Since childhood, living beings aroused interest and admiration in me. I spent my childhood in the north of California, where I often played in nature among plants and animals.
My friends and I watched the bees as they pollinated the flowers and trapped them in ziplock bags to get a better look at their obsidian eyes and golden hairs, and then let the insects free to go about their daily activities.
Sometimes I made a bow and arrow from the bush that grew on our site, used bark from the same bushes as a bowstring, and the leaves from them went to the feathering of the arrows. On trips to the beach with my family, I quickly learned to find crabs and arthropods in their nooks, observing the bubbles in the sand after the tide of the next wave. And I vividly remember how we went on a hike in the eucalyptus grove in Santa Cruz in elementary school, where thousands of migrating Danaid butterflies stopped to rest. They clung to tree branches in large brown lumps, resembling dried leaves. And then some butterfly began to move, and it turned out that the inner part of its wings was fiery orange.
These moments, as well as many of David Attenborough's films, have reinforced my fascination with the living world of the planet. While my younger brother was enthusiastically engaged in the K'Nex kit presented to him, painstakingly building roller coasters or a railway, I tried to understand how our cat works. How does she see the world? Why is she purring? What are her fur, claws and mustache made of? I once asked for an encyclopedia about animals for Christmas. Having ripped the brown paper off a massive book that weighed about half of me, I sat by the tree for several hours reading it. So it’s not surprising that I ended up making a living writing articles about nature and science.
But recently I had an epiphany that made me take a fresh look at why I love all living things so much, and think in a new way about what life is. The fact is that all the time that people study life, they still cannot give it a clear definition. Even today, scientists do not have a convincing and universally accepted definition of life. Thinking about this problem, I remembered how my brother was enthusiastically playing construction set, and I was curious about the cat.
Why does it seem to us that the constructor is inanimate, but the cat is alive? Aren't both the first and the second machines in the end? Of course, a cat is a much more complex mechanism, capable of amazing deeds, which the designer will never be able to repeat. But at the most basic level, what is the difference between an inanimate machine and a living organism? What, people, cats, crabs and other creatures belong to one category, and constructors, computers, stars and stones to another? My conclusion: no. Moreover, I decided that life does not really exist.
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Let me explain
Formal attempts to give a precise definition of life were undertaken even in the days of ancient Greek philosophers. Aristotle believed that, unlike the inanimate, all living things have a soul, and the soul is of three types: in plants, in animals, and a rational soul, which is exclusively in humans. The Greek anatomist Galen suggested a similar, organ-based system of "life spirit" in the lungs, circulatory and nervous systems. In the 17th century, the German physician and chemist George Erns Stahl and other scholars advanced a theory that was later called vitalism.
Vitalists argued that "living organisms are fundamentally different from inanimate entities, because they contain some intangible element, and they are governed by different principles than in inanimate things", and also that organic substances (molecules containing carbon and hydrogen and created living organisms) cannot be synthesized from inorganic (these are molecules where there is no carbon, which appears mainly as a result of geological processes). Subsequent experiments showed the complete inconsistency of vitalism: inorganic substances can be converted into organic ones both in laboratory conditions and outside the walls of laboratories.
Instead of instilling in organisms "some intangible force", other scientists have tried to derive a certain set of physical characteristics that differentiate the living and non-living. Today, due to the lack of a concise definition of life in Campbell's books and other widely used biology textbooks, there is an extensive list of defining characteristics, for example: order (the fact that many organisms consist of either one cell with different divisions and organelles, or groups of ordered cells), growth and development (change in size and shape in a predictable manner), homeostasis (stability of the composition of the internal environment, which differs from the external one, as well as the balance of biophysiological functions, for example, regulation of acidity and salt concentration), metabolism (expenditure of energy for growth and for slowing down aging),reaction to stimuli (change in behavior in response to light, temperature, chemicals and other components of the environment), reproduction (vegetative reproduction or mating in order to produce new organisms with the transfer of genetic information from one generation to another) and evolution (change over time of genetic characteristics of the population).
The logic of such lists can be very easily refuted. No one has ever succeeded in compiling such a set of physical properties in which all living things are combined and everything that we call inanimate is excluded. There are always exceptions. So, most people do not consider crystals to be alive, but they are highly organized and they grow. Fire also consumes energy and increases. Conversely, bacteria, tardigrades, and even some crustaceans can hibernate for a long time, and at this time they do not grow, they do not metabolize, and they do not change at all, although they cannot be called dead either.
What category can we classify a leaf that has fallen from a tree? Most people would agree that a leaf attached to a tree is alive. Its many cells work tirelessly to convert sunlight, carbon dioxide and water into nutrients, among other functions. When a leaf breaks off a tree, its cells do not immediately cease their activity. Does he die when he falls to the ground, when he touches the ground, or when all his cells die? If you rip a leaf from a tree and place it in a nutrient medium in the laboratory, where the leaf cells are full and happy, is this life?
Almost all the proposed characteristics of life fall into this predicament. Response to the environment - this property belongs not only to living organisms. We have invented countless machines that do the same. And even reproduction is not a defining feature of life. In many cases, an individual animal cannot reproduce on its own.
It turns out that two cats are alive, because together they can give birth to new cats, and one cannot, since it cannot reproduce on its own and transmit its genes. Remember also the immortal jellyfish turritopsis nutricula, which can endlessly return from the "adult" stage of the jellyfish to the "child" stage of the polyp. It does not reproduce offspring, does not vegetatively reproduce, and does not even age in the traditional way - however, most people would agree that this jellyfish is alive.
What about evolution? The ability to store information in DNA and RNA molecules, transmit this information to offspring and adapt to changing environmental conditions by changing genetic information - of course, not only living beings possess these talents. Many biologists have focused on evolution as a key and distinctive feature of life.
In the early 1990s, Gerald Joyce of the Scripps Research Institute was part of an advisory group to John Rummel, who at the time ran NASA's extraterrestrial biology program. During discussions about the best ways to find life in other worlds, Joyce and colleagues created a very popular working definition of life today: an independent system capable of Darwinian evolution. The definition is clear, concise and comprehensive. But does it work in practice?
Let's see how this definition applies to viruses, which most of all complicate the search for a definition of life. Viruses are, in fact, strands of DNA or RNA packed in a protein coat. They have no cells, no metabolism, but they have genes, and they can develop. However, as Joyce explains, in order to become a "self-contained system," an organism must contain all the information it needs to reproduce Darwinian evolution. He states that because of this condition, viruses do not fit the working definition. After all, the virus must invade the cell and capture it in order to reproduce itself. “The viral genome only develops within the host cell,” Joyce said in a recent interview.
Tardigrade
But when you think about it, NASA's working definition is no better at capturing the ambiguity of a virus than any other proposed definition. The parasitic worm living in the human intestine, which many consider to be a disgusting, but quite real form of life, has all the genetic information necessary for reproduction. But the parasite cannot reproduce in any way without cells and molecules in the human intestine, from which it steals the energy necessary for survival. In the same way, a virus has all the genetic information it needs to reproduce, but it lacks the necessary cellular machinery. The claim that the situation with the parasitic worm is radically different from the situation with the virus is a rather weak argument.
Both the worm and the virus multiply and develop only within their "host". In fact, the virus replicates much more efficiently than the worm. The virus gets down to business immediately, and it needs only a few proteins inside the cell nucleus to start multiplying on a large scale. And the parasite needs a whole organ of another animal for reproduction, and the worm will achieve success only if it is able to survive until it grows and lays eggs. So if we use NASA's working definition to exclude viruses from the living, we also have to exclude all the other larger parasites, including worms, fungi and plants.
Defining life as an independent system capable of Darwinian evolution also forces us to admit that some computer programs are also alive. For example, genetic algorithms mimic natural selection to find the optimal solution to a problem. These bitmaps encode traits and properties, evolve, vie with each other for reproduction, and even exchange information. Likewise, software platforms like Avida create "digital organisms" made of digital bits that mutate in much the same way DNA mutates. In other words, they also evolve. "Avida is not a simulation of evolution, it is an example of it," Robert Pennock of Michigan State University told Carl Zimmer in his Discover program. - There is a process of natural selection. All the components of the Darwinian process are present there. These things reproduce, they mutate, they compete with each other. If this is the main thing in defining life, then these things must also be taken into account."
I would say that Joyce's laboratory itself dealt a devastating blow to NASA's working definition of life. He, along with many other scientists, gives preference to the theory of the origin of life called the "World of RNA". All life on our planet depends on DNA and RNA. In modern living organisms, DNA stores the information needed to create proteins and molecular mechanisms that work together to form a fussy cell. At first, scientists thought that only proteins, enzymes, could act as a catalyst for the chemical reaction needed to build the cell structure.
But in the 1980s, Tomas Cech and Sidney Altman discovered that, by interacting with different protein enzymes, many types of RNA enzymes, or ribozymes, read the information encoded in DNA and build different parts of the cell step by step. The RNA World hypothesis states that the earliest organisms on our planet performed all these tasks of storing and using genetic information exclusively with the help of RNA and without the help of DNA and a whole suite of protein enzymes.
How could this happen? That's how. About four billion years ago, free nucleotides from the primordial soup of the earth, which are the building blocks of RNA and DNA, combined into ever-longer chains and over time produced ribozymes that were large and complex enough to create new copies of themselves. Thus, they were much more likely to survive than those unable to reproduce RNA. These early enzymes enveloped self-assembling membranes, forming the initial cells. Ribozymes not only created more RNA, but could also link nucleotides into DNA strands. Nucleotides could also spontaneously form DNA.
Anyway, DNA has replaced RNA as the main molecule for storing information, because it is more stable. And proteins have begun to play the role of catalysts, since they are very diverse and easily adaptable. However, the cells of modern organisms still contain remnants of the original RNA world. Thus, ribosomes, which are a set of RNA and proteins that synthesize proteins from amino acids, are ribozymes. There is also a group of viruses that use RNA as the main genetic material.
To test the RNA World hypothesis, Joyce and others have attempted to create the types of self-replicating ribozymes that may have once existed in the primordial soup of the Earth. In the mid-2000s, Joyce and Tracey Lincoln created trillions of random and unrelated RNA sequences in the laboratory, similar to early RNAs that could compete with each other billions of years ago.
In addition, they created isolated sequences that accidentally showed the ability to connect two other pieces of RNA. By opposing such sequences to each other, this pair eventually produced two ribozymes that could reproduce each other indefinitely, as long as they received enough nucleotides. These naked RNA molecules are capable of not only reproducing, they can also mutate and evolve. Ribozymes, for example, have altered small segments of their genetic code to adapt to changing environmental conditions.
“They fit the working definition of life,” says Joyce. "It's an independent Darwinian evolution." However, he cannot say for sure if the ribozymes are alive. In order not to turn into Doctor Frankenstein, Joyce wants to see how his creation takes on completely new properties, and not just modifies what he already knows how to do. “I think the missing link here is that ribozymes have to be inventive, have to create new solutions,” he says.
But it seems to me that Joyce is not doing justice to ribozymes. Evolution is genetic change that occurs over time. To see evolution in action, you don't have to wait for the pigs to develop their wings and for the RNA to assemble into the letters of the alphabet. The blue eye color, which appeared 6,000-10,000 years ago, is just another type of iris pigment. This is the same grounded example of evolution as the first feathered dinosaurs. If we define life as “an independent system capable of Darwinian evolution,” then I see no compelling reason to deprive the title of living of self-replicating ribozymes or viruses. But I also see no reason for a complete rejection of this working definition and all other definitions of life.
Why is it so difficult to define life? Why have scientists and thinkers for centuries not been able to find a specific physical property or a set of properties that can clearly distinguish living from non-living? Because there are no such properties. Life is a concept that we have invented. At its most basic level, all matter in existence is an organized set of atoms and their constituent particles. This is an incredibly complex set that contains things like the elemental hydrogen atom and the most complex brain.
Trying to define life, we arbitrarily drew a line in this complex set and declared: everything above it is alive, and everything below it is not. In fact, this distinction exists only in our brains. There is no threshold beyond which a cluster of atoms suddenly revives, there is no clear distinction between living and nonliving, there is no proverbial Frankenstein spark. We cannot give a definition of life, because there is nothing to define here.
I nervously explained these ideas to Joyce over the phone, expecting him to laugh and call them absurd. After all, it was he who helped NASA develop the definition of life. But Joyce called the "ideal" argument that life is just a concept or idea. He agrees that defining life is, in a sense, an empty idea. The working definition exists simply for linguistic convenience. “We were trying to help NASA find extraterrestrial life,” he says. "We couldn't use the word 'life' in every paragraph without defining it."
Carol Cleland, a philosopher at the University of Colorado at Boulder, who has spent many years researching attempts to describe life, also finds it wrong to try to define it precisely. But she is not yet ready to deny life in her physical reality. “To conclude that there is no true nature of life is as premature as defining it,” she says. "It seems to me that the best option in such conditions is to consider the final criteria of life as hypothetical and speculative."
What we really need, Cleland writes, is "a sufficiently grounded and adequate general theory of life." She makes comparisons with chemists from the sixteenth century. Before scientists realized that air, dirt, acids and all chemicals are made up of molecules, they could not define water. They could list its properties - wet, transparent, tasteless, freezes, can dissolve many other substances - but they were unable to accurately characterize it until the researchers discovered that water is two hydrogen atoms in conjunction with an oxygen atom.
Salty, dirty, tinted, liquid, frozen - water is always H2O. It may contain other elements as an impurity, but the three atoms that make up what we call water are always present in it. Nitric acid may resemble water, but it is not water, because the two substances have different molecular structures. A much larger sample size will be required to create a theory of life that fits molecular theory, Cleland says. She claims that so far we have only one example of what life is - this is earthly life, which is based on DNA and RNA. How can you create a theory about mammals by observing only zebras? This is where we find ourselves in our attempts to define what makes life life, Cleland concludes.
Cluster of bacteriophages, viruses that have evolved
I disagree with her. Of course, the discovery of samples of extraterrestrial life on other planets will expand our understanding of how what we call living organisms works, and above all, how they evolved. But such findings are unlikely to help us develop a new revolutionary theory of life. The chemists of the 16th century could not say how water differs from other substances, because they did not understand its fundamental nature: they did not know that every substance is made up of a specific and ordered set of molecules. And modern scientists know exactly what creatures on our planet are made of - cells, proteins, DNA and RNA.
The difference between the molecules of water, soil and silver from cats, humans and other living things is not "life", but the level of complexity. Scientists already have enough knowledge to explain why so-called organisms can do what most non-living things cannot. They can tell how bacteria make new copies of themselves, how they quickly adapt to their environment, and why stones cannot. But at the same time, they may not say that the living is this and that, and the inanimate is that, and that this pair will never unite.
Recognizing life as a concept and an idea, we in no way deprive it of its inherent magnificence. It's not about the absence of material differences between the living and the non-living. Most likely, we will never find a clear dividing line between them, since the concept of life and non-life as certain categories is just a concept, not reality. Everything that fascinated me in wildlife in childhood is equally surprising now, even with my new understanding of life. I think that those things that we call living, in fact, unite not only some of their inherent properties; rather, they are united by our understanding of them, our love for them and, frankly, our arrogance and narcissism.
Firstly, we announced that everything on Earth can be divided into two groups - living and non-living, and it is no secret which group we consider the highest. Further, we not only placed ourselves in the first group, we insisted that all other life forms on our planet should be judged in relation to us. The more this form resembles us - the more it moves, speaks, feels, thinks - the more alive we consider it. But at the same time, a specific set of properties and characteristics that make a person a person is far from the only way (and far from the most successful in terms of evolution) of describing a living thing.
In truth, what we call life is impossible without and is inseparable from what we consider inanimate. If we could somehow peep at the fundamental essence of our planet, understand its structure at all levels at the same time - from microscopic to macroscopic, we would see the world as an innumerable number of grains of sand, like a giant trembling sphere of atoms. A person can build castles on the beach from thousands of almost identical grains of sand, make jellyfish and everything else that he can only imagine.
Likewise, the countless atoms that make up everything on our planet continually gather, disintegrate and create an ever-changing kaleidoscope of matter. Several of these particles become mountains, oceans, and clouds; others produce trees, fish and birds. Some sets remain relatively motionless and inert; others change with unimaginable speed and puzzle over the complexity of their constructions. Something makes the K'Nex constructor, and something cat.
Original publication: Why Life Does Not Really Exist