissolve away because there's nothing alive inside them. And what, what fascinates me is that we sort of take it as sort of an objective, obvious fact that each of us are alive. So I know, I'm alive. You know, you're alive. But we never sort of question how we know that. Because we think it's obvious. There are living people for whom it's not obvious. And in fact, what's obvious is the opposite. And so what I think this points do is that there are actually they're actually, you know, brain circuits, deep in our evolutionary history that aren't responsible for sort of monitoring our own interactivity. And we have sort of taken those as being sort of kind of an objective reality. And that are knowledge, you know, whereas they're just these kind of, you know, signals that our brains are using to then come up with other decisions on a very deep, subconscious level. So the fact that we know we're alive is really more of a an unconscious response of our brains and you can pull out as it were, that's part of that circuit, and the brain will keep working language will keep working, and yet people will no longer recognize themselves as alive. So, so I you know, I think that's maybe the first opportunity to the most intimate opportunity to stop and say like, wait a minute, We're taking life and being alive and knowing that even yourself are alive for granted. So let's let's dig deeper.


Nick Jikomes 10:10

Yeah, so humans clearly have the sense we know that we're alive. We have rituals built around the birth of new life, and also the passing of people that were alive. And I'm curious, to get you talking about how how deep this sense goes, how far back in evolutionary history, it goes. At one point in the book, you talk about something called primate thanatology. So what is that? And what does it tell us about the evolutionary history of this awareness of life and death?


Carl Zimmer 10:37

So thanatology is the science of death? And so in a way, the question of what is life can be flipped on its head and say, Well, what is death? There was a French physician in the late 18th century named Javier be shot who, who said essentially, that, that life is the means by which death is resisted. So once death is no longer resisted, you're not alive anymore, you're dead. And so, so scientists have been wondering, well, what is our What is the means by which we know that others are dead? You know, people who have guitars and drums think themselves are dead? But how do we recognize death in others? And again, that might seem obvious, but you know, we we actually, we humans, in the 24th century, we'll we'll use all sorts of various means to determine death, we might check someone's pulse, and might say, like, well, if they don't have a pulse, then they must be dead. Unless, of course, the pulse is too weak, because you know, maybe they have hypothermia or something like that. And we just can't hear the pulse can't feel it. So. So it's actually difficult sometimes to determine if someone is dead. This this actually led to lots of practices, even in the 19th century where caskets coffins would be rigged up with strings and bells, so that if someone was accidentally given a premature burial, they could ring the bell when they woke up, and they could be taken out of the ground. Now. So So then this, this sort of raises the question, well, how far back in human history where we aware of, of death? And it one of the best ways to look at this question is to compare our behavior with the behavior of our closest relatives, the primates. And so even before Darwin, there were British naturalist who were, as they were starting to move into British colonies, like India, for example. And they would observe primates, monkeys, for example. And they'd be really struck sometimes by how the A troop of monkeys might have some sort of visible change when one of the members of the monkey troop died. And they would describe it like it was grieving that they would make the sounds that sounded like wailing, like grief. Now, you know, how much of that is anthropomorphism? How much of it isn't? Well, that was a question that Darwin got really interested in, because he believed that human behaviors connected with primate behaviors through evolution from a common ancestor. And much more recently, scientists like Jane Goodall would go and just spend time with wild communities of primates, in her case, chimpanzees, and just try to quietly observe them for long, long periods of time, and every now and then a chimpanzee would die. And she would sometimes notice that they would do very unusual things like a mother might carry their dead infant in a kind of peculiar way for a couple days after it had died. So more recently, we've gotten a lot more data on this, and so it does. And there are some pretty elaborate explanations for how it is that humans apes and monkeys share some of the same kinds of responses to death. And there there's some pretty elaborate explanations that are being explored now. So part of it is that, again, you know, death is a flipside of life in the sense that,


you know, we have, in our brains very sensitive biological circuits for detecting life around us. So we have this feeling that we're alive. And but we can also look around and we, we can pick out evidence of life very quickly, not in a sort of rational deductive way. But we just, we are tuned to certain kinds of features of biology, like biological motion. So it's so when when animals move around in a sort of purposeful way, boom, like certain parts of our brains light up in a way that's different than if we just see like a rock rolling down a hill, just passively obeying physics. So, so we see that in each other, and not only do we see that kind of biological motion, but all primates are very keenly tuned to faces. And so you know, you have, you know, you're looking at the the movements of the face, you're looking at the expression of the face. And again, we have, you know, quote, unquote, face circuits in our brains, which are very keenly tuned to that. And that's, that makes you aware of other living things that surround you socially. So you know, you want to be aware of predators, and might come and jump at you, but you also want to be, you know, very carefully keeping track of what, you know, members of your troop are up to, you know, how are you going to fit in the hierarchy and so on, are you going to get ambushed by a rival or something like that. So when death comes, that you have this member of your group, and it's, it's looking at that that member of your group, it lights up some of these biological circuits, but they're, they may not be moving at all, if they've sort of collapsed, they may, their body may be in some strange position that's very unfamiliar, you know, you don't can't fit that into your sort of biological shape recognition as it were, and their faces are no longer responding in the way that you're used to. And so, it could be that, for monkeys, at least, like, you know, why is it that monkeys will spend a long time near other dead monkeys? Well, maybe they are needing time to, to process this mismatch. And then eventually, they can recategorize members of their troop as dead. Now, in, in our own lineage, you know, that became more elaborate. So you have, you know, human relatives who start to bring dead members of their own species to, to these caves and seem to be leaving their, their bodies there. You know, is that a way to just to kind of get them out of the way? Or is it some sort of ceremony? We don't know. But, but then more recently, you know, in that within the past 100,000 years, you do start to see evidence of what we'd agree are sort of full on funerals, where, you know, bodies are being put somewhere in the ground, and some cases, being decorated with flowers with other objects. And so, you know, it could be that, you know, through language, we develop a more elaborate conception, both of life and of death. And then we carry that on till now.


Nick Jikomes 18:53

So, like the basic recognition that a conspecific has passed away, might go back 10s of millions of years in primates, but the fully modern conception of death that that we would call the fully modern human view of death goes back perhaps 100,000 years.


Carl Zimmer 19:14

That yes, I mean, again, when you're talking about human evolution, you often are having to just, you can't like point to a day on the calendar, you often have to like make brackets over periods of time, because all you got to go if R is the evidence, and the evidence can be fossils, the evidence can be comparisons between different species. In some cases, when you're lucky, it can be looking at genes. So So yes, I mean, there there are there are clearly funerals in the archaeological record within the past 100,000 years. So you know, it's there, then the question is, well, how far back do you go to the first funeral how wasn't that simply hauling a body and putting it, say in a cave chamber? Sort of changes from that and into into funerals with rituals? And and you know, what does? And why did it change? Did it change because we started to think about life differently in life in terms of, you know, our existence within a society? You know, these are, these are fascinating questions, some of them will get answered, some of them may never be,


Nick Jikomes 20:33

yeah, I'm really interested in that conception of life that you mentioned a couple minutes ago, of life being the resistance to death. And I think I think that actually ties into concepts that people who think about this have brought up in the past around, you know, life being, in some sense, a resistance to entropy, to the natural tendency of things to become more disordered in the physical world, and the homeostatic regulation of various forms of chemistry, that seem to be characteristic of life. Before we get there, I wanted to ask you about a couple other things. There's various places in the book, where you sort of look at really extreme out there forms of life to get out this question of like, how, how far can life go? At one point, you bring up crypto biotic species? So what is that? And how do their extreme abilities allow them to defy defy death in ways that are remarkable.


Carl Zimmer 21:29

So this is a, you can think of it as a third state between life and death. Now that might seem impossible, like either you got life or you got death. And that's it. But the world doesn't really care about the lines that we might want to draw. And the first person to really appreciate this, this third state was Anthony, Vaughn, Leona, who is one of the pioneers of microscopes, he built himself microscopes, and use them to discover all sorts of things in the 1600s that no one else had seen before. Discovering different kinds of cells, just discovering lots of single celled organisms. And, you know, really kind of opened up this hidden world to all of us. And he, he noticed one day, it was a kind of a, you noticed his gutter on his house was full of water. So it's like, well, let's see what's in there. So He scoops it up, and puts up dropping and his microscope and he sees, you know, odd little animals with what look like wheels on their heads. They're spinning around the microscopic animals called rotifers. He's like, Oh, that's cool. Okay, so I got this little zoo in the gutter of my house. Very interesting. What was really surprising was that, you know, the summer comes, the water dries out, and there's sort of like a, you know, kind of red, caked, dried mud in this gutter. So it's like, Huh, I wonder what are what's what that's like now, so he just goes any cracks off a bit of this dried mud. And he, you know, he's looking at it, and he and he decides to wet some of it and put it in, put it under a microscope. And he sees these rotifers again, but they're, they're just totally they look, there, they have a strange shape and sort of squashed in there. And they seem like as far as he can tell they're dead. But when he takes a look at them later, they're their wheels are spinning on their head again, like and they're just spinning around. And he's like, Well, wait, whoa, like, these were dead. And now they're not dead. You know, it's one thing for, you know, seeds, for example, to go dormant. You know, and that's, that's a remarkable thing in itself. But, you know, we think of animals as well, you know, like, they, they they are irreversible. Right, right, you know, they've got all these cells that are being very active, you know, and then they may have circulatory systems, they have to keep up their metabolism, like, you know, you're an animal like you're not, you're not a seed, right? So, what was going on with this animal? And so then, in the 1700s, other scientists would slowly discover other examples of this little nematode worms, for example, that they could be dried out and be in this weird state for years and then just throw them in water and boom, they're back in action. And so so so they're actually like been just, you know, this has been such a point of fascination and controversy for for centuries actually, because biologists just saying like, what is going on? And it is they have a better idea of it now, but I wouldn't say that it's all figured out. My favorite example, has to do with another little animal called tardigrades. And if you've ever seen a picture, tardigrades, you know you, you'll love them forever. They're so cute and they're, they're there. Some people call them water bears and some people call moss piglets, because they've got these eight legs, no eyes, just this giant sort of sweared snout with a sort of circular sucker shaped mouth. And they just amble around on their eight legs and, and eat and just do their thing. On loss in the ocean all over the place. They're everywhere. So what's kind of astonishing is that the you can, it's really there, it's really hard to kill a tardigrade.


So for example, like if you like, if we just, you know, locked you in a room and didn't give you water, you would eventually die of thirst. You're no it particularly help if you know, we turned up the heat in the room to me, basically, what would happen is that, you know, the water content in your body would start going down and down and down. And, you know, many of the proteins that carry out essential chemistry in your body just don't work anymore, unless they unless they're in enough water. So chemistry fails, you die, we're done. And, and you know, that's irreversible. Like, it's, you know, I can't come back in the room and just dump a bucket of water on your face and say, like, Hey, wake up, like, That's it, goodbye. But that's not the case for tardigrades. So tardigrades, they can dry out and they can stay dried out for decades, we actually don't know how long they can stay dried out. At the I believe, I, someone recently told me that the record is a century but I need to double check on that. I know that it's like, over 50 years. But anyway, like, it's kind of a hard thing to prove, since it takes so long to test. But point being these are amazing animals. So how do they do it? Well, it appears that as they're starting to dry out, they can start making a kind of a replacement for water, they make a certain kind of sugar, which has some of the chemical properties of water. And it can, so it will sort of sub in for water, it doesn't evaporate so so it's not going to disappear the way water did. But this in a way, you know just buys the tardigrade time, because the long term solution that the tardigrade is creating for itself is it starts making a very special kind of protein that just starts to sort of fill up its cells and basically kind of lock in all the other molecules in place. And so it's kind of like glass. So eventually, you can have a completely dried out tardigrade that is been turned into this kind of protein glass that they they will just hang in there in this state for a very long time. And you can you can meet scientists have put tardigrades on rocket ships, and sent them into Earth orbit that gone into space, you bring them back. And then you do give them some water, they're fine. They can they can handle the the vacuum of space, they can handle the radiation they encounter there, because they are frozen into this third state. And what scientists will call crypto BIOSIS. And there are probably a lot of other animals, some plants and some fungi that can do something similar. We can't but you know, who knows? Maybe there are some things we can learn from crypto BIOSIS that we can harness for medicine, for example. Maybe there are some secrets there for keeping organs more viable for transplantation or, you know, some other types of secrets for preservation. But who knows, who knows.


Nick Jikomes 29:36

One of the other interesting life forms that you spend some time on in the book are slime molds. And a couple phrases that popped out to me or you know, one person described them as a moving stomach. And for those who don't know what a slime mold is, we'll unpack it but you know, it's it's mold. It's basically slime. And so it's not that impressive at first blush, but these organisms can actually do very impressive things that we might characterize as problem solving, and they have you say, a brainless kind of memory. So what are slime molds? And how do they work?


Carl Zimmer 30:12

So, people may have seen slime molds without knowing what they were seeing. They if you've walked in the woods on a summer day, you look down at a log or on the ground, you may have seen a strange little spattering of brightly colored blobby things. And those are slime molds. And there are hundreds and hundreds of species of slime mold around the world. And all sorts of great names. My favorite is dog vomit. Because that's what it looks like, it looks like you are some dog going in the wood threw up, but it's actually a living thing. And actually, when you're looking at that, dog grommet, or some some bright yellow, strange or spattering of tentacles, these things can get to be the size of a teacup, or even a placemat, you're looking at a single cell is just one cell. It's not a multicellular organism the way we are, or the way a mushroom is, or a plant. It is one huge cell. And, you know, it's got DNA in it actually has its DNA, like ours is packed into a nucleus, but it may have millions of nuclei inside of it. So that if, if, if the weather gets dried, part of it may break off and fly away. And if it lands in a wet place, it's got those genetic instructions inside of it to just start growing again into another giant cell they went to slime molds meet, they can have a very bizarre form of sex where they actually create an entirely new cell that has a combination of their genes, and then that goes off and does its thing. So so I really fascinated by slime molds because of the fascination that a lot of scientists have with it. It's because scientists you know, they would they would take some take a sample of slime mold and bring into their lab to understand well, how does it go about its life. slime molds eat bacteria that they find growing on surfaces in the woods. And so you can actually feed slime molds in a lab. I spent a lot of time at a lab of a scientist named Simon Garni a who's at the New Jersey Institute of Technology and his favorite food for slime molds is just a few flakes of oatmeal. Because oatmeal like lots of other things has bacteria growing on it and the slime molds love it so so if you put a little little pile of oatmeal in a petri dish and you put a little slime mold and on the other side of the petri dish, very slowly like over the course of hours, that slime mold will extend tentacles and will eventually reach the oatmeal and will envelop it in the Simon Ghani. He studies a particular kind of slime mold called Fae serum which has these beautiful yellow tentacles. And so you know, within a day that lump of oatmeal is going to be turning gold, because the slime mold is feeding off of it. So that silo had to find it. And it doesn't have eyes, so it can't see it. And what it seems to do is it seems to it seems to have a kind of maybe think of it as a sense of taste so that as molecules are diffusing from the oatmeal across the petri dish, it can taste the flavor of this oatmeal from a distance and try to follow that taste towards the source. So that's pretty complicated. That's pretty interesting. But that's just the same old just getting started honestly. So for example, scientists can build a maze for a slime mold. So you just take little pieces of acetate and you lay them down on a dish and in basically create walls of a maze. And you put the slime mold at the entrance of the maze and you put oatmeal at the exited the maze, and then just go away.


And when you come back, maybe in a day, the slime mold will have spread out and explored the maze, and eventually made its way to the oatmeal. And what's amazing is if you take sort of stop action pictures of this, you can see that once it's found the oatmeal, it retracts, its tentacles that we're going off into dead ends. And so what you're left with is the shortest path through the maze, to the food, you know, you can have mazes where, you know, there are several different potential paths you can take and but some are longer than others, it will sort of pare itself down to the shortest path. Because that's efficient, you know, you doesn't have to like waste energy maintaining a bigger body, it can just focus on eating the food. So So scientists are finding that, that these, these sign molds are doing all sorts of astonishing things. They're almost like mathematical, they seem to be doing a kind of computation. And you mentioned memory. I talked about this in the book that they you think, oh, go ahead. A slimeball had memory, it doesn't have a brain doesn't have a hippocampus, like there's no place for memory inside a slime mold. And the fact is that slime molds may not need to actually store their memories inside of themselves, they can create sort of an external memory. So what they do is they you know, they extend their tentacles in different directions. And when they retract their tentacles from a dead end, they can still sort of sense the the flavor of having been there, they leave behind a signature of themselves. So sign models tend to avoid where they have been before. And this is like, so they can remember, in a sense, their failed explorations. And so if so that means that you don't waste time going in places that you tried to go before and didn't manage to get any payoff for it. So you know, you can. So slight, you can sort of build like, a very simple experiment is to put some oatmeal on one end of a dish, but the slime on the other the dish, and then create a kind of a U shaped trap between the slime mold and the oatmeal. So the owner of the SIBO, goes straight for the oatmeal as you're going to hit this wall, the end of this U shaped barrier. So what the Simonds will do is they'll they'll go, hit the wall, and then they'll sort of start to explore around a little bit. And eventually, they just shoot around, they go backwards, back around the the edge of that U shaped wall, hook the hook around on the outside of it, and then go around the outside to get to the food. So they're actually able to override that natural movement towards the food, they actually go backwards, because they're just also trying to avoid where they've been. So that combination lets them do this incredible sophisticated navigation through through this space. So why is this in a book about life? Well, in life sedge, i in terms of coming to terms with life, I wanted to look at what we think of as some of the hallmarks of life what are what are things that, that things that seem to set life apart from everything else, and what it's a combination of hallmarks. And one of those hallmarks that you see again and again, is that living things have what Simon Gournay who studies the slime molds would say is intelligence. He has a very broad definition of intelligence, he thinks slime molds are intelligent. And intelligence is basically arrangements of molecules in a sense that are able to take in information from the environment, use that information to make decisions about what to do next. And those decisions are better than random so that you know a slime mold is not just like going around randomly a slime mold does a really good job of getting to food. And and you can study that down to these molecular components with a slime mold because you don't have to worry about a brain or anything complicated like that. It's just one cell. And and it's, it has intelligence and it's almost in its purest form.


Nick Jikomes 39:59

Yeah, one The things I want to talk about is the concept of homeostasis. And this is often brought up as one of these essential features of life. So we can look at something like a slime mold, or tardigrade or a human. And all of these things are very, very different in terms of how they behave, and how they work. And yet, they all share something in common. And that has to do with the fact that we share a common evolutionary history ultimately. And you know, we're not completely different. There's actually a lot of shared processes there. And you talk at one point in the book about this concept of homeostasis, and there's a quote you have in there from a very famous biologist from the 19th century, named Claude Bernard. And he said, quote, all vital mechanisms, however, vary, they may be have only one object, that of preserving constant the conditions of life in the internal environment. So what is homeostasis in simple terms, and can we point to that as perhaps being the essence of life?


Carl Zimmer 41:02

Well, certainly is a hallmark of life. And what homeostasis is, is this stability. So So life is remarkably stable, even when its environment is changing. And that stability can take many different forms. So you know, for us, for example, we have lots of different kinds of homeostasis. And if we lost any of them, we would die. So for example, our body temperature, generally stays incredibly stable, even when it gets cold or hot around us, we have all sorts of ways of keeping our body temperature at a very narrow band of temperature. And that's a temperature at which our proteins are, you have evolved to work at an optimal way, our blood sugar level stays at an optimal level, you know, and that's true, even if we haven't eaten for hours, or if we're in the middle of eating, you know, a candy bar, like we can keep our blood sugar very stable. And there are all sorts of other parameters that, that we keep really tightly controlled all the time. And, you know, it's just kind of an, these are things that just sort of keep themselves stable, they have to be maintained. Now, you, there are different different species will have different kinds of homeostasis, because they have different needs for staying balanced. But it's just a theme that you see over and over again, throughout life. One example I really like is with bats. So when I was working on the book, I went to visit some hibernating bats in an abandoned mine in New York. So here were bats that were surviving winter in the Adirondacks. And they were doing it by going into this abandoned mine, where the temperatures you know, maybe around 4045 degrees, and they had dropped their, their body temperature to that level. Now, they were still breathing slowly, but you know, they were still managing the oxygen levels in their body. So they had that kind of homeostasis, their their hearts were still beating, so they still had to serve a certain blood pressure they're maintaining. So they have that inner balance, they just shifted it to a different kind of homeostasis that would let them survive for months without eating a thing. And, and yet, you know, in the summer, you know, these bats, they, they they rouse out of their hibernation, they fly around, and, and they still keep their inner environment stable. Their their temperature is much warmer. But, but it's stable at that new setpoint. You can even think about the way they fly. Like in order to fly, you have to stay stable, even if you're being buffeted by by winds and such and bats are amazing at doing that you you blast a gust of air at them in my tip one way, but then they very quickly write themselves. And that is actually I think, one of these universal principles of life. How does life keep homeostasis? It uses things like negative feedback. So it's the same same kind of system that your thermostat uses to keep your house at a certain temperature, or your car uses with cruise control. So if you're going up hills and down hills, it senses that you're, you're moving away from the setpoint. And has these negative feedbacks built in, to bring you back. And so, so that so again, you know, the these, we need these stable inner environments to sustain life. But we also need to sustain the inner environments with negative feedback.


Nick Jikomes 45:32

So what about evolution is Darwinian evolution, perhaps another one of these hallmarks of life, I know that at least at one point, NASA more or less defined life as any self replicating, self replicating entity capable of undergoing Darwinian evolution. And, you know, defining life actually is really important. If you think about the, the search for life elsewhere, we need a good definition of what it is before we can find it at least if it looks, you know, very, very alien compared to life on Earth. So do you think that Darwinian evolution is another one of these hallmarks of life?


Carl Zimmer 46:08

Well, I have a chapter in the book about evolution, because certainly, you know, everything that we call alive, is the product of evolution. And evolution is not something that just happened a long time ago, and then stopped. Evolution is is built in to the process of life. Living things can reproduce. And in the process of reproduction, they pass down the genetic information that allows their own cells to function their bodies to function. And when you copy genetic information, it's just a series of chemical reactions. And there are there can be what we would call mistakes. In other words, the, the new copy of the of some DNA won't be totally identical to the original one. And so these are mutations. And so this is the raw material for for, for genetic variation. And when you have this kind of variation that can be passed down to for to future generations, you have the opportunity for natural selection. So, you know, if there's any mutation that is really harmful, or is really beneficial, evolution just takes over, it will make those mutations either less common or more common. So evolution is sort of built in, to, to, to life as we know it. So NASA, in the early 1990s, you know, they they brought some scientists together to talk about looking for life on other planets. And the scientists after a while realized, you know, we really need to agree on what we're talking about before we start looking for it. And so they had a very long conversation, you know, they wanted they wanted to, they, they weren't philosophers, you know, they just what they said they just wanted a working definition, something they could use for research. So something short, sweet, practical, and really focused on what NASA wants to do. And so they came up with this definition of life being self sustaining chemical system that's capable of Darwinian evolution. So the self sustaining part is, is, you know, takes into account the homeostasis, we've talked about metabolism. And then, you know, being capable of Darwinian evolution, there's, you got the reproduction there, you've got the genetics built in, you know, you're not saying that it has to be made of DNA, but we're gonna, you know, they're just saying, like, we're gonna call life this. It's, you know, it, but it's, it's tricky. I mean, obviously, it's really important to understand evolution to understand life. But if you go to another planet, do you have and see something you're like, Hmm, I think that might be live. I don't think that's a rock. I think that's a living thing. Well, do you have to prove that it is capable of evolution before you will really demonstrate that it's alive. I mean, I can't prove to you that I can evolve like I can't evolve. I'm just a I'm just a guy I've got I'm a father of two kids. I don't know what they, their descendants work. Like, who knows? Here I am. So how so? So in a way, it's a even though supposed to be a working definition. In a way it's kind of impractical because it doesn't really tell you Well, you're supposed to do with this concept of evolution, you know, when you when you're on another planet and trying to figure out what you're looking at?


Nick Jikomes 50:10

So, is there anything special about the type of chemistry that we find in life? So, you know, you mentioned DNA, and the ability to replicate imperfectly, which gives rise to the evolutionary process, but is the type of organic chemistry that we see in life somehow different from the type of chemistry we see in the nonliving world?


Carl Zimmer 50:33

Well, certainly, you know, when you look at some of the, you know, molecules that make us up, or that make up, you know, a tulip, they are different than the kind of molecules that you will find floating around in a interstellar gas or deep inside the earth, you know, living things can build stuff, they are sort of these these, you know, you know, blind Lego builders, you know, like they can, they can assemble very complicated proteins and other structures. And so some scientists have argued that have put forward something. This is a leak crown in years of Glasgow, Sarah Walker, Arizona State University and Kate harmala. At university, Minnesota, they've been exploring something called assembly theory, which is basically a theory that posits that maybe we can distinguish life from non life by trying to determine how many steps it takes to build stuff. Now, these can be steps that just happen without the presence of life, just you know, if you go to some hydrothermal vent, for example, where lots of minerals are surging around at the bottom of the ocean, there's a lot of weird chemistry there. And chemical reactions can build on chemical reactions, and ended up building something really weird, you know, so these giant chimneys, you know, they're not, they weren't built by living things, it's just geology, geochemistry. But you know, a big DNA molecule, or even a hemoglobin molecule, a protein like these things, that it takes many steps to build them. And they're built in a very specified way. So yeah, so that's, that is, that is one way that, you know, maybe not only can we start to think about life, theoretically, but also, it could be a very practical way of looking for life on other planets, you might actually just not even have to visit a planet, maybe you just look with your telescopes at the atmosphere of a planet, maybe in another solar system. And then if you can figure out the composition of some of the molecules in the atmosphere, well, maybe those are the kinds of things that have to be assembled, they don't just sort of form through atmospheric chemistry.


Nick Jikomes 53:18

So the idea is that life assembles things, the assembly of those things, proteins, or whatever, takes many, many steps. And it just, it sounds like it's just sort of a fact. And so far, as we understand that, you know, there's certain things certain forms of chemistry that can't happen in just one step. It takes many, many steps. And that seems to somehow be tied to


Carl Zimmer 53:42

life. Yeah, I mean, in one sort of preliminary study, the Quran and other scientists said, like, let's just, let's just take a look at a huge number of compounds, just a vast, vast number of compounds, some of them will be things we get from living things, some of them will be stuff, we get pulled out of meteorites, some of them will just pull out of Iraq or out of the air or whatever. And, and they had a sort of a procedure for basically, you know, determining the number of steps through chemistry that could produce them. And, and basically, like, anything that wasn't alive, they couldn't find anything that required more than 15 steps. Only if you go above 15, only living things are there. So there are some there are some components, some organic molecules that you don't need 15 steps, there are some there on the simpler end of things, but But life is capable of this incredible elaboration. Because once you got the system set up, essentially, I've just I mean, you know, I mean, there are the chemistry that we use and that life as we know it use, it's just basically me taking an amino acid, and I'm going to stick it on another amino acid. And I'm gonna go look for another amino acid, I've got, you know, 20 or so to choose from, and I'm is different kinds, stick that one on the on the end, and just keep sticking them until I got a chain, and then they let it go. And then it spontaneously folds into some very cool shape. And then, you know, other proteins might sort of fine tune it. And there you go. So So, so that, so it's an open ended process, I mean, you can keep just sticking on more and more amino acids onto a protein. And then you can then cut that protein into two different pieces. If you want or combined two proteins together. It's an open ended creative process. That's one of the things that makes life so wonderful. And and you can encode those proteins with with genes where you got these genes have these four different sort of genetic letters as it were, and you just have this amazing, open ended capacity to rewrite genes to make new to make new proteins or new RNA molecules. And so So life has this capacity for assembly far beyond anything else that we know of in the universe.


Nick Jikomes 56:28

So is anyone using this notion, this idea of assembly theory to actively look for potential signs of life in planets outside of Earth right now?


Carl Zimmer 56:42

They are there they're trying to develop some strategies for for that search for extraterrestrial life. A lot of this is really going to scale up in an exciting way as astronomers put new generations of telescopes into space.


Nick Jikomes 57:03

So one of the things that I wanted to ask you about is something that a lot of people have been thinking about for the past year, which is viruses. So we talked about homeostasis, we talked about Darwinian evolution with those things in mind, are viruses alive or not?


Carl Zimmer 57:23

Well, it does come down to what we think of as being alive. And so we just talked about the NASA definition of life. So there, according to NASA, in their working definition, in order to be alive, you have to be a chemical subsystem, a chemical self sustaining system capable of Darwinian evolution. So if we look at viruses, well, Darwinian evolution, they've got that covered, because viruses mutate very quickly, and they, you know, every time that they infect a cell and produce new viruses, those viruses often have a lot of new mutations. Most of those mutations are harmful, a few may be beneficial. And so those beneficial mutations can help the virus adapt. It can adapt within our own bodies. And it can also adapt over longer timescales going from person to person. And so, you know, it's it is this evolution that gave us our COVID 19 pandemic by a a Coronavirus that adapted to humans, having been in bats originally, and the Coronavirus is continuing to evolve. And it's branching into new lineages, the those some of those lineages have mutations that are increasing their transmission, and those are becoming dominant. So we can see that viruses are capable of Darwinian evolution right in front of us. They're not self sustaining, in the sense that maybe NASA meant NASA was thinking, you know, the scientists, if you ask them, they were thinking about homeostasis and metabolism. And so that's taking place inside of cells with all sorts of complex chemistry, you know, proteins that are regulating each other feedback loops built in genes making new proteins when they're needed, proteins being destroyed when they have to be gotten rid of this incredible choreography that goes on inside of all of our cells. Viruses, you look inside a virus, there's none of that. There's some genes usually those genes are held in place by a protein, and that's it. So the way that viruses are able to replicate and to evolve is that they just go into a host cell, they infect a cell, and their genes are used by the cell to basically rejigger the cell itself. So that it is now all about making new viruses. One of the scientists involved in the NASA working definition of life is named Gerald Joyce. And he was once asked, Well, what about viruses? And he was, he was very clear about he said, Well, according to this working definition, viruses don't make the cut. So you know, a lot of a lot of scientists will tell you categorically that viruses are not alive. However, there are viruses there are. However, there are scientists who will tell you that viruses are definitely alive. Because they can evolve because they're so important in ecology, because they are so important to the to the web of life. You know, we have viruses, viruses have contributed to our own genome, for example, we have hundreds of 1000s of pieces of our DNA that came from viruses.


And so, so you've got those people arguing that viruses are alive. And then you have people who are kind of in between. So there's one French scientist named Patrick forterra, who argues that, well, when a virus is just floating along on its own, it's it's not alive. However, when it goes into a cell and infects itself, that combination of the virus and the cell is now a new, distinct living entity. So the virus gains this this life every time it infects a cell. That in a sense, that cell is the virus because the purpose of that cell is now to make new viruses. And it is remarkable when you look at what happens to a cell when it gets infected by say, the new Coronavirus SARS cov. Two you can see it just it looks different in the cell builds little incubation chambers for before creating and and fine tuning new viral genes. It doesn't do any of the of the stuff it needs to do for itself to grow and to divide that all stops because it's now all of its chemistry is in the service of making new viruses. So this question is vire is a virus live is not settled, but it's unsettled in the most interesting way possible.


Nick Jikomes 1:02:57

One anecdote that I did want to ask you about that came up in the book. It was from an episode in the early 20th century. So there's this guy named Henri Bergson. And he wrote a book. This was a philosopher, I think he wrote a book called Creative evolution, and apparently was a best seller at the time. It was like a popular science book. And, and you even write that when he traveled to New York to deliver a series of lectures. He reportedly caused one of the city's first big traffic jams. So what was who was this guy? What was this book about? And why was there so much discussion and debate around it?


Carl Zimmer 1:03:36

Let's Henri Bergson was a French philosopher, who was um, he wrote a number of books what the perhaps the most famous one is the English translation is creative evolution. And in in a way, Berkson was was reacting against the modern science of biology that was emerging at the time. There there had been attention ever since the scientific revolution in the 1600s, about the nature of life. Because in the 1600s, you know, physicists were starting to think about the the world is matter in motion. So you have matter that is passively being moved around by mechanical forces. And you can explain all sorts of things this way you can you can explain how planets go around the sun, you can explain why it is that a cannonball fired at a certain angle will land in a certain place, very far away. It's predictive, it's powerful, and it leads some natural philosophers. These are the predecessors of scientists to argue that well, living things are also explained through these mechanical forces. These would sometimes be called mechanist. And did Rene Descartes was perhaps the most famous of the people who saw nature this way, and really saw animals and other living things as essentially machines that merely in a sense that that you could explain them as, as parts that were moving to carry out certain functions in response to just just understandable forces, mechanical forces. And there was an opposition to him. From other thinkers who argued now well, there's got to be it, that's you can't just reduce life, to mechanical forces. It's not just matter in motion. And, and these people would sometimes speak of a vital force, sir, there was some sort of vital force that that organized living matter, that gave living things, they're the goals that they they sought, you know, they they're, they're eating to survive to reproduce. It's not just a random movement of particles. Eventually, these people can became to be known as vitalists, often in a derogatory sense. And this tension between sort of mechanism and vitalism it really It's fascinating how that that's the debate over what is life for for centuries. So getting back to Henri Bergson, you know, Bergson is writing at a time when, you know, the mechanist have been turned into biochemists. In other words, they have been able to understand living things in terms of enzymes, they're they're starting to see some of the reactions that carry out these these vital, essential processes in life. And so on the one hand, this is very exciting. I mean, the Board did the biochemistry saying, Look, we are unpacking the secret of life. But for a lot of people this was very unappealing, it really felt like you're just again, reducing nature to just these molecules that are passively responding to these forces. And so that you know, were quote unquote, just chemistry. So any Bergson not, he was not a scientist, but a philosopher, but he built into his philosophy, what you might call Neo vitalism, in the sense that he still believed that there was there was something something beyond this kind of physics and chemistry that define life, and in fact that there was something that propelled life forward that that evolution in a way, like have there was some sort of almost mystical force that was propelling evolution to more and more higher and higher levels of complexity. And maybe the, you know, the universe was was, you know, carried along in some some direction. Now, his his arguments, I'm not doing justice to his arguments, because they're a lot more complex than that, but But I will say that,


you know, he, he was incredibly popular, I mean, creative evolution was a huge bestseller. And as you mentioned, he may have created the first traffic jam in New York when he showed up to talk about it. So people were hungry for this kind of view that sort of elevated their lives above biochemistry. But the biochemists themselves, you know, I mean, I think they found that talking about vital forces, and vitalism just wasn't helpful. Because all these people were doing is just pointing to things that we do not yet understand. And that's not, that's not really evidence of anything. It's sort of like saying, like, well, we don't know how whales evolved because whales couldn't have evolved. Because we don't have any fossils of whales with legs. That's true until you find a whale with legs, which scientists did in the 1990s. So that's, that's no longer a thing. So but at the same time, you know, the spirit of vitalism, I would argue, and I talked about and I've said it indoors, because, you know, you do need to talk about life beyond just Newton's laws of motion or new rules about you know, the bonds between atoms. There are, there are higher levels of organization in life that you have to recognize in order to make sense of life. You don't have to be a vitalist to recognize that, but But you know, in a way, you know, you some of the questions that vitalists had even, you know, 100 years ago, have yet to be answered.


Nick Jikomes 1:10:20

Do you think that perhaps a modern, a modern answer to the vitalists comes from things like assembly theory, where it's not, it's not merely mechanistic, and just pointing to the biochemistry, but a key aspect of that theory is that there is something special about the way that information flows through things that we call living systems, that the information is in a sense, controlling the way that the matter organizes itself. Is that does that seem right to you? And can you just unpack what that thinking is around assembly theory?


Carl Zimmer 1:10:58

Well, assembly theory, in a way, kind of sort of takes it as a given that, that life is capable of the sort of open ended creativity that we're just talking about. How how that how life was able to start assembling in the first place, actually, is a very hard question. But But certainly, you know, you mentioned information and information probably is, you know, part of the answer to these big the, to these big questions. People think of information, as, you know, in terms of like, you know, computer files, and so they might think of information strictly in terms of what what's the information encoded in genes. It's a legitimate thing to think about information that way, but, but physicists will also talk about information in lots of lots of other contexts. They will often talk about, well, if you know about something, how much uncertainty does that reduce about what you know about something else. And you can actually sort of trace that reduction of uncertainty through a system. And physicists will call that a flow of information. And cells have very interesting flows of information, the ways that proteins interact with each other and interact with genes, proteins, switch some genes on and turn them off, in response to things that are happening on the outside, the flow of information is very peculiar, it's and that might be a clue about, you know, a theory of life that is that is based on information, that full theory does not yet exist. You know, there are other theories that involve the dissipation of energy, for example. So you know, if you got a tree growing in your yard, energy is coming into the tree, the tree is absorbing energy through photosynthesis, and then as eventually releasing that as heat. But it's really good at that that dissipation of energy. And in the process, it is. It is building, it's wood, it's bark, it's leaves and, and flowers and making new trees as well. So there may be something about how it is that matter dissipates, dissipates energy, that under certain conditions, you get this sort of self organization that is so distinctive of life. And so it just may just happen spontaneously, when you get the parameters just right. A scientist named Jeremy England has done a whole lot of work on this particular view of life. And it might help to, sort of, in a sense, show how life is an inevitable and then an inevitability. You know, you set up a universe like ours, sooner or later, somewhere you're going to get life.


Nick Jikomes 1:14:22

I wanted to ask you a little bit about your writing process. So you've written a large number of books at this point, would you say that you have a writing process for your books, like do you? Do you decide that you're going to write a book about a topic? You start writing and then you finish writing and you're really focused on that book? Or is it more of an organic thing where you're sort of writing articles and taking down notes over time and eventually you have an assemblage that you then recognize is potentially book worthy.


Carl Zimmer 1:14:57

Sometimes I I just decide I want to write a book on x. And I just go out and I do some research on x. And just outline the structure of a book and then write it. So for example, some years ago, I got interested thinking about the brain. And how was it that we came to see ourselves, in a sense, as our brains or that, you know, our brains were where things like emotion, took place, reasoning, and so on, like, when did the brain become the center of our existence? And that will that would be, I'd be curious to know what the answer is. And I started looking around doing, you know, starting to go beyond the history of neuroscience as I, as I knew it, then and just started digging deeper. And just was fascinated to discover how neurology was invented very quickly in the in the mid 1600s. By just a few people, most interestingly, an English physician named Thomas Willis, he actually coined the word neurology and published the first book just about the brain, it included the first really accurate pictures of the brain made by Christopher Wren, and, and I just like, wow, I, I want to write about this. And there was no book about this before. So I wrote a book called Solomon flesh. So there was a case where I just said, Okay, I'm interested in this subject, I'm going to do a book on it. There are other cases that are more like what you just described, where I will just be going about my business as a journalist writing newspaper articles, articles for magazines, like the Atlantic or National Geographic. And then after a while, I might kind of look back at you know, at a few years of material and think my seem to be continually gravitating back to a certain question, maybe, maybe it would be good to just stop and take all that material as the starting point, and dive deep, and really, really try to bring it together into some kind of a unified story. So I would say live sad. Just one example of that. Also, my, the book I wrote in 2018, my previous book, she has her mother's laugh, and they're, you know, I was writing about heredity, which was something that I had been writing a lot about in various forms, you know, you know, what, as, as scientists were able to analyze, you know, DNA from millions of people and start to reconstruct our genealogy and look at how it related to each other. What was that telling us, both about heredity, and about maybe are the the ideas about heredity that we share that we're wrong about? Things like epigenetics, which were really coming to the fore, and I'd been writing about, and CRISPR, in a sense, is the manipulation of the future of heredity. And I was writing a lot about crisper as well. So I want one I just said like, wait a minute, like, you know, these 20 or 30 articles, it's all about the same thing. So maybe I need to create one thing where I really explore this in a unified way and dive into the history as well.


Nick Jikomes 1:18:37

So when you're actually in the midst of writing a book, say, so you've decided that you're writing the book, you know, you're writing it, maybe even have a, you know, a deadline you have to hit. It's no longer just an idea how regimented is your writing process? Are you you know, some writers describe a very regimented process where every morning for four hours sit down and write no matter what, and others have a very different style. How would you describe your approach to actually doing the writing and sitting down to get it done once the project is underway? Well,


Carl Zimmer 1:19:12

you know, certainly once you sign a contract, that that does clarify the mind. And you know, you're not going to just sit around at that point and just wait for inspiration to strike or or you know, you're not going to only write when you when you only feel at your very best. So yeah, I mean, you need to you need to be writing or researching every day, whether the muses are looking kindly on you on that day or not even even allows a day of writing is better than not writing at all. And you know, at least you have raw material that you can go back to and look at and say like, oh, wait a minute. I can see why this is so terrible, because there's something else I need to be Talking about her if I flipped this chapter upside down, it's great. So, yeah, so I'm I don't think I'm, I, I do hear sometimes stories of writers who say like, you know, I get up at five in the morning and you know, have a bowl of muesli and then from 6am to 10am I do nothing but write. That's not me. You know, I might, some some days I'm writing very late at night. Other days it's the morning when I'm writing some days it's you know, it's kind of a slow creeping crawling process, maybe I'm, I'm just realizing have to stop writing and do a lot of reading or call someone because I don't really understand what I'm writing about. And then other days, I'm just on a, just on a tear. And I really, you know, I'll tell my wife like, I'm coming out for dinner, but then I'm going back in because like, I, you know, I don't want to I don't want to waste this, this this role that I'm on.


Nick Jikomes 1:21:06

Interesting, and how is there anything else actually, that you want people to know about life's edge that maybe we didn't cover?


Carl Zimmer 1:21:16

Well, one thing that, that I find interesting is that, you know, I went and spent time with a lot of scientists who are


who are really thinking about life, at its at its extremes. So for example, one person I wanted to talk to with was Laurie barge. She's an astrobiologist at NASA's Jet Propulsion Laboratory. And what she's been doing a lot of work on recently is thinking about these icy moons, such as Enceladus, around Saturn as possibly being where you can find life. So Enceladus is covered in ice, but there's a liquid water ocean under there. And as the moon goes around Saturn, there are these tremendous tidal forces that kind of stretch out the core of the moon, in different directions. And that generates heat. So you have this energy source, and a liquid ocean, and lots of interesting chemistry happening. So what's going on under the ice, she would like to know. And so you know, NASA is thinking about maybe sending some probes back out to places like Ansel it is to really figure that out. And what's fascinating to me is like, I will talk to someone like her and say, like, Well, do you do use this NASA definition of life that, you know, what do you what what, what would something have to be for you to decide that it's life? And you know, she says, Actually, she tries to avoid definitions of life, because they get in the way. She wants to just learn as much as she can about what's going on on Enceladus. What is the chemistry there? You know, it could be very interesting chemistry, that's just the result of these hydrothermal vents at the bottom of its ocean, could forget some very weird structures and very weird molecules. And that'll be cool. And maybe some of them will be starting to replicate in some peculiar way. But she's not about to sort of say like, well, if it's not life, I don't care. She wants to go there and get some ideas about life that maybe she didn't already know. And when you talk to philosophers, they will actually say that, well, we should have this more sort of open ended exploration of life rather than arguing about what is the definition of life. One philosopher I spoke to named Carol cleeland. Put it this way, she's like, essentially, it's like, you're in 1500. And you go to an alchemist and say, Well, what's your definition of water? And you actually, you can look at books of alchemy and see what they said. They would say things like, well, water is something that's transparent, and is liquid, you know, water is wet, it can dissolve certain things. That's just the list. Like saying like life is something with homeostasis and metabolism and evolution. And the problem with lists is that well, you have a situation like okay, the water gets cold and now you have ice is that water? The Alchemist has to look at his list and say, No, I guess not because it's not liquid, so it's not water. And also you have different kinds of water. You have things that are transparent and liquid, and can dissolve metal. So you need to give them so they must be a certain kind of water, a royal water, it might be called. Now, if you want for 300 years, you know, you could talk to a chemist and a chemist has a theory of matter now, a theory of atoms and molecules, elements, you know, atomic starting to get ideas of how atoms form molecules. And now you can start thinking talking about water in terms of a molecule of hydrogen and oxygen to hydrogen through an oxygen. And, you know, eventually through experiments and, and modeling and so on, you can understand how it is that what we call liquid water becomes ice through the change in the arrangement of those molecules.


So, you know, we're kind of at the point now, where we're, we're in the before times were before that theory, that could account for life. And there are people working on that theory, right now. It's very exciting, but we're not there yet. And, you know, maybe we will live long enough to see it. You know, there were people who, you know, for whom superconductivity was it was a mystery. And then Einstein tried to explain it, and he couldn't, and others try to explain they couldn't, and then someone did. And then we had a Theory of Superconductivity, which is a really important thing for physicists. So maybe one day we'll have a theory of life, and that will be better than a definition of life.


Nick Jikomes 1:26:39

Well, Carl Zimmer, thanks for your time. The book is life's edge, the search for what it means to be alive. It's a great read, and it's out now. So thanks for writing the book, Carl, and thanks for talking to me today. Thanks a lot.



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