Foreign organisms should solve the riddle of life
Life is everywhere on Earth, but despite centuries of diligent efforts, no one has come to any universal definition of what separates life from everything else. We don't know what to look for when trying to find life on alien planets. The researchers will have to create life in the laboratory before learning to differentiate between the living and the non-living.
The boundary for life goes between viruses and bacteria
Biologists usually emphasize three things in living organisms: They have metabolism, can reproduce and are delimited from their environment, for example, by a cell membrane, which makes them individuals.
Not alive: Crystals grow
Crystals can grow by attaching new atoms to the structure, but they have no metabolism.
Not alive: Prions can spread
So-called prions spread by changing the structure of other proteins in their environment. But like crystals, they have no metabolism.
Not alive: Viruses reproduce
Viruses contain reproducible genetic material, but help from a living organism is required.
Live: The bacteria has the whole package
Bacteria have metabolism and cell membranes and can reproduce. Therefore, they are life.
Plants, fungi and animals are colonies of life
Almost all plants, fungi and animals are multicellular organisms. Each cell lives up to the three criteria of life, even though the organism only works when they work together as a collective.
When 22-year-old chemist Stanley Miller set up a pair of round glass flasks connected to tubes in the laboratory of the University of Chicago, USA, his experiment didn’t look like much to the world.
One of the flasks he filled with the gases methane, ammonia and hydrogen, while in the other there was only water. It became a bit more spectacular when he lit the bunsen burner under the piston.
Via one of the pipes, the steam from the boiling water took over to the flask with the gases, where it was illuminated by small flashes that Miller created by energizing two electrodes.
Now it was just to wait for the result.
After a day, Miller could see that a pink liquid had formed in the tube that went from the flask with gases to it with water.
Since the system was closed, it would probably be because some substances had been formed in the process.
Miller had the trial run for a week.
When he analyzed the substances, he found what he had hardly dared to hope for; several biomolecules, including amino acids that are essential building blocks in all life we know.
Miller’s famous 1952 trial proved that simple chemical compounds could be transformed into complex biomolecules in an atmosphere similar to that of young earth billions of years ago.
Miller had thus imitated the first step towards the emergence of life – but only the first.
Although many researchers have followed Miller’s footsteps and made far more advanced experiments, it has proved to be very difficult to move on and take the following, absolutely crucial steps.
The emergence of life is still one of science’s greatest mysteries. If we could solve it, we might also get answers to other universal questions, such as if life on Earth is unique. Is it a result of pure coincidence that we exist or is it a consequence of regularity in nature?
Finally, perhaps we could end a multi-hundred-year discussion of what life is. Then it would be easier to discover life if we came across it elsewhere.
Philosophers give life goals and meaning
As biologists of our time trying to define life, they encounter the same problems that history’s greatest thinkers have wrestled with.
The Greek philosopher and scientist Aristotle initiated the discussion of the nature of life when he made a distinction between the “mineral kingdom” on the one hand and the “animal and plant kingdom” on the other.
According to Aristotle, life required a “soul”. He did not use the word soul in the same sense as we put it today, but defined it as the ability to reproduce, nourish, perceive the outside world and think.
He considered the first two abilities to be most essential, as they apply to all animals and plants.
Aristotle had a so-called teleological view of nature, which means that there is an underlying purpose – perhaps even an end goal – with all processes in nature, especially in the processes of what is living.
A few millennia later, the French philosopher and mathematician René Descartes tried to settle the teleological thinking.
In the 17th century, when machines such as artificial mechanical watches and church organs were developed, he regarded living organisms in the same way.
There was no overall purpose for the processes that took place in living organisms, which he thought were just advanced machines.
Everything could be described based on the physics that were emerging in Descarte’s time and which came to fruition later in the 17th century when Newton formulated his physical laws.
However, the teleological idea would prove to be resilient. Already in the 18th century, it reappeared, then with the well-known German philosopher Immanuel Kant.
For him, life was so advanced that it couldn’t be explained by simple physics. He even went so far as to say that “there will never be a biological Newton”.
Kant said that all processes in classical physics could be described based on cause and effect, but he did not think so with life.
He formulated it with the words that “a living organism is in itself both cause and effect”, that is, a “chicken and egg construction” where cause and effect cannot be separated because they are each other’s prerequisite.
In the following decades, the teleological view of nature as opposed to the idea that modern physics was the key to understanding everything, even the processes of life.
Darwin formulates the laws of life
* A book published in 1859 put the discussion of life’s essence at an end.
The British geologist and naturalist Charles Darwin About the origin of the species were written in a language that everyone understood, so the message was spread quickly.
The content of the book was not only discussed by scientists, but also by the public and in religious and philosophical circles.
With Darwin’s theory of evolution, the teleological view of nature had to give way. Darwin’s explanation of how life develops was in good agreement with modern physics.
The reason that life processes in all organisms appear to be so well-balanced and effective is simply that organisms with endless processes have died out.
Darwin gave the biologists what physicists had for a couple of hundred years.
With the theory of evolution, they were given a set of rules that explain how plants, animals and other organisms will look and function as they do.
Life is continually experimenting with new variants in the form of mutations, while the natural selection ensures that only the most efficient survive and can reproduce.
Thus, the fact that all life forms seem to be perfectly adapted to their environments is not the result of any targeted development. Evolution shoots from the hip, and we only see the hits.
So Immanuel Kant was not right, because with Darwin we just got “a biological Newton” – at least concerning the regularities of life on Earth. Although not all were satisfied, the discussion continued until the 20th century.
The famous Austrian physicist Erwin Schrödinger was, surprisingly, one of the sceptics who questioned whether modern physics could accommodate and explain life’s processes.
Schrödinger focused on the ability of living organisms to hold on to a complex organization in its interior and even expand it over time.
In all other closed systems, the second law of thermodynamics applies, which is also called the law of increasing entropy.
According to this law, order with time is followed by chaos, which when a click of cold, white cream in a cup of hot, black coffee ends as a lukewarm, light brown mixture of both.
This is not the case in living organisms. For Schrödinger, this fact was so important that he thought it should be at the heart of the very definition of life, as it sets it apart from everything non-living.
However, Schrödinger’s thoughts did not lead to a good definition of life.
If that were the case, it would have been something like “life is systems that do not follow other laws of thermodynamics”, or “life is systems that cannot be explained by classical physics”, formulations that say more about what life is than what it actually is is.
All known life forms are astonishingly similar
We can describe life, but it is not the same as defining what it is.
When biologists characterize life as we know it, they highlight a number of characteristics that are common to life on Earth.
The most important is the ability of living organisms to convert energy in connection with their metabolism and to reproduce themselves.
At the same time, the living must be able to be separated from its surroundings, for example, with a cell membrane, so that it can be regarded as an individual.
With these characteristics, we can roughly distinguish the inanimate from the living. Bacteria, for example, live up to these three criteria. However, they do not make viruses because viruses cannot reproduce on their own, but only with the help of other organisms.
The same applies to so-called prions, unique proteins that cause diseases such as mad cow disease in cattle, scrapie in sheep and Creutzfeldt-Jakob disease in humans.
Prions can “reproduce” by causing other proteins to fold in the same way as the prions themselves.
In this way, the prions can spread in an organism, but they have no metabolism themselves and are thus not classified as living.
Crystals can also grow by attaching atoms to them and building on the structure, but that in itself does not make the minerals alive.
On the living side of the border, we find everything from single-celled organisms, plants and fungi to animals and humans.
Life, which has incredible diversity, has adapted to almost every environment found on the planet.
But when biochemists and molecular biologists zoom in on the fundamental processes of cells, life is astonishingly similar.
In all living organisms, it works with the help of DNA strands, which are made up of the same substances and have the same four bases. The DNA strands contain the genetic code with the recipe for the proteins needed to keep the organism alive.
All life forms build their proteins from the same 20 amino acids, even though there are over 100 to choose from.
In comparison, Stanley Miller created eleven different amino acids in his simple experiment in 1952. When he later repeated the experiment with slightly more ingredients in the glass flask, he came up with 23 pieces.
The biochemical similarities between all life on Earth have made biologists convinced that it has the same origin.
If we go back in history, then all branches of life’s large family tree meet at one point.
By analyzing the differences that eventually have arisen in the organism’s genetic material, biologists have concluded that everyone’s common ancestor was a single-celled organism that lived about four billion years ago.
It has been appropriately named Luca, an abbreviation of the English last universal common ancestor, “the last universal common ancestor”.
All Lucas descendants use the same self-building machine, in which DNA, RNA and proteins work together in a process where none of them can be avoided.
That’s why biologists have an ongoing discussion about what came first, yet another “chicken and egg problem” in the style of what Immanuel Kant encountered over 200 years ago.
The fact that all the life forms we know are related to one another is evident even when the researchers zoom further into life’s building blocks.
Five elements form the backbone of all known life forms: oxygen, hydrogen, nitrogen, phosphorus and carbon. Besides, there are a number of other substances that may vary from organism to organism, but the five above are central to all life.
So we know about the essential ingredients of life, but finding the recipe, and thus the definition, of life is a long way off.
In a famous essay published in 1970, American astrobiologist Carl Sagan addressed the problem systematically and tangibly.
He then examines a wide range of definitions of life and finds all unsatisfactory. The story divides the definitions into five groups: Physiological, metabolic, biochemical, genetic and thermodynamic.
For all of them, they are either too wide or too narrow – in some cases both.
Life contributes to the disorder of the universe
Life can reproduce itself. Does that mean that the mule, which is sterile, is not alive?
With this provocative example, Carl Sagan pulls the rug under the genetic definition of life.
He gives the same treatment the physiological and metabolic definitions, which focus on movement and energy conversion.
The story claims that they are wide enough to include machines such as cars but at the same time so narrow that they do not contain plant seeds and fungal spores, which can remain completely still and survive for hundreds of years without energy conversion.
The story says that it is wrong to try to define life based on a single example, and that is only what we know about: Life on our planet.
“Since there is only one kind of life on earth, we lack the larger perspective,” he says.
In his essay, Carl Sagan also agrees with Schrödinger’s idea that life can be defined as a system that violates the other law of thermodynamics.
The story points out that it is only on a small scale that organisms appear to adhere to and develop order. When an organism creates order in its interior, it happens at the expense of the order that surrounds it.
For example, an alga absorbs carbon dioxide. It uses the carbon to build more advanced molecules, but this can only be done through photosynthesis that uses energy in the form of sunlight.
The sun’s radiation is created by the fusion processes in its interior, which increase the disorder. It means that the order that life creates locally on Earth is matched by a much larger disorder that spreads in the sun.
The system thus moves towards an ever more significant disorder, just as the other law of thermodynamics states.
Carl Sagan’s view of life as a more extensive, coherent system is in line with the thoughts of British scientist James Lovelock, one of Sagan’s colleagues at Nasa who, in the 1960s and 1970s, developed his so-called Gaia theory of the Earth and its life.
The biosphere is a single massive organism
On December 30, 1968, Nasa published history’s most famous image from space.
It was taken a few days earlier by the astronauts on Apollo 8, who orbited the moon in their spacecraft.
Just before they flew in over the back of the moon for the fourth time, they looked around and saw the Earth above the sterile, crater-covered lunar landscape.
The world became as enchanted by the image. The sight of the little blue oasis against the dark infinity of space became a powerful symbol, which among other things reinforced the message of environmental and peace movements to take better care of our common, fragile home in the universe.
See the story of Apollo 8 and the image “Earthrise” here:
The picture fit into both the spirit of peace, love and harmony and the view of nature represented by Gaiate theory.
According to the theory that Lovelock named after the Greek word for “Mother Earth,” life is not just a random collection of species, but a single large organism that fills the entire planet.
This organism, also known as the biosphere, consists of the Earth’s surface, water, atmosphere and life. The biosphere is self-regulating and continually strives for an equilibrium position with optimal living conditions.
An example is the oxygen content of the atmosphere. Life itself creates oxygen in the atmosphere through the photosynthesis of plants, and the oxygen is consumed when other organisms breathe, and when organic matter breaks down.
The balance between production and consumption of oxygen is so delicate that the oxygen content of the atmosphere over the past 400 million years has been around 20 percent, which is perfect for all organisms that depend on oxygen – even for us humans.
“Life does more than just adapt to Earth. It changes the Earth to its advantage,” Lovelock said.
Gaia theory can recall Aristotle’s teleological view of living organisms, though on a larger scale.
The theory suggests that there is an overall purpose behind life’s processes, that the organisms act in a way that guarantees excellent opportunities in the future.
Gaiate theory got a lot of attention when Lovelock presented it in the 1960s, but today it does not have many followers.
One inherent problem, in theory, is that it does not explain how all organisms that make up the Earth’s life could act as “sensibly” as the theory indicates.
On the contrary, many researchers believe that life is anything but sensible, that it even tends to destroy itself.
Life makes life miserable for itself
2.5 billion years ago, life on Earth was about to go down.
In a world where the atmosphere was dominated by carbon dioxide, cyanobacteria suddenly began to produce oxygen through photosynthesis.
The new organisms converted carbon dioxide to oxygen at a rate that the other organisms in the world could not adapt to. For them, the oxygen was poisonous. The result was that almost all life died out.
Development for the new cyanobacteria was also catastrophic because, in the end, they had absorbed so much carbon dioxide from the atmosphere that the natural greenhouse effect plunged and threw the Earth into a global ice age that hit them hard.
American palaeontologist Peter Ward uses this example to argue that life on Earth is its own worst enemy, much worse than other threats such as volcanic eruptions and asteroid strikes.
In 2009 he presented his so-called Medea Theory, which is a direct answer to Gaiate theory. According to Medea theory, life is not at all self-regulating, but somewhat self-destructive.
That is why Ward named the theory after Medea in the famous greek tragedy – the woman who kills her children.
As a palaeontologist, Ward specializes in the Earth’s great mass extinctions, and his point is that several of them have been caused by life itself.
According to Ward, it was pure luck that life did not wipe itself out 2.5 billion years ago. He argues that if life, in general, has any form of built-in automation, it is not to save oneself but rather the opposite.
“Life is actually trying to wipe itself out – not consciously, it just happens,” he says.
Ward claims that life on Earth has on several occasions destroyed its living conditions to such an extent that the planet has looked entirely different from what it does today.
It has at times been so inhospitable that an astronomer had not called it an earth-like planet.
Of course, it does not make it easier for astronomers to discover other living planets in our own or another solar system, because then it is not enough to look for planets similar to the Earth of our time.
It becomes even more complicated if the life forms that can exist in other parts of the universe are entirely different from ours.
In astronomers’ search for alien life, they have traditionally been looking for planets similar to our own, that is, planets or moons at a suitable distance from their star, so that liquid water can appear on the surface.
It is, however, an overly one-eyed view, says American philosopher Carol Cleland, a professor at the University of Colorado Boulder, who works for Nasa, among other things.
Cleland, who has written several books on the search for life on other planets, warns to look only for life conditions similar to those we have on Earth.
Life as we know it depends on water, but we do not know if it also applies to other planets. Maybe there are other fluids that alien life can use instead if the organisms are built up in a different way.
Our science gives us a tunnel end
In 2017, the Alma telescope saw evidence that the Saturn Moon Titan had large amounts of the substance acrylonitrile in its atmosphere.
There is no particularly exciting information for the search for life, but it is relevant on Titan. Titanium is an ice-cold planet with temperatures around 172 degrees Celsius, so it has no liquid water that life could utilize.
In return, there are large amounts of liquid methane. For fatty cell membranes, which use the life of the Earth, liquid methane is devastating, but the substance acrylonitrile has the properties required to form similar membranes.
This means that it may be that life on Titan is based on substances other than those on Earth, since the conditions there are completely different. What water and fat substances are for the life of the Earth is perhaps liquid methane and acrylonitrile for life on Titan.
Two years before Alma’s discovery, a group of biochemists and astronomers at Cornell University in the United States had explored that idea.
They created a computer model of an artificial cell with an acrylonitrile cell membrane and concluded that it would actually work.
If the scientists had not happened to make that discovery, it is not certain that the NASA scientists at the Almatella telescope had established that they had found the substance in Titan’s atmosphere.
The example shows how important it is to have both imagination and an open mind in the pursuit of life on alien planets. That’s exactly the philosopher Carol Cleland’s point.
She entirely agrees with Carl Sagan that we know far too little about the nature of life to define it and that an overly narrow definition can be negative in what she calls “the hunt for life we do not know”.
It can only give us the tunnel end.
“How can we generalize all life from a single example, which may not be representative?” she asks.
“If we use a fixed definition of life, we will automatically exclude life that differs from ours when looking for life on alien planets.”
For the same reason, Cleland believes that even here on Earth, we should not draw too sharp boundaries between the living and the inanimate.
Before we learn more about life’s innermost being, we should at least place viruses and prions in a grey zone, for example. It keeps several other researchers looking for alien life with it.
While astronomers are looking for life far out in space, other scientists have followed Stanley Miller’s footsteps and explored life in the lab.
One of them is the American biochemist Steven Benner. He is a pioneer in the field of synthetic biology, where scientists on the chemical pathway create systems that mimic the complex processes of living organisms. In the laboratory, they build biomolecules like amino acids, proteins, RNA and DNA and study their function.
So far, however, they have failed to create a system that is self-functioning as nature’s organisms are.
Benner not only tries to copy life’s known building blocks but also to construct entirely new ones. In 2019, he led a group of scientists who created artificial DNA that is even more advanced than what has been developed on Earth.
The DNA strand, as we know, it has four different bases that can be considered as letters in the “alphabet of life”.
The order of the bases determines the function of the genes and contains the recipe for the proteins that the cells produce. We, humans, contain about 25,000 genes.
Some of them have a few hundred bases, while others have over two million. In total, there are about three billion bases in human heritage.
The base combinations are incredibly large, but with Benner’s artificial DNA, they are even bigger.
In the laboratory, he has created DNA strands which, in addition to nature’s own four bases, have another four. Because of the eight bases, this artificial DNA has been named Hachioji, a compound of the Japanese word for “eight”, Hachi, and “letter”, moji.
Benner’s artificial DNA is, like nature’s DNA, a twisted double strand and Benner’s attempts have shown that it can carry and transmit information in the same way.
The purpose of his project is to show that life’s central building blocks can look different in ways that we know.
“By analyzing the shape, size and structure of the Hachioji DNA, we can increase our understanding of the molecules that may contain information about alien life on other planets,” he says.
Benner’s work is supported by NASA, which wants to use the results to improve the instruments sent away to other worlds to look for life.
The wider they can search, the less likely they are to overlook alien life – if it now exists.
Benner himself hopes that research in synthetic biology will lead to a universal definition of life. Like the Nazi philosopher Carol Cleland, he warns that we take it for granted that the ingredients in foreign life are the same as in life on Earth.
The definition has reached a dead end
We still lack knowledge of life to define it and thus formulate a universal difference between the living and the inanimate.
Although biologists and chemists have given us deep insights into the processes that keep the Earth’s organisms alive, we still do not know how life has come into being.
Therefore, we also do not know how rare life is. Some researchers believe that it arises as a natural regularity, as long as the conditions are right and there is enough time.
If so, we must count on it to have happened countless times in Earth’s history and that it still does, even on alien planets.
Here on Earth, it was our common ancestor Luca who was best suited and thus won in competition with other forms of life.
Since then, Lucas’s descendants have become even better adapted, so that nature’s later attempts to start over from the beginning in new ways have not been given a chance.
Of course, it could also be that life arose on Earth by an unlikely coincidence and that it only happened once. Then there is not much chance of finding it anywhere else.
Another critical issue is how robust life is. If it has a built-in self-preservation drive, as the Gaiate theory holds, it increases the chance that we can encounter life on other planets.
By contrast, as Medea theory claims, the chance decreases if it is self-destructive. In that case, it is a real hit that life remains on Earth, and most likely it has gone elsewhere.
In the pursuit of life on alien planets, it will be a great advantage to have a universal definition of life, so that we can identify it when we encounter it.
However, it is impossible to formulate such a definition, since we do not know of any other life than that on Earth – the chicken and the egg again.
The problem can only be solved when researchers have more than one example of life to work with. We can hope that astrobiologists manage to find life in alien worlds in space – or that it appears in the flask of biochemists.
Steven Benner is confident about who will win the race: “Our first meeting with foreign life will take place in a chemistry lab,” he says.