Life is older, weirder, and more interconnected than we ever thought

What does it truly mean to be alive? Nobel Prize-winning geneticist Paul Nurse answers biology’s most fundamental (and elusive) question in his full interview with Big Think. 

Drawing from decades of research, Nurse explores how five core ideas redefine life, from the hidden power of the cell to the bizarre machinery inside us all.

PAUL NURSE: My name is Paul Nurse. I’m a geneticist and cell biologist, and I run a biomedical research institute in London called the Francis Crick Institute. And I’ve just written a book, it’s my first book, and it’s called “What Is Life?”

– [Narrator] Chapter one, what is life?

– I call this book “What Is Life?” because it’s the fundamental question in biology. I mean, biologists study living things. And a critical question for us is really, what is the difference between something that’s alive and something that isn’t alive? Now, you might think this is a rather simple question to answer, but in fact, it’s not so easy to answer. And if you look up this in Wikipedia or in a dictionary, you often get rather complex answers. And I wanted to address this question, use my background experience thinking about biology for many years to see if I could have a go at answering what I think is one of the key questions in biology. Should be said, I’m by no means the first person to have considered this question. And I have built on others. In particular, a very famous book, also called “What Is Life?” by Schrodinger, the great physicist in the 1940s. There isn’t really a very good definition of life. And the way I approached it in this book was to take five of the great ideas of biology, ideas that most scientists would be very happy with agreeing to, that some of them have, in fact, been around for a number of centuries. And exploring those five ideas, which are the idea of the cell, the idea of the gene, the idea of evolution by natural selection, the idea of understanding life in terms of chemistry, and the fifth idea, understanding life in terms of information. All of those ideas are well accepted. Maybe the last one, life is information, is not perhaps as fully accepted as the others, but by exploring those five ideas, explaining where they came from, because that’s quite interesting. And then seeing what you could extract from those ideas to get some principles about what life was. And that’s the approach I decided to use.

– [Narrator] Chapter two, the cell where life begins.

– The cell is really critical for the way I think about life. And why is that? It’s because it’s simplest entity that you can unambiguously think is clearly alive. There’s some other life forms where there’s some argument about that, such as viruses, which maybe we can talk about a little later. But everybody will agree that the cell is the basic unit of life. It’s not only the basic structural unit of life, which is how it’s often taught at school, for example, it’s also the basic functional unit of life, by which I mean it is the simplest object that expresses characteristics of life. It can grow, it can divide, it can reproduce. And single-celled organisms, organisms, living things that are only made up of one cell, have all the characteristics of other life as well. It’s just much simpler. So I sometimes think of the cell as life’s atom, just like the physicists would’ve talked about the atom, particularly at the beginning of the 20th century. The cell is the equivalent. It’s life’s atom. We don’t read too much about cells. Perhaps we do a bit more in recent times when people have talked about stem cells and the possibility of using cells to replace damaged tissues, damaged organs, and other medical uses. But what I wanted to concentrate on was really the cell as something which is fundamental to life. Because all living things, myself, a plant, an insect, everything that we see around us that is alive is either a single cell or made up of groups of cells acting together. So that’s why it’s really critically important. And something we humans perhaps sometimes forget is that all of us were once a single cell. When, we were once a fertilized egg in our mother’s body. It was a single cell. And really, if we were all one cells, isn’t that something we should be really interested in? Is it something that we haven’t fully appreciated? Well, for me, I think it is, and I just wanted to really explain that is the case. So you’d better be interested in cells, because you were always once one. And if we understand cells, we’ll understand ourselves and every other living thing on this planet a bit better. I saw my first cells when I was at school. I forget quite how old I was, 12, 13, something like that. I had an inspirational biology teacher. His name was Keith Neal. I still think of him as Mr. Neal. And he explained to us that if we took a tip of a root of a plant, it was an onion plant, and squash it and look at it under the microscope, we would see the cells that made up that root. And indeed, he was right. There they were. I looked under the microscope, and there were these rows of box-like structures which were cells. I found it quite wonderful. Looking down that microscope when I was a schoolboy and seeing those cells was a revelation for me, but it was an even greater revelation for the person who first saw a cell. And that was Robert Hooke, who, in 1660s, working in Oxford where I live, used a very simple microscope, took a thin slice of a plant and looked under the microscope, and there he saw the cells. He published them in a very famous book called “Micrographia” in 1665. And it looked just like what I was looking at under the microscope centuries later. It’s wonderful to see these things, ’cause one thing about being a scientist is that we look around us. If we now explore it and using instruments and techniques like a microscope, we discover so much more about it. And one of the things that of course I discovered, 12 or 13, was the cell itself. And it must have had an impression on me, because I’m still studying it over 50 years later.

– [Narrator] Chapter three, the ancient connection between yeast and humans.

– One thing that’s important about cells is that they reproduce themselves. They undergo division from one to two. And of course, reproduction is a characteristic of all life. It’s one of the things that all living things do. And you see it in its simplest form in the division of a cell. That particularly fascinated me, because when I was seeking a research project in my 20s, so quite a long time ago, I wanted to study a fundamental process. And for me, cell reproduction represented exactly that, because all living things reproduce, and we see it there in its simplest form. And so I thought if we could understand how cells control their reproduction, then we get to one of the central mysteries of life. And the consequence of that is really, I have spent my own life studying this important question of life. When I thought about how to study cell reproduction, I thought, let’s take a simple system. A system which we can work on easily. A system where we can do lots of experiments, a range of experiments, cell biological, biochemical, genetical, so that we can use every tool that a biologist has to understand what’s going on. And I decided to work on yeast. Most people are not excited by yeast. I have to say I am excited by yeast. They just think, well, it’s good for making beer and wine and bread maybe, but actually it’s more than that. It’s very good as a model for other cells in all sorts of more complicated living things like our cells. Cells from yeast through to human cells are called eukaryotes. That’s a technical term, which just means their chromosomes, their DNA is wrapped up in a nucleus. And such cells are called eukaryotes. But what I decided was, if I could understand the process of cell reproduction in a yeast, maybe that would illuminate how our own cells control their division. That in fact turned out to be true, because later in life, I worked on yeast for some years, together with my colleagues discovered the molecules that controlled that whole process of cell division, and then went on to show that exactly the same processes worked in human beings as well. And that was one of the most exciting discoveries of my life, quite frankly, because it had a consequence. If the same process was controlling the reproduction of a yeast cell as controls the reproduction of a human cell, then this means this was a control that was extremely ancient, because yeast and human beings diverged at least a billion years ago and probably one and a half billion years ago. So it meant that this control process was put in place one and a half billion years ago, 1,500 million years, just to hear how long it was. And just to put that in perspective, dinosaurs just went extinct only 65 million years ago, and this is far, far longer. We were back in the depths of time, deep time, that when this mechanism was established, and it’s still the same today in yeast and in humans. It is so similar that the way, in fact, I showed it and my laboratory showed it, was to show that we could identify the gene in humans and put it into a yeast cell, and it would entirely substitute for the yeast gene. In other words, the process was still so similar after 1,500 million years, they were completely interchangeable. It would be like taking, let’s say, a piston from a car engine, a Maserati, and putting it into a Ford. Except the difference between the Ford and the Maserati would be 1.5 billion years. And it still worked. Another great advantage about yeast is that it is fantastic for genetics. It’s very easy to cross different yeast strains, to follow the track of different genes through those crosses, to mutate, as we call it, to damage genes so that we can see what effect it has on our yeast cells work. It’s a beautiful system, particularly for studying something that you don’t understand, because I was interested in how cells control their division, how they go through the process from the birth of a cell through the growth and then undergoing division. And to study that, what I did was to look for mutant yeast cells that couldn’t undergo that process. They could still grow, actually, but they couldn’t reproduce. And what this meant was that the cells would just get bigger and bigger. ‘Cause although nothing was affecting their growth, they were unable to divide themselves. So what I did, and I did this as a postdoctoral worker, that’s when I started it. I looked for mutant yeast cells that would simply get bigger and bigger and would never divide. And by doing that, I could define some of the genes that were necessary for the whole cell reproduction process. And that reproduction process from the birth of a cell to its division is something we call the cell cycle. And so I call these genes, cell division cycle genes, or CDC. And I was actually following with this work, work done by a friend of mine, Lee Hartwell, who worked on a different yeast to me, but he’d done this a few years before I did it. And he called his gene CDC genes as well. And so we had two yeasts, which were rather different, in which we were revealing some of the genes, some of the, which encoded components, that were required for the division of a cell from one to two. And so that was the start of trying to work out what controlled that process.

– [Narrator] Chapter four, genes, DNA, and evolution.

– I mentioned that cells were discovered in the 17th century. And the first work that revealed something about what genes were were from the 19th century, from work carried out by a monk, a monk called Gregor Mendel. And he worked in a town in the Austro-Hungarian Empire called Brno. It’s now in the Czech Republic. And he belonged to an order of monks that taught in local schools and encouraged research. It really did encourage research. And Mendel was interested in looking at the genetics of plants, of peas. And what he did is he crossed different peas with different characteristics, like tall pea plants and short pea plants, which had flowers of different colors or seeds of different shapes and so on, and studied how when you cross plants that were different in these different characteristics, what were the progeny like that grew up from those crosses? And he was trained as a physicist, actually. He was an amateur astronomer. He did maps of the moon and also studied weather as a meteorologist. Rather curiously, I’m an amateur meteorologist and also astronomer myself, so I feel a lot of affinity to Gregor Mendel. But I mention that, because perhaps unlike most biologists at the time, he was quite quantitative. He counted things. Biologists tended to describe things rather than count them. Physicists tend to measure things and therefore count. And he counted the different types of offspring that were produced from these pea plants. And he began to notice, there were very clear ratios, famous ratios, like three to one, and nine to three, to three to one. And he realized, being a physicist, that these were sort of very simple combinations. And he had this idea that maybe what was responsible for inheritance of a tall plant or a short plant could be described in terms of a gene that influenced that process, a sort of unitary particle, which could be passed down from one plant to another. He didn’t get it quite right. That took another 30, 40 years when others rediscovered what he had done and they interpreted it more clearly. But it was the first evidence that there was something particulate, which we now would call a gene. And isn’t it interesting where it came from? It came from work on a humble plant, the pea plant, carried out by a humble monk in a humble small town in the middle of Europe. And that has led to the discipline of genetics. But what was unfortunate was that nobody took any notice of what Mendel did. None whatsoever. So they knew it had been published. Even Charles Darwin had been sent to paper. And I noticed that it was in the Encyclopedia Britannica, 10, 15 years later, but nobody took any notice of his interpretation. And then in the early 1900s, three different botanists did the same sorts of experiments, got the same results, but they then had to admit it all had been described by the gardening monk 35 years before. So he got no credit for his discovery, but now of course a central part of genetics is called Mendelism in his honor. So genes were studied for quite a long time in the 20th century. They were rather abstract. Nobody knew what they were. And then a very famous experiment was carried out in Rockefeller University in New York. I’m very fond of this experiment because I was once president of Rockefeller University and lived in New York for nearly 10 years. And this was an experiment that showed that genes were made of DNA, deoxyribonucleic acid. And this was done by a scientist called Avery. And what he did is he found that he worked on a bacteria, pneumococcus, and the bacteria could make smooth colonies or rough colonies. And if he took, extracted the chemicals that make up the bacterial cells from, say, a smooth colony and then added it to the rough colony, sometimes they were turned into smooth colonies. And so what he did was he looked to see what chemicals were responsible for that, and he found that DNA was the chemical that caused it. This, once again, just like Mendel, was taken no notice of. This turned out to be controversial, too. ‘Cause DNA isn’t a complicated molecule. And quite a few scientists at the time thought, oh, this isn’t sufficiently elaborate or sufficiently complicated to encode all the information that has to be kept in genes. But in fact, he was absolutely right. DNA was the responsible for inheritance. It was the material that made up the gene. And that was shown in another set of beautiful experiments, this time on the other side of the Atlantic in Cambridge, and involved a variety of people. Watson and Crick working in Cambridge, Wilkins and Franklin working in London. And some beautiful X-ray pictures were taken of DNA by Rosalind Franklin, and Wilkins, a little before her, was also involved in this. And that revealed a very interesting plot, a diffraction image, which if you could interpret it properly, would indicate the structure of the molecule. And that’s what Crick and Watson did. They interpreted those spots and revealed that they could be explained if the structure of DNA was a double helix. And what is a double helix? Well, the way I like to explain it is it’s like a ladder with rungs going across. And each rung consists of two bases. And these bases, one comes from one side of the ladder, and the other from the other side of the ladder. And there’s four types. But a base, which begins with the word A, is always linked to a base beginning with the word T, and another pair is G and C. And what this meant is then that the side of the ladder was string of letters, A, G, C, T, and the complement was on the other side of the ladder. And this ladder was then twisted into a helix and is in fact what DNA is made up of. Now, there’s two interesting features about this, which is really, really exciting. The first is it conveys information. And the information is conveyed in the order of those bases that make up the side of the ladder, the order of the A, the G, the C, and the T. So this is basically a digital information storage device. Now, this is a very economical way of storing information. If you read a book, they are made up of letters, which are digital information. In the computers we use, it’s all made up of bytes, which is digital information. And we like to think that we humans invented this, but actually it was invented by life probably 3.5 billion years ago. And so that’s the information storage part of DNA. But it’s even more beautiful than that, because the structure of it means it’s extremely easy to copy precisely. Because if you tear that ladder apart, then you will have on one side, half a rung. So you will have one of the bases, like A, T, G, C, and the other side will have the complementing bases. So now you can copy those molecules, those sides of the ladder, and make a very precise copy of producing not just one double helix, but two. And that is the basis of heredity. It’s very, very seldom that a particular experiment, the beautiful X-ray structure discovered in London, the interpretation of it discovered in Cambridge, which explains the nature of heredity, how information is encoded, and how that information is then precisely copied. It’s a beautiful, beautiful experiment. DNA is central to understanding the cell cycle at the reproduction of the cell. Because every time a cell reproduces itself, it has to copy that DNA, and it has to be segregated into the two newly divided cells. ‘Cause otherwise, they would lack genes, they would lack hereditary material, and they wouldn’t be able to survive. So that means this copying process, which occurs every cell cycle, it’s in a process we call S phase for DNA synthesis, every single base pair in the genome has to be replicated, has to be divided, and it has to be done with extraordinary precision. Because if it made mistakes, then genes would be damaged. And if they’re damaged, then the cell can’t work properly. And so it’s very precise in the copying, but there’s a little bit of damage that occurs, and that turns out to be important for evolution, as I’m sure we’ll talk about a little later. The idea of evolution by natural selection is, for me, probably the most beautiful idea in biology. There’s two things we have to think about here. One is evolution itself. Do animals and plants evolve? And the second is, what is the mechanism by which it takes place? The idea that animals and plants might evolve is actually quite old. Even Aristotle had some thoughts about it. But particularly in the 18th century, there were a number of people who speculated looking at fossils and so on, that animals and plants had evolved over time. But it took Charles Darwin in the middle of the 19th century to come up with a mechanism, evolution by natural selection. And he came to this position because of a five-year voyage he took in a small royal naval boat called the Beagle that went round the world, and he collected animals and plants and birds and studied the geology, made lots of natural history observations. And this eventually led in the 1840s to his idea. He didn’t publish it till 1865. And it provided a mechanism by which animals and plants could evolve. And really, you could get design without having a designer. Now, how is that possible? It’s a very clever idea, and it’s based on the genes that I’ve already talked about and cells, really. And what he speculated was that all living things have hereditary material. He didn’t know about genes, but he speculated that they had hereditary material. That if this hereditary material had some differences that resulted in the living thing, living organism being different, then it is possible that what was produced was perhaps a plant or an animal that could grow faster or produce more offspring. And that would mean that in the next generation, there would be more of that particular variant. And if it was advantageous in the environment in which it found itself, then it would eventually take over the whole population. So you get a change from one type of organism into one that was a bit different. I sometimes think of a very simple model just to explain that. Imagine a single-celled organism which had a red coat outside the cell, or a yellow coat. Let’s say that the red coat got eaten by some other living organism. Then, if there was a mutation which resulted in the red coat turning into a yellow coat, then the yellow-coated cells would survive better than the red-coated ones. And as a consequence, they would take over the population. And that’s just a very simple example of evolution by natural selection. But it’s also built on the ideas that I’ve been talking about. It’s built on the idea of genes and the fact that genes determine the properties of cells. And if you have genes determining the properties of cells and the genes show some variability, then you will simply get evolution by natural selection. And this was the idea Darwin had. He didn’t know about genes, but he did know about hereditary material. And this has been a revolutionary change in our understanding in biology. Because what it leads to is a better designed living thing without having a designer. It can occur just randomly by selecting naturally for those changes that are more advantageous for that living thing. It really truly is a beautiful idea. One of the consequences of evolution is that we’re related by descent. Because if we all can trace back our ancestors to a common place, then every living thing on this planet is related. This is rather profound, really. The relatedness of all living things and particularly of humans to other life forms was brought home to me in a very special way, almost spiritual, when I was trekking through a Uganda rainforest looking for gorillas. And we came across a whole family of them, 20, 25 of them. I sat down behind a tree. I was a little bit separate from the rest of my group. And then this very large male gorilla sat down in front of me and looked at me, deep brown eyes. He put out his arm, he bent down a sapling tree, two, three, four inches in diameter. I’m sure he was telling me, you know, “I can do this. I’m not sure you can do this.” But sitting there, just looking at him, seeing the similarities, his deep brown eyes, and they were really staring at me, it just looked like we were having a conversation somehow. He sat and looked at me for a while, then he climbed up the tree, and then he peed all over me. So I don’t really know whether he was trying to put me in my place, but for sure it was a magnificent interaction. Here I was almost talking to a sort of relative diverged five million years ago, 10 million years ago. And it made me think of the deep connection between all life on this planet. It’s just that we diverged further and further back. And that experiment that I described of taking the human gene and putting it into a yeast cell and showing that that human gene could control the reproduction of the yeast cell just as well as it could control a human cell even though they diverged 1,500 million years ago, is another example of the extraordinary similarity between living things. And I think about this quite a lot, because doesn’t it mean, if we’re related to every living thing on the planet, do we not have a special responsibility for every living thing on this planet? They are really all our relatives. Do we not really have a responsibility to be a steward for all animals, plants, fungi? We have a responsibility for them. And it’s the best argument I’ve seen in an abstract sort of way that we should take care of every living thing that we can on this planet because we’re related to every living thing on the planet. They are our relatives.

– [Narrator] Chapter five, life equals chemistry plus information.

– So understanding life, understanding that the basic unit is a cell, understanding that hereditary central and that involves the gene, and then evolution by natural selection, can give you diversity of life. It doesn’t actually tell you quite how life works. And for that, there’s two ideas that I think are important. And the first of those is life as chemistry. Life is made up of molecules and chemicals, and what we, if we study this, and that’s the study of biochemistry, then you quickly learn that what happens in living cells and therefore in all living things is that a huge amount of very complicated chemistry going on all the time in a little cell. There are many thousands of chemical reactions going on all the time. And they’re catalyzed by something we call enzymes. And these are long molecules called protein. And they’re chains of amino acids, just like the DNA is a chain of bases. Proteins are chains of amino acids. And in all cells, the chain of different bases in the DNA specifies the type and order of the amino acids in proteins that are encoded by those genes. And it’s in fact those proteins that are fundamentally responsible for the characteristics of life. And these proteins, and there’s many thousands of them, even in the yeast cell, the simple yeast cell that I studied, there’s 5,000 of them in genes, encoding proteins. These proteins and enzymes are doing a myriad of chemical reactions at the same time. And the reason they can do that is that these proteins are made up of amino acids, and there’s 20 of them. And those amino acids have different chemistries. They can be basic, or they can be acidic. They can like water or dislike water. They can be bulky or not bulky. And all this means is that the proteins can fold up into complicated shapes with different chemistries in different places in that molecule, which allows them to be an incredibly efficient catalyst. And a catalyst can change one chemical into another chemical. And it’s the basis of the majority of chemistry that we see in life. Once we’ve understood that, we begin to see this relationship between heredity and how the cell works. The genes encode the proteins, and the proteins do chemical work. This is a very powerful system of producing very elaborate chemistries that can undergo all the chemical reactions that life needs. And I want to emphasize how fantastic that is, because I want you to imagine in a tiny, tiny cell that you need a microscope to see, there’s thousands of chemical reactions, all very different, going on all the time, very, very close to each other. Now, when we make us human beings a chemical plant to do something similar, we only perhaps do 20, 30 chemical reactions. We have to put a lot of heat in or make it very acid or make it alkali. And these wonderful little cells can do all of this simultaneously with lots and lots of different proteins at the same time in almost the same space. And it’s almost miraculous how that works. And that is a foundation of life. That chemistry is responsible for growth of the cell, the reproduction of that cell, for its ability to capture energy and use energy. All of that is explained by this chemistry. One of the ways in which cells can carry out all of this chemistry, in fact, different chemistries, in fact, it’s the main way, is that the cell is made up of lots and lots and lots of little compartments. The proteins themselves isolate one chemistry from another chemistry because they keep that separate. And then we have structures in cells, which are membrane-bounded components where you have one set of chemistries different from another set of chemistries. And it’s compartmentation that allows all these different chemistries to occur simultaneously in such a small space. It’s miraculous that it can work like that. But what we have to realize is when we look down at a cell and see all these different structures within it, which we can see when we use a powerful microscope, what we’re looking at is lots and lots of different chemistries going on in the same place within different compartments. So it’s really compartments which are necessary for that huge range of chemistry. So life is chemistry, but life is also built on information. And this is perhaps the idea that is less well-accepted. It’s one that I think is very important, because life has to constantly manage information. I described that in the book in terms of a yellow butterfly, a brimstone yellow that I saw when I was a child and started me thinking about life, coming in, flying into the garden, settling, taking off, going, and leaving the garden. And all the time, that butterfly was sensing its environment around it, taking in that information, turning that information into knowledge of what was happening, and then changing what it did maybe to avoid being eaten by a bird or a shadow, or to find a flower to eat some nectar. And you can understand how the butterfly is operating in terms of information, capturing information and managing it and then changing what you actually do. And that is central to life. Life is, and can only operate through information. Let’s take what I said about chemistry and all those different compartments that allow different chemistries to take place. That can only occur if there’s an information transmission from different compartments to keep the whole thing coordinated. If you didn’t keep it coordinated together, then the whole thing could fall apart. It’s very important to manage information for life to work. And I use two examples to really illustrate this, and one is back to DNA. DNA is a digital information storage device. And the structure of DNA is such that it’s very stable and it has the, it can store that information for a very long time. But it has to do work. And to do work, that DNA is copied into protein structures, as I’ve explained. And that has a much more elaborate chemistry that can do work, but it is transferring information from the DNA to the protein so the protein could work. Another example was really just regulating how the chemistry works in a cell. And what I described is a control system. A control system where, let’s say, a gene is making a substance, and it wants to keep that substance at a constant level. Then cells have a very clever mechanism whereby as that substance increases, it switches off the gene, and when it decreases, it turns the gene on. And so it maintains the substance at a certain level. Now, this is what we would call a negative feedback control, and that maintains homeostasis. It keeps the whole thing constant. But you only really understand that when you realize there’s information going on here, information management, that is to say the cell is measuring how much stuff it’s got and then using that to determine whether the gene should be switched on or switched off. So information management is critical to understanding life, and I’m going to push it one stage further than that. When we study the chemistry of life, what we do is describe the molecules and how they interact with each other and how the atoms are spaced one with another. But if we have to understand what that means in terms of biology, then nearly always it has to involve information. If we take DNA, we could understand the structure of DNA and describe it in terms of how one base is related to another base, but it only makes biological sense when it is seen to be a digital information storage device. So to understand how life works, you need a combination of chemistry and combining that with information. And I think that is the basis of all life. When we think of information, we often think of the computer age and so on, artificial intelligence and machine learning and so on. Now, that is related to the information management we see in life. It’s done by completely different processes. When we do it in a computer, in silico, you have hardware in the computer and we have software that drives it. In life, living things, it’s a little different. I sometimes use the word, I didn’t invent it, but in living things then, it’s not hardware, it’s wetware. Because actually what’s happening is that the way information is managed is by communication and using molecules from one part of a cell to another. And that’s diffusing through liquid, through water. And that allows you to rewire the connections. And so you don’t just change by software, you change also how things work by rewiring connections. I see the digital world that we think about with computers as more as a metaphor rather than a direct analogy. Putting all these five ideas together, I think some principles emerge. I tried to summarize them. Living things are bounded physical entities. I mean, they are working as themselves. And that’s important, because otherwise you get mixed up with computer games and so on sometimes, which have evolution built into them. These physical bounded entities separate a living thing from the rest of the world, so you can increase your order within the cell as a consequence of disorder outside the cell. And this means that you don’t have any problems with the second law of thermodynamics of physics. So you can deal with that too by being a bounded entity. The bounded entity is described as a chemical and informational machine, and that emerges from the ideas that we’ve talked about. And then critically, that informational chemical machine in a bounded entity has a hereditary system that determines how it works, which has variability. And therefore the whole thing can evolve by natural selection. And that means that the living thing can acquire purpose, a purpose to be better adapted in the life state it finds itself. And so we can evolve living things from one type into another. And for me, that bounds everything together, not in a neat dictionary way, like a dictionary definition, but in a way that emphasizes the core principles underlying life.