Many of us do not need to think too much to distinguish living things from non-living things. A man is alive, a stone is not. It's that simple! However, scientists and philosophers do not believe that such a simple distinction can be limited, pardon the pun. They've spent thousands of years trying to figure out what makes us alive. Great minds, from Aristotle to Carl Sagan, have offered their explanations - and still have not come up with a definition that would satisfy everyone. In the literal sense, we do not yet have a "meaning" in life.
If anything, the problem of defining life has become even more difficult over the past 100 years or so. Until the 19th century, one of the common ideas was that life becomes animated through the "spark of life." Now, of course, this idea has lost its weight in academia. More scientific approaches have taken its place. NASA, for example, describes life as "a self-sustaining chemical system capable of Darwinian evolution."
But NASA's attempt to crush life with one simple description is just one of many. Over 100 definitions of life have been proposed, most of which focus on a handful of simple attributes like replication and metabolism.
To make matters worse, scientists from different disciplines have different ideas about what is needed to define something alive. Chemists say that life boils down to certain molecules; physicists discuss thermodynamics.
To understand why life is so difficult to define, let's meet some of the scientists who are working to define the boundaries that separate living from non-living things.
Virologists: studying the gray area on the borders of life we know
In schools, children are taught to remember seven processes that supposedly determine life: movement, breathing, sensitivity, growth, reproduction, excretion and nutrition.
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While this is a useful start for defining life, it doesn't stop there. There are many things that we could fit into this box and call them alive. Some crystals, infectious proteins - prions, and even certain computer programs will be "alive" if we proceed from these seven principles.
The classic borderline example is viruses. “They are not cells, they have no metabolism and they remain inert until they encounter cells, so many people (including many scientists) conclude that viruses are not living,” says Patrick Forter, a microbiologist at the Pasteur Institute. in Paris, France.
Forter himself considers viruses to be alive, but admits that the decision depends on where you decide to put the cutoff point.
While viruses lack many of the things needed to enter the club of life, they have information encoded in DNA or RNA. This is a strong marker of life that any living creature on the planet possesses, which indicates that viruses can evolve and multiply - albeit by breaking open living cells and invading them.
The fact that viruses - like all life we know - carry DNA or RNA has led some to think that viruses should take a place in our tree of life. Others have generally stated that viruses keep the secrets of the very appearance of life. And then life ceases to seem black and white and becomes rather a vague size with not quite living and not quite dead boundaries.
Some scientists have adopted this idea. They characterize viruses as existing "on the border between chemistry and life." And this raises an interesting question: when did chemistry become more than the sum of its parts?
Chemists: study the recipe of life
"The life we know is based on carbon-based polymers," says Jeffrey Bada of the Scripps Institute of Oceanography in San Diego, California. From these polymers - namely, nucleic acids (the building blocks of DNA), proteins and polysaccharides - literally all the diversity of life has grown.
Bada was a student of Stanley Miller, one half of the duo who was behind the Miller-Urey experiment in the 1950s - one of the first experiments to figure out how life emerged from non-living chemicals. He has since returned to this famous experiment and demonstrated an even greater range of biologically suitable molecules that are formed when electricity is passed through a mixture of chemicals believed to have existed on primitive Earth.
But these chemicals are not living. It's only when they start doing some interesting things like excreting or killing each other that we allow them that honor. What does it take for substances to make the leap to life? Bada has a pretty interesting answer.
“An imperfect replication of information molecules could herald the origin of life and evolution and thus bring about this transition from non-living chemistry to biochemistry. The beginning of replication and, in particular, replication with errors marked the beginning of "offspring" with different abilities. These molecular offspring could then begin to compete among themselves for survival.
“This is essentially Darwinian evolution at the molecular level,” says Bada.
For many chemists, it turns out that replication - a process that viruses can only do with biological cells - helps define life. The fact that informational molecules - DNA and RNA - provide replication suggests that they are also an essential feature of life.
But characterizing life for these specific chemicals does not open up the bigger picture. The life we know may need DNA or RNA, but what about life we don't know yet?
Astrobiologists: hunt for strange aliens
Determining the nature of alien life is not easy. Many scientists, including Charles Cockell and colleagues at the Center for Astrobiology at the University of Edinburgh, use microorganisms that can survive in extreme conditions as test specimens of extraterrestrial life. They believe that life elsewhere could be in completely different conditions, but will most likely inherit key characteristics of life as we know from Earth.
“But we have to keep our minds open to the possibility of detecting something that is completely outside this definition,” says Cockell.
Even trying to use our knowledge of earthly life to try to find aliens can lead to mixed results. NASA, for example, believed it would do a good job of defining life in 1976, when the Viking 1 spacecraft successfully landed on Mars, equipped with three experiments for life. One test, in particular, showed that there was life on Mars: the level of carbon dioxide in the Martian soil was high, which means that microbes lived and breathed in it.
But the carbon dioxide seen on Mars is now widely explained by the much less exciting phenomenon of nonbiological oxidative chemical reactions.
Astrobiologists have learned from these experiments and narrowed down the criteria they use to find aliens - but so far, their search has been unsuccessful.
However, astrobiologists shouldn't narrow down their search criteria too much. Sagan considered the carbon-centric search for aliens "carbon chauvinism", believing that such an approach would be very narrow-minded.
“People assumed that the aliens could be silicon-based, or use other solvents (not water),” Cockell says. "They even talked about extraterrestrial intelligent cloud organisms."
In 2010, the discovery of bacteria with DNA containing arsenic instead of standard phosphorus amazed many astrobiologists. Although this finding has been questioned more than once since then, many quietly hope that life will not follow the classic rules. At the same time, some scientists are working on life forms that are not based on chemistry at all.
Technologists: Build Artificial Life
Once upon a time, the creation of artificial life was completely at the mercy of science fiction. Now it is a full-fledged branch of science.
For the time being, new organisms in the laboratory can create biologies by simply piecing together parts of two or more known life forms. But this process can be more abstract as well.
Ever since Thomas Ray's Tierra computer program attempted to demonstrate the synthesis and evolution of digital "life forms" in the 1990s, scientists have been trying to create computer programs that truly imitate life. Some are even starting to create robots with life-like traits.
"The general idea is to understand the essential properties of all living systems, not just living systems that have been found on Earth," says artificial life expert Mark Bedo of Reed College in Portland, Oregon. "This is an attempt to take a very broad view of what life is, while biology focuses on the real-life forms that we are familiar with."
Of course, many artificial life researchers use everything we know about life on Earth as the basis for their research. Bedo says the researchers are using what is called a "PMC model" - programs (eg DNA), metabolism, and a container (eg, cell walls). “It is important to note that this is not a general definition of life, just a definition of minimum chemical life,” he explains.
Working on non-chemical life forms, scientists are trying to create software or hardware versions of PMC components.
“Basically, I don’t think life has a clear definition, but we have to strive for something,” says Steen Rasmussen, who is working on artificial life at the University of Southern Denmark in Odense. Groups around the world have worked on individual components of the PMC model, creating systems that demonstrate one aspect or another of it. So far, no one has managed to put it all together into a functioning form of synthetic life.
“It's a top-down process, lining up piece by piece, he explains.
Research into artificial life can also be beneficial on a broader scale, creating life that is completely alien to us. Such research helps us refine our knowledge of life. But it is too early to talk about the results.
Philosophers: trying to solve the riddle of life
Well, even if those who are looking for - and trying to create - new life are not worried about its universal definition, should scientists stop worrying about reducing all definitions to one? Carol Cleland, a philosopher at the University of Colorado at Boulder, thinks so. At least for a while.
“If you're trying to generalize mammals using a zebra, which trait would you choose?” She asks. “Definitely not her breasts, as only half have them. Their stripes seem like an obvious choice, but they are just a coincidence. This is not what makes zebras mammals."
It's the same with life. It may be that the things we think are important are really only life on Earth. After all, everything from bacteria to lions descended from one common ancestor, which means that in the universe our life is just one point in the data.
As Sagan said: “Man tends to define in terms of the familiar. But fundamental truths may not be familiar."
Until we have discovered and studied alternative life forms, we cannot know which traits important to our life are truly universal. Making artificial life may offer a way to explore alternative life forms, but at least in the short term, it's not hard to imagine how any life created on a computer would affect our beliefs about living systems.
To define life more accurately, we need to find aliens.
The irony is that trying to define life before we find them can make it harder to find them. How tragic it will be if in the 2020s a new rover passes by a Martian simply because it does not recognize him as a living being.
Finding a definition of life can get in the way of finding a new life. We need to move away from our current concept and be open to discovering life, even if we don't or don't know it.
ILYA KHEL
