Most of us probably do not need to think too hard to distinguish living things from the "non-living". A human is alive; a rock is not. Easy!
Scientists and philosophers do not see things quite this clearly. They have spent millennia pondering what it is that makes something alive. Great minds from Aristotle to Carl Sagan have given it some thought – and they still have not come up with a definition that pleases everyone. In a very literal sense, we do not yet have a "meaning" for life.
If anything, the problem of defining life has become even more difficult over the last 100 years or so. Until the 19th Century one prevalent idea was that life is special thanks to the presence of an intangible soul or "vital spark". This idea has now fallen out of favour in scientific circles. It has since been superseded by more scientific approaches. Nasa, for instance, has described life as "a self-sustaining chemical system capable of Darwinian evolution".
But Nasa's is just one of many attempts to pin down all life with a simple description. In fact, over 100 definitions of life have been proposed, with most focusing on a handful of key attributes such as replication and metabolism.
To make matters worse, different kinds of scientist have different ideas about what is truly necessary to define something as alive. While a chemist might say life boils down to certain molecules, a physicist might want to discuss thermodynamics.
For a better idea of why life is so difficult to define, let's meet some of the scientists who are working on the frontier that separates living things from everything else.
Virologists: exploring the grey area at the edge of life as we know it
Did you meet MRS GREN at school? This handy mnemonic is a way for children to remember the seven processes that supposedly define life: movement, respiration, sensitivity, growth, reproduction, excretion and nutrition.
Over 100 definitions of life have been proposed
While this is a useful starting point for defining life, it is not definitive. There are plenty of things we would not traditionally class as living that can tick these boxes. Some crystals, infectious proteins called prions, and even certain computer programmes are "living" according to MRS GREN.
The classic borderline case is viruses. "They are not cells, they have no metabolism, and they are inert as long as they do not encounter a cell, so many people (including many scientists) conclude that viruses are not living," says Patrick Forterre, a microbiologist at the Pasteur Institute in Paris, France.
For his part, Forterre thinks viruses are alive, but he acknowledges that the decision really depends on where you decide to place the cut-off point.
While viruses lack virtually everything that we might think is required for membership of the life club, they do possess information coded in DNA or RNA. This blueprint for life, shared with every living thing on the planet, means viruses can evolve and replicate – albeit only by hijacking the machinery of living cells.
The very fact that viruses – like all life as we know it – carry DNA or RNA has led some to suggest that viruses must belong in our tree of life. Others have even claimed that viruses hold clues to understanding how life began in the first place. If this is the case, life begins to look less like a black-and-white entity and more like a nebulous quantity with confusing not-quite-alive, not-quite-dead borders.
Some scientists have embraced this idea. They characterise viruses as existing "at the border between chemistry and life". And this raises an interesting question: when does chemistry become more than the sum of its parts?
Chemists: exploring the recipe of life
"Life as we know it is based on carbon-based polymers," says Jeffrey Bada from the Scripps Institution of Oceanography in San Diego, California. From these polymers – namely nucleic acids (the building blocks of DNA), proteins and polysaccharides – virtually the entire diversity of life is built.
Bada was a student of Stanley Miller, one half of the duo behind the Miller-Urey experiment in the 1950s – one of the first experiments to explore the idea that life arose from non-living chemicals. He has since returned to that famous experiment, demonstrating that an even greater range of biologically relevant molecules are formed when electricity is shot through a mix of chemicals thought to have existed on primordial Earth.
Life as we know it may require DNA or RNA, but what about life as we do not know it?
But these chemicals are not alive. It is only when they start doing certain interesting things like excreting and killing each other that we accord them that honour. So what is needed for chemicals to make the leap and spring to life? Bada's answer is surprising.
"Imperfect replication of informational molecules would have marked the origin of both life and evolution, and thus the transition from non-living chemistry to biochemistry," says Bada. The beginning of replication, and specifically replication that involves errors, leads to the creation of "offspring" with different levels of ability. These molecular offspring can then compete with each other for survival.
"This is basically Darwinian evolution on the molecular scale," says Bada.
For many chemists, then, it is replication – the process that viruses can undertake only with a helping hand from biological cells – that really helps define life. The fact that informational molecules – DNA and RNA – enable replication suggests they are an essential feature of life too.
But characterising life by those specific chemicals fails to take in the bigger picture. Life as we know it may require DNA or RNA, but what about life as we do not know it?
Astrobiologists: hunting for weird aliens
Second-guessing the nature of alien life is a tricky business. Many researchers, including Charles Cockell and his colleagues at the UK Centre for Astrobiology at the University of Edinburgh, use microorganisms capable of surviving in extreme environments as proxies for extra-terrestrial life. They reason that life elsewhere may inhabit very different conditions, but probably still maintains the key characteristics of life as we would recognise it on Earth.
Sagan referred to a carbon-centric view of alien life as "carbon chauvinism"
"[But] we must keep an open mind to the possibility of finding something that falls outside of that definition," says Cockell.
Even attempts to use our knowledge of terrestrial life to try and spot aliens can throw up confusing results. Nasa, for instance, thought they had a good working definition of life in 1976 when the Viking 1 spacecraft made a successful landing on Mars, equipped with three tests for life. One test in particular seemed to show that there was life on Mars: carbon dioxide levels in the Martian soil were high, suggesting there were microbes living and breathing on the surface of the Red Planet.
In fact, the carbon dioxide the observers saw being released is now almost universally ascribed to the far less exciting phenomena of non-biological oxidative chemical reactions.
Astrobiologists are learning from these experiences and narrowing down the criteria they use to search for aliens – but for now, that search remains unsuccessful.
The creation of artificial life is now a fully-fledged branch of science
Perhaps astrobiologists ought not to narrow their search criteria too far, though. Sagan referred to a carbon-centric view of alien life as "carbon chauvinism", suggesting that such an outlook could hold back the search for extra-terrestrials.
"People have suggested that aliens could be silicon-based, or based on different solvents [other than water]," says Cockell. "There have even been discussions about extra-terrestrial intelligent cloud organisms."
In 2010, the discovery of bacteria with DNA containing arsenic in place of the standard phosphorus had a lot of astrobiologists excited. While these findings have since been called into question, many are still hopeful for demonstrations of life that does not follow conventional rules. Meanwhile, some scientists are working on life forms that are not based on chemistry at all.
Technologists: building artificial life
Once the preserve of science fiction, the creation of artificial life is now a fully-fledged branch of science.
It's trying to take a very broad view of what life is
At one level, artificial life can involve biologists creating new organisms in labs by stitching together parts of two or more existing life forms. But it can also be a little more abstract.
Ever since the 1990s, when Thomas Ray's Tierra computer software appeared to demonstrate the synthesis and evolution of digital "life forms", researchers have been trying to create computer programs that truly simulate life. There are even teams that are beginning to explore the creation of robots with life-like traits.
"The overarching idea is to try and understand the essential properties of all living systems, not just the living systems that happen to be found on Earth," says artificial life expert Mark Bedau at Reed College in Portland, Oregon. "It's trying to take a very broad view of what life is, whereas biology focuses on the actual forms we are familiar with."
That said, many artificial life researchers use what we know about life on Earth to ground their studies. Bedau says the researchers use what he calls the "PMC model" – a program (for example, DNA), a metabolism, and a container (for example, a cell's wall). "It's important to note that this isn't a definition of life in general, just a definition of minimal chemical life," he explains.
Maybe the things we think are essential are really just peculiar to life on Earth
For those artificial life researchers working on non-chemical life forms, their task is to create software or hardware versions of these PMC components.
"Fundamentally, I don't think there is a sharp definition [of life], but we need something to aim for," says Steen Rasmussen, who works on creating artificial life at the University of Southern Denmark in Odense. Teams from around the world have worked on individual components of the PMC model, making systems that demonstrate one or other aspect of it. So far, however, no one has assembled them all together into a functioning synthetic life form.
"This is a bottom-up process, building it piece by piece," he explains.
Artificial life research might ultimately work on a broader scale, building life that is completely alien to our expectations. Such research could help redefine what we understand by life. But the researchers are not at that stage yet, says Bedau. "They don't have to worry about defining all forms of life; maybe they'll talk about it over a beer but they don't need to include it in their work," he says.
Philosophers: trying to solve the riddle of life
So if even those searching for – and building – new life are not yet concerned about a universal definition, should scientists stop worrying about trying to come up with one? Carol Cleland, a philosopher at the University of Colorado in Boulder, thinks so. At least for the time being.
Man tends to define in terms of the familiar. But the fundamental truths may not be familiar
"If you're trying to generalise about mammals using zebra, what feature are you going to choose?" she asks. "Certainly not their mammary glands, because only half of them have those. Their stripes seem the obvious choice, but these are just an accident. They aren't what make zebra mammals."
And it is the same with life. Maybe the things we think are essential are really just peculiar to life on Earth. After all, everything from bacteria to lions is derived from a single common ancestor, meaning that on our chart of life in the Universe, we only really have one data point.
In the words of Sagan: "Man tends to define in terms of the familiar. But the fundamental truths may not be familiar."
Until we have discovered and studied alternative life forms, we cannot know if the features we think are essential to life are actually universal. Creating artificial life might offer a way to explore alternative life forms, but at least in the short term it is easy to imagine how any life form dreamed up inside a computer will be influenced by our preconceptions about living systems.
The definition can actually hinder the search for novel life
To properly define life, we might need to find some aliens.
The irony is that attempts to pin down a definition of life before we discover those aliens might actually make them more difficult to find. What a tragedy it would be if in the 2020s the new Mars rover trundles straight past a Martian, simply because it does not recognise it as being alive.
"The definition can actually hinder the search for novel life," says Cleland. "We need to get away from our current concept, so that we are open to discovering life as we don't know it."
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