What is life? Carl Zimmer, a writer popularizing the natural sciences, challenges our stereotypical thinking about what is alive and what is dead.
I think each of us has heard the words Well, that’s life. But what exactly is life? We have been asking ourselves this question since the dawn of time; the most eminent philosophers, biologists, biochemists, astrobiologists and physicists have all pondered it... There were so many definitions that scientists had to start classifying them. Now Carl Zimmer – a journalist dealing with the natural sciences, author of 14 award-winning books, including She Has Her Mother’s Laugh and Parasite Rex – has plunged himself into the mix. To find out what life is, Zimmer grew bacteria in a laboratory and researched their evolution; he walked in abandoned mines and studied wintering bats; he also came face to face with a python. Apart from this, he followed the work of eminent scientists. The result of these adventures is his latest book Life’s Edge: The Search for What it Means to be Alive.
Jan Dzierzgowski: Scientists are great at creating definitions, so they probably established what life is a long time ago, right?
Carl Zimmer: You would think so, but science has come up with many definitions of life – and as far as I can tell, there’s no consensus. Every biologist I talk to has their own favourite definition. You ask one of them, they will explain to you what their definition of life is. You turn to another one and hear that the first definition is ridiculous. So it’s a fascinating state of chaos. Imagine that physicists did not agree on the definition of an electron. That would be strange, but that’s where we are with biology.
How come? After all, we were all taught in school that every living organism has strictly-defined characteristics. It must, for example, assimilate food and perform metabolism, reproduce, react to stimuli...
When scientists talk about what life is, there are certain hallmarks that come up again and again. These include metabolism, evolution and homeostasis. However, if you make such a list and start looking at individual organisms or cells, you quickly run into trouble when you find entities that have only some of those hallmarks. According to some researchers, in order for something to be alive, it has to have genes. However, white blood cells have DNA, while red blood cells do not. So, are red blood cells alive or not? Viruses can also have genes, but they do not metabolise. Should they therefore be considered alive? As you can see, things start to fall apart when you try to create a definition of life that is just a list of hallmarks – and this might tell us something about the problem with definitions in general.
You describe many of these kinds of complications in your latest book, Life’s Edge. At times, one may get the impression that you are more interested in them than the answer to the question about the essence of life, and certainly more than in definitions.
It’s very easy for humans to assume that a simple understanding of the world is correct. We have to recognize that there might be something waiting out there that could throw our assumptions upside down. Moreover, we are very often guided by an intuition that we know already what it means to be alive. But actually, this is more of a biological process inside of us. The human brain is tuned to the processes taking place inside our body and to the biological entities around us, especially to those belonging to our own species. Some parts of our brains are specialized in perceiving movements or recognizing faces. That is why the sight of a dead person shocks us: our brains are oriented to look for signs of life and expect to see them, while we do not see anything like that in a corpse. We have a variety of intuitive assumptions about life and death, and we think that science will simply fill in some gaps and confirm what we already know. Maybe our intuitions are not always as reliable as they seem.
This is indeed problematic because the question of what life is has an important political dimension. For example, we want to know when a person’s life begins. What does science have to say about this?
Many politicians in the US claim that human life begins at conception, but they do not define what life itself is, so we are really back to square one. We confuse different aspects. What do we mean when we say ‘life’? If we only mean that there is a cell that metabolises, that takes in nutrients, that builds molecules, that expels waste, then conception has nothing to do with it. According to this definition, the unfertilized egg and sperm are as alive as possible. Some go further and claim that at the moment of fertilization a new genome is formed. But can the genome be the basis for the definition of life? Let us suppose a fertilized egg divides and a woman becomes pregnant with twins. In this way, we get two separate cells and two genomes. Are each of these cells half alive? Will the twins who come into the world not be fully human? Sometimes, on the other hand, we have two undeveloped embryos with two distinct genomes that fuse together and from which one embryo is formed. This embryo can grow into a perfectly healthy person. So, did any of the original embryos die?
We have similar problems with defining the moment of death. Different approaches were taken in different eras.
Yes, the definition of death has changed with the development of science. Hundreds of years ago, it was very difficult to unequivocally know if someone was dead or not. Therefore, physicians came up with the strangest methods to dispel doubts. One doctor advocated the use of a tobacco smoke enema. If the patient did not react to it, it meant that they were dead. Furthermore, people were afraid that they would accidentally be buried alive and wake up in a coffin. Things moved on when we began to better understand our physiology and learned about the processes that occur in cells after death. Science was advancing and new devices such as ventilators were being developed. When a person cannot breathe on their own – for example, as the result of an accident – a ventilator is used instead. This may also apply to people who, for example, have suffered from serious brain damage. The heart is beating, but the brain has stopped functioning and there is no hope of returning to normality.
Therefore, in the 1960s a new definition of death appeared. It was then concluded that the work of the heart is less important and that brain death has significance. But this approach is a matter of choice made by scholars, doctors and politicians, rather than one of strict, biological laws. We recognized that a human being is not only a collection of cells, but also a being who experiences the world through their brain. This is what human life is all about. When the brain dies, life ends.
However, there are disputed cases. One girl in the US had to be put on a ventilator. Brain death occurred. The state of California issued a death certificate to the child, but the mother could not accept it and had her daughter flown to New Jersey, where other regulations apply. The girl spent several years connected to the ventilator, she even went through puberty. Her heart was beating, her cells were dividing. Certain parts of the brain were still active, but the higher brain functions stopped forever and the patient never regained consciousness. Eventually, she died of internal bleeding. Then the state of New Jersey issued her another death certificate. The fact that one person can receive two death certificates illustrates how difficult it is sometimes to draw a line between life and death.
As I read your book, I often realized that some of my beliefs about what the world of living beings looks like were too simplistic. For example, the question of reproduction. On the surface, the ability to reproduce seems to be a pretty good criterion for considering a given being as alive. However, you describe various exceptions to this rule.
Reproduction is one of the most common hallmarks of living beings. But if a person doesn’t have any children and decides not to have them, does that mean they’re not alive? If I were to get a vasectomy and was incapable of having children, I don’t think I would be declared dead. So the ability to reproduce is somehow essential and yet optional for defining life. Let’s cite another example: bacteria. Unlike us, they do not reproduce sexually, but by division. They live, among other places, on the ocean floor, where access to energy sources is difficult; they have very little energy and food. They are barely metabolising at all and sometimes it can take thousands of years before a single cell divides. Should we therefore consider the bacteria to be alive or dead?
The eminent biochemist Albert Szent-Györgyi posed a provocative question: should a rabbit living alone, and therefore unable to reproduce, be included in the group of living beings?
Yes, a single rabbit cannot reproduce. However, people replied to Szent-Györgyi that reproduction is a hallmark of the whole species, not a single individual – because there are male rabbits and female rabbits, and if two of them get together, as they often do, they can reproduce. Thus, we bypass the problem of the single rabbit. And yet there are other, much more fascinating exceptions to this rule, such as a fish called the Amazon molly, which lives in the southern US and Mexico. It is a hybrid fish that evolved from the combination of two pre-existing species and it reproduces through a specific form of parthenogenesis. All Amazon mollies are females. They have no sons, only daughters. To put it simply, they make clones of themselves. To do this, they mate with a male belonging to one of the two ancestral species and then they destroy its sperm. At the same time, females get a kind of a signal, thanks to which their eggs begin to develop and their daughters can come into the world. Therefore, the Amazon molly is a bit like the single rabbit we talked about a moment ago. It represents a species that cannot reproduce on its own. But all you have to do is look at her when she swims in a fish tank to immediately say: “It is obvious that this is alive.” Our definitions break down yet again.
Doesn’t this really frustrate you?
No. I wanted my readers to admire such strange wonders and appreciate the beings that break the rules. When we try to understand living creatures, we are constantly surprised by them. I believe that this is also one of the essential features of life.
You also encountered ‘breaking the rules’ when you decided to look at various ways of metabolism, that is, a process considered one of the determinants of life. This led you to the land of an animal that I am terrified of: the snake.
The metabolism of snakes is a real marvel of nature. Especially in boa constrictors, which eat very large prey whole and digest them in a couple of days, and can then go weeks without eating anything. Snakes are vertebrates, just like us, but they can do things that we could never do. If you wanted to treat yourself to the equivalent of a boa constrictor meal, you would have to eat a whole sheep at once – from head to tail. Snakes can feed in this way because they are adapted to it. As soon as an animal is in the mouth of the snake, its physiology starts to change rapidly. Its intestines double in size; the intestinal walls double in thickness.
Double? How is that possible?
Yes, the intestinal walls thicken to digest the eaten animal as quickly as possible. The heart also gets twice as big, the liver grows. This involves a huge expenditure of energy. If you film a digesting snake with a thermal imaging camera, you will see that it gives off a lot of heat. It then has the same metabolic rate as a galloping horse and maintains it for two or three days. And then, amazingly, all that’s left from its prey is a little bit of hair that comes out the other end; soon after, the snake’s heart shrinks again, as does the liver and intestines. Everything goes back to normal. Scientists are trying to understand this process. They wonder what genes need to be activated so that these physiological changes can be made safely, especially after each meal. Who knows, maybe at the same time we will discover something that will be applicable in medicine? It is true that the human being and the snake are very different from each other when it comes to how we eat a meal, and the human heart cannot grow twice as big – except because of cancer – but we have that underlying similar biology.
On the one hand, life on Earth is incredibly diverse, and on the other hand, we all descend from the last universal common ancestor that inhabited our planet some 3.5 billion years ago. Therefore maybe, as some scientists say, the research sample of organisms and cells that we have at our disposal is too limited, which makes us unable to find an answer to the question of what life is. It is possible that if we encountered it on another planet, we would not recognize it at all.
The study of the universe is certainly important. The Perseverance Rover on Mars is currently looking for life forms similar to those that exist on Earth. It’s understandable that NASA has chosen this approach. But maybe a life has developed there that we can’t yet notice.
Some scientists say that we should think about life more broadly, so that we don’t get stuck on the details. Perhaps we should focus, for example, on the search for complex molecules. Molecules found on Earth, which were formed without the help of biology, are quite simple. Their synthesis usually requires a few (or at most, a dozen or so) steps. In contrast, biomolecules, such as DNA or large complex proteins, turn out to be extremely complicated. If we discover their presence on other planets, we will be able to conclude that there is probably life there, because there’s nothing else that we know of that can synthesize molecules that big.
Sometimes astrobiologists in general shun the question of what life is. They fear that any answer could dangerously narrow their horizons.
It’s true. On the other hand, other scientists say that our goal should not be to develop a definition or to make a list of essential characteristics that must be met in order to talk about life, but rather to create a theory about how certain patterns emerge in matter. A few hundred years ago, alchemists wondered what water was. They said that water is wet, transparent and that it can dissolve certain things. They made a list of hallmarks for water. But on this basis, so-called royal water – that is, a mixture of hydrochloric and nitric acid – was also classified as water. Finally, in the 19th century, a breakthrough in chemical sciences took place. A new theory of matter appeared and since then, instead of defining water by reference to its hallmarks, scientists have begun to talk about the interactions between oxygen and hydrogen atoms. It would be great if we lived to see a theory that naturally gave us an understanding of life.
Here we probably come to another paradox, because if such a theory arises, it will probably be a very complicated biochemical theory, not very understandable for lay people and worthless on a daily basis. Perhaps it will not be able to influence what our intuition tells us about life.
Not necessarily. Today, everyone knows that water is H2O because they teach us that in schools. Kids learn things that used to go beyond the understanding of someone like Leonardo da Vinci. Maybe the layman 300 years from now will be learning about life based on a theory that we just cannot grasp yet.
In that distant future, children will probably take for granted what is beyond the minds of our time...
But really, my book is about failures; about the hard work of scientists that so far has not resulted in any success. I hope that readers will also see how delightful and mysterious the world of biology is, although no-one should expect simple answers. Simple answers do not exist.
Indeed, in Life’s Edge we have many examples of failures. Dealing with the essence of life seems very risky from the point of view of a scientific career.
For a biologist or biochemist, the safest bet would be to choose a protein that others have already studied and that is applicable in medicine. They will be able to research it for many years and everyone will applaud them. They may not achieve anything important, but they will provide themselves with a safe and comfortable life. If they instead announce, “I’m going to create a life in my lab from scratch”, they won’t necessarily make a career in science. When a scientist gets to this borderland between the living and the non-living, they put themselves at risk. The object of research can deceive us. If you had studied zoology in the early 1870s, you would certainly have studied a species called Bathybius haeckelii. It was discovered and named by one of the most prominent biologists at the time, Thomas Huxley, who studied a sample of slime from the sea floor and believed that he saw a kind of a blobby net, a network of protoplasm. It was not meant to be an organism made up of cells, but something called a ‘living paste’. According to Huxley, life on Earth could have evolved from it. Except that the species Bathybius haeckelii did not exist at all. It turned out to be an artefact which was produced by the chemical preservatives that people would put on the sea floor ooze to preserve it for the journey, which resulted in a slimy goo that Huxley mistakenly mistook for an organic substance.
Indeed, when the error was demonstrated, Huxley admitted his mistake in the journal Nature and recanted his discovery. Yet scientists researching life also encounter another problem – namely, their research belongs to the sphere of so-called basic research. It is not as sexy as applied research, which has various practical benefits.
That’s true, but basic research is extremely important. It is easy to forget that every modern appliance which makes our daily lives easier has its origins in scientists just asking very basic questions about the universe. You and I can have this conversation even though we live on different continents, not only thanks to engineers or computer scientists, but also thanks to quantum physicists. Without discovering what atoms and energy are, computer chips would not have been created. Everything has its roots in scientific research, which on the surface seems completely pointless.
Does this also apply to the study of life?
Absolutely. An example that I find fascinating is a new method of DNA sequencing, namely nanopore sequencing. It is being used during the current pandemic to study and sequence samples of coronavirus genomes. A device smaller than a cellphone is used for this. All you need to do is put a sample in it, plug it into a computer, and it can very quickly sequence huge amounts of DNA and deliver it to the data. One of the fathers of this technology was David Deamer, a scientist who tried to understand the beginnings of life. He wondered what the first, simplest cells might have looked like. Each of them must have been surrounded by a membrane that let the desired things inside, did not let in undesirable things, and allowed the cell to remove unnecessary or dangerous products of chemical reactions outside. Deamer focused on simple molecules called lipids. Maybe a lipid bubble could serve as a very simple, partially permeable sheath of protocells. When Deamer studied how genetic material gets inside such a lipid sheath, it occurred to him that DNA or RNA squeezing through tiny pores would block them and cause changes in the flow of the current. If you measure the characteristics of these changes, you can tell which of the four DNA rules you are dealing with at any given time. In other words, changes in the flow of current can be translated into the language of genes. Deamer wrote this down in his notebook and many years later, also thanks to the work of other scientists and engineers, nanopore sequencing devices were created.
Yet we still do not fully know what life is, which is a pity. But reading your book, I had the impression that regardless of the definition, one thing counts: connections. No organism is a lonely island. We all interact with the environment. We live through different internal and external relationships.
One trouble we have with life is that we often try to think about whether an individual living thing in isolation is alive or not. When, in fact, a feature of life is that connection. It’s the connection between cells and a body. It’s the connection between individual organisms and species. It’s the connection between different species. These connections extend across the whole world. Maybe we actually need to think about life on a more planetary scale. The thing that we’re trying to understand is perhaps not life, but the living world.
Parts of this interview have been edited and condensed for clarity and brevity.
Writer, columnist for The New York Times. Lecturer at the Yale School of Medicine. Author of 14 popular science books.
Edited by Agata Masłowska, Mikołaj Gliński and Richard Greenhill
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