It may seem logical to start the story of the search for life at … well … the beginning. Yet it won’t make much sense without first knowing what life actually is and what it needs to survive. It isn’t pretty and is chemistry heavy, but do not worry – it will prove to be interesting and provide a solid background to what makes life possible. On our planet, living organisms have spread to every nook and cranny that can sustain them – in every direction you look; across land, water and air; from the driest deserts, balmy tropics, and tallest mountains to the coldest Arctic ice; on, within and under rocks and deep within nuclear reactors; there are even living beings inside the bodies of other living beings. Yet even with all this life, we still cannot quite explain it. The question ‘What is life?’ seems so simple to answer. If something is alive, it is obvious – a chair isn’t alive, although the cute little kitten asleep on it is. What distinction causes us to consider one to be living and one not? We only have direct knowledge of one form of life, life on Earth, and have only one data point from which to extrapolate theories about its chemistry and essence. Until life is encountered elsewhere, or aliens finally contact us, we will not have an independent second data set. Even then we may not, if the alien life itself shares an ancestor with life on Earth.
So What is Life?
To truly understand life, we need to explore what it is made of, what it looks like, and what it does. One of the oldest philosophical enigmas ever posed is that of life, and remains to this day a query with no (or quite possibly many) answers. Life on Earth is recognisable; we normally know a living organism when we see it as our brains identify a number of qualities all living organisms display. However, just because an entity presents one or all of these, there is the chance that it is still not technically a living organism. We cannot rely on our gut instinct and say something is alive because we know it is such – we need proof.
To start with, we can look at what life is made of. All life that we know about is formed of a close-knit family of units, popularly known as cells. Simple life forms are mostly made up of one single lowly but powerful cell (they are called unicellular), while advanced life forms, such as humans, are built of a magical mix of many millions and millions of cells working together (unsurprisingly, they are called multicellular). Cells do absolutely everything; they provide structure for the body, take in nutrients from food, convert those nutrients into energy, and carry out specialised functions that keep their host bodies working and running smoothly. They can do this because each individual cell is bound by a wall called a membrane that acts as a choosy barrier to the outside environment, sometimes letting in molecules and ions that the cell needs or pushing them out to keep the inside of the cell working properly. Cells are also the librarians of life. They contain the story of the organism they are a part of, holding its hereditary material within deoxyribonucleic acid or DNA and storing an instruction manual on how to re-build every aspect of that particular life form. Owing to life’s dependence on the cell, materials in living organisms are always seen to exhibit some kind of unit order, and so we believe this order is a necessary condition for life to exist. Since cells and order break down over time, however, death for sure is also an inescapable characteristic of life, but alone does not make something alive in the first place. Besides, we hardly want to wait around for a potential life form to die, just to prove it was once living. As an example, this book has a structural unit order of paper and pages and this order can be broken and destroyed over time, but we would not consider it alive, or in fact dead.
For cells to do their jobs, and run the organisms they are a part of, they need energy and this is a common element always found flowing through living systems. Cells gobble up chemical compounds from the environment, transform them and spew out waste products as residues. In this way, cells obtain the basic materials to build their own body parts, and at the same time gain the energy needed to carry out the thousands of biochemical reactions that must happen every day, such as reproduction, growth, thought and movement. The need for a life form to get around is actually one of the most important uses of its energy. To enable it to survive, an organism must look for food, escape from predators and react to changes in the environment – if it starts to get too hot or cold for survival, then being able to move and find shelter or hunt for more comfortable conditions becomes really rather important. Sometimes an organism does not have to move physically but can interact with its surroundings and actually respond to the changes imposed on it. For example, when it gets hot humans and horses instinctively sweat, dogs pant, and elephants and rabbits send more blood flow to their ears to carry the heat away from their bodies. At a basic level, cells also respond to physical and chemical environmental stimuli and can communicate among themselves about how best to react through small signalling molecules. Using energy, enacting movement and responding to a changing environment are all activities that we have observed in living organisms, but those actions or responses cannot be used as universal indicators or markers to define life. A refrigerator utilises energy to regulate the temperature of its environment and a thermostat in your home increases the heat coming out of the radiators when it senses the room getting colder. Although we appreciate these objects and even in extreme cases rely on them for our own survival, they are most definitely not living beings.
The cellular custodians of life’s instruction manual can be reproduced and in doing so pass on this hereditary information to the next generation of cells. Reproduction is when cells duplicate their genetic material and divide to produce two new ‘daughter’ cells, similar to their mother. All living organisms reproduce or at least are the products of reproduction. One exception to the former statement is mules, which are the hybrid products of reproduction between a horse and a donkey. Mules cannot reproduce themselves: they are sterile. Does this mean they are not alive? We remain reasonably certain that they are. Viruses are tricky; they do reproduce, as the rapid spread of the common cold shows, but they cannot do it alone – they need to hijack the molecular machinery of a host they have infected. Does this mean they are alive or not? Defining life using an entity’s ability to reproduce itself is a tough ask, as it means a computer virus could technically be considered a living organism.
Life grows and develops in patterns, yet the final forms, the offspring, are never a completely identical copy of the original. This is the cornerstone of evolution. Evolution is the passing on of heritable traits or characteristics, both good and bad, from parent to child and occurs within every generation born. This transfer of information to build a new being is, however, imperfect. In some circumstances, a mutant gene is introduced that imparts either a survival advantage or disadvantage for the newly created life form, making it better equipped to survive in the world it is entering, or possibly cause its untimely death. Evolutionary adaptation over generations is regarded as the most fundamental and unifying of the properties of life mentioned thus far. One of the most famous British scientists, Charles Darwin (1809–1882), has become synonymous with this fascinating skill of life. His research into what he termed ‘natural selection’ highlighted that those individuals in a population whose traits best enable them to survive and reproduce will go on and produce more offspring, which in turn will survive to reproduce. This is where the phrase ‘survival of the fittest’ came from. Evolution and an organism’s chance of survival is essentially a genetic lottery.
There are many forces found in nature that fulfil nearly all of the above characteristics of life and could theoretically be classed as living organisms but we would still, in the face of all this proof, not say they are alive. Fire is a perfect example – it grows and moves as it encounters material to consume, providing it with more energy. As such it is self-sustaining, as long as the food remains available. It breathes oxygen and responds to changes in its environment. It excretes waste products of ash, heat and carbon dioxide by some of the same reactions that run the cells in our bodies, and it is even able to reproduce itself and make little fire babies. Like any life form, it can be brought into existence from a parent fire, or born from just striking a match. Even with all this evidence, we still do not say it is alive. Why? Is it because it hasn’t demonstrated any intelligence or shown itself to have some kind of spirit or soul?
What if, instead of this list of physical characteristics that we can easily find exceptions to, we try and define life by what it does, rather than from its composition. I like this idea as it can incorporate life forms from the past, those being created now and those that will possibly be designed in the future. Who is to say that a robot or hologram that can think and make decisions, and may even be able to feel emotions or pain, is not alive, just because it is not made up of cells; for growth requires installation of a new part or algorithm instead of growing one itself; is born of a creator instead of a genetic parent; and does not store its blueprints within DNA or is silicon- not carbon-based? The question of what life really is remains by far one of the hardest, but also the most excitingly challenging to answer.
Carbon, Carbon Everywhere!
It is difficult to comprehend, but at the heart of all life is a single element: carbon. Every life form on Earth is built upon a skeleton of carbon and some even need a source of carbon-based food for energy. Since all atoms are essentially put together in the same way – with a nucleus housing a variable number of neutrons and protons, orbited by shells of electrons – what makes the carbon atom so special that life put all of its hopes for survival on it? The answer lies in the periodic table, an organised list of all the currently known elements in existence. As you read the table from left to right, both the number of protons within and electrons around the nucleus of each atom of each element increases. The table is also broken into columns, grouping together elements that have a similar number of electrons present in their outer shells. Typically, it is only this outer coating of electrons that gets involved in chemical reactions and is important for this story. Carbon has four electrons out of a possible eight in its outermost shell – it is half full. As the most stable configuration is to have all eight electrons present and accounted for, each atom of carbon has the ability to form up to four bonds with electrons orbiting nearby atoms to achieve this stability. The ability to form four bonds is not restricted to carbon – it’s a property of every atom with four outer electrons, including those that sit below carbon on the periodic table, such as silicon, germanium, tin and lead. What’s special about carbon is that it can form complex molecules built of double bonds, sharing more than one electron with other atoms, and these bonds are very strong. The simplest carbon molecules consist of a carbon skeleton bonded only to hydrogen atoms, unsurprisingly called hydrocarbons. There are more than a million known carbon compounds, broadly termed organic molecules, and it is finding these molecules that drives our current exploration of the Solar System and beyond. These organic building blocks are rife throughout the Galaxy, strengthening the idea of carbon-based life existing outside of the Earth.
The Units of Life
The cell mentioned earlier can be imagined as a membrane-bound miniature chemical reactor containing a library of genetic information, and is the building block of every living thing on Earth. The first known cells are thought to have originated in the oceans of the early Earth about 3.8 billion years ago. These were the prokaryotes, organisms made up of one single cell, such as bacteria and archaea. These cells do not have any internal organs, nor do they have a nucleus to house their hereditary material. Instead, their genetic instruction manuals float freely as a twisted closed loop of DNA within a watery cytoplasm inside the cell. For a billion years, the prokaryotes reigned supreme throughout the waters of the Earth. More or less 2.7 billion years ago, however, the Earth and evolution decided their dictatorship was over and introduced cyanobacteria to the world – tiny bacteria capable of converting energy from sunlight into food. They not only produced food but also became food themselves. The larger original prokaryotes found these tiny new bacteria highly appetising, and enveloped them in their plasma membranes (mouths had not yet been invented). This resulted in a larger prokaryote with a smaller cyanobacterium inside; instead of the former digesting the latter, a symbiotic relationship developed whereby both organisms mutually benefitted from the new shared living situation. Over generations, these cyanobacteria stopped being a separate organism and became a part of the cell itself.
A second type of cell with internal organs, or more correctly organelles, had arisen through evolution and this step changed the course of life on Earth forever. These cells, known as eukaryotes, have since been used to build everything from fungi to plants to humans. Most eukaryotic cells, or animal cells, are invisible to the naked eye but can do everything from providing structure and stability to creating energy and a means of reproduction for an organism. We could not have evolved without eukaryotic cells, although interestingly in the average human body prokaryotic bacterial cells vastly outnumber eukaryotic human cells – so maybe we are more bacterial than human. Eukaryotes are much more complex than prokaryotes and have a genome that is up to 10,000 times larger, housed in a control centre called a nucleus. Eukaryotes evolved thanks to the predatory actions of prokaryotes gobbling up other prokaryotes who, instead of lunch, became a part of the original organism. However, even after their initial inception this eating of other organisms continued, and rather than being digested provided the eukaryotes with even more organelles, enabling them to evolve into ever more sophisticated cells. Inside eukaryotes there are mitochondria, which are the powerhouses of the cell, and perform reactions that extract energy from food. Within plant and algae cells there are chloroplasts, which perform the light-harvesting reactions of photosynthesis. Neither of these organelles is native to eukaryotic cells and chloroplasts are believed to have once been those free-living cyanobacteria that were engulfed by early prokaryotic cells but not digested. Over evolutionary timescales, the symbiotic relationship between the host cell and visiting bacteria developed to a point at which they were inseparable and required each other for survival.
Food for Thought
Every living organism – from the smallest bacterium to the tallest tree and fastest mammal – needs a source of food and an input of energy flowing through its system to survive, driven by an intricate process of chemical reactions within each cell known as its metabolism. A strong metabolism allows organisms to grow and reproduce, digest and transport substances into and between different cells, maintain their structures, and respond to their environments. To power this metabolism, humans eat food, plants absorb sunlight, and microorganisms use energy produced from chemical reactions through a process called chemosynthesis. To make these reactions take place, a very special molecule called ATP (adenosine triphosphate) transports the energy created by photosynthesis in plants, by cellular respiration in animals and by chemosynthesis in bacteria and archaea. As such, life cannot only be characterised by what it looks like, what it can do or whether or not its cells have a nucleus, but also by how it uses carbon and energy to metabolise. There are four categories that can be used to describe a life form based on the sources of carbon and energy available to them, and they are extremely helpful for hypothesising how and what type of life might be able to exist on other worlds.
Life can use carbon directly from the environment by drawing it from carbon dioxide found in the atmosphere or dissolved in water. This is how plants find carbon for their energy and so are classified as autotrophs. Other life forms derive their essential carbon from consuming pre-existing organic compounds through eating, such as animals and many microscopic organisms and these are called heterotrophs. Human beings are heterotrophs. There are also two sources of energy available to life: plants use the power of sunlight through photosynthesis, termed photo-; and animals use chemical energy from reactions that happen when organic compounds are eaten, termed chemo-. Combining these carbon and energy sources we find photoautotrophs – organisms such as plants and microbes that absorb their energy from sunlight and carbon from carbon dioxide in the environment. Chemoautotrophs draw their energy from chemical reactions using inorganic chemicals and carbon from environmental carbon dioxide. These organisms need neither food nor sunlight to survive and are found in environments where most other organisms would perish (we will meet a lot of these hardy little life forms later on). Photoheterotrophs get their energy from sunlight and carbon from consuming other organisms. This is rare but possible by some bacteria, such as Chloroflexus. Finally, chemoheterotrophs get both their energy and carbon from food – we humans are chemoheterotrophs. In general, eukaryotes can only feed on certain carbon sources, which can be pretty restrictive, but bacteria and archaea are truly remarkable as they can live off almost any imaginable foodstuff out there.
Molecules of Life
Life on Earth is built from only 24 of the greater than 100 known elements on our planet, each with properties that seem to be essential for a healthy metabolism. With so many elements available it almost seems a pity, and a tad risky, that just four elements – oxygen, carbon, hydrogen and nitrogen – dominate and control 96 per cent of the mass of a typical living cell. Life uses a polymer-based chemistry that includes nucleic acid polymers, DNA and RNA, to store and transmit information; carbohydrates for energy; and, when these are scarce or running low, fats. Yet the most diverse and multitasking of life’s molecules are proteins, performing a vast array of functions within living organisms, including catalysing (speeding up or slowing down) metabolic reactions, replicating DNA, responding to stimuli, transporting molecules from one location to another and even building muscle. All proteins within living organisms on Earth are made up of the same 22 amino acids, from a selection of more than 500 found in nature. The key elements of every amino acid are the same as those of cells, namely carbon, hydrogen, oxygen and nitrogen. A fascinating quirk of amino acids is that there are two types; they are considered to be asymmetric molecules or chiral. To explain this, hold out your hands in front of you. Human hands are perhaps the most universally recognised example of chirality: no matter how you try and orientate both hands it is impossible for all the major features of both hands to match up – a truly opposite mirror image. The same can be said for amino acids. There are, therefore, two possible forms characterised as either left-handed (L) or right-handed (D). Amino acids occur in both L- and D- chiral forms, but nearly all life on Earth uses the L-form. Sugars are also found in both chiral forms, although terrestrial living cells use the right-handed D-forms exclusively. Why life chooses these particular mirror images over the other is not yet understood. Organic material found in carbon-rich meteorites also seems to have a bias towards a similar handedness. The evidence is mounting that a predisposition to one form over another occurs naturally across all bodies in the Solar System and adds weight to the idea that the precursor molecules for early life are perhaps related to those in space or may even have come to Earth from the cosmos.
Could alien life have the same left-handed L-form amino acids as we do or would they use right-handed D-form instead? Perhaps they use left-handed L-form sugars instead of right? If all life in the Universe, both terrestrial and alien, spawned from the same pool of early molecules, then theoretically every molecule in the Universe would have the same chirality as is found on Earth. It would therefore be quite hard to tell if the life were truly alien, or just a very, very distant relative. The universe of chemical possibilities is huge. For example, the number of different proteins that can be built from combinations of just the naturally occurring 22 amino acids is larger than all the number of atoms in the cosmos. Life on Earth certainly did not have time to sample and test all possible sequences and combinations to find the best. What exists in modern terrestrial life must therefore reflect some chance events in history that led to one choice winning over another, whether or not the choice was ideal. Perhaps some features of Earth’s biochemistry emerged because of some now – unknown selective pressures on early life that no longer exist. Today’s protein make-up and handedness may therefore not represent the finest design for survival in the modern world, but rather be a vestige of optimisation in an ancient one, such as is the case with the human appendix – once possibly needed in early humans for digesting leaves, today it is just a leftover of an ancient organ that has lost most or all of its original function. Who is to say, therefore, that life in the Universe may, or may not, have followed a similar pattern?
All life forms contain some form of DNA – the vessel containing all of the information-storing genes – life’s genetic blueprint. It also has RNA (ribonucleic acid), which transfers information from the genes to enable the production of the cells’ proteins. DNA is shaped like a long ladder twisted into a spiral – a double helix (RNA looks like one half of this ladder). Each strand of DNA’s ladder has a carbon backbone of sugar molecules and phosphate groups and attached to it, making up the rungs, are chemical subunits known as bases. DNA is composed of four bases: Adenine (A), Thymine (T), Cytosine (C) and Guanine (G). These letters represent the code for building amino acids, that themselves make up proteins. The bases bind the two DNA strands together, with an A always bonding to a T on the opposite strand (and vice versa), and C and G doing likewise. A big question asked by biochemists is why DNA uses these precise bases, in particular adenine, when there are better alternatives? Maybe it was chosen over other available candidates by a freak accident, and was kept because later on it was thought too difficult to replace without losing fitness of the existing life forms. Potentially it was because adenine can be made prebiotically (chemically and before the formation of the first life forms) from ammonium cyanide, and had a much greater availability during the earliest eras in Earth history, making it a better choice in those times for starting life – even though a different contender might now be preferable. Fun fact: if the entire DNA in just one of your cells were unpacked and stretched out straight, it would be nearly 2m (7ft) long. Since you have about five trillion (5,000,000,000,000) cells in your body and just over 2m (7ft) of DNA in every cell, the total length of DNA packed into our bodies would stretch from here to the Moon and back 1,500 times.
No experiments can presently test all of the theories as to why life is built the way it is and uses certain molecules over others in its construction. Trying to understand the possible reasons for such choices allows us to appreciate how easy alternative explanations are, and therefore the number of alien life forms imaginable.
Is Carbon Really the Only Option?
If for some now hidden reason life chose this amino acid over that one, left-handed over right, DNA over RNA, then what if it had not chosen carbon? For years, scientists and science-fiction writers alike have dreamed about the possibility of life based on some other element. To replace life’s dependency on carbon would require a carefully chosen competitor. This challenger would have to be an element that is found in abundance across the known Universe and behave in a similar way to carbon, if life as we know it is still to function. Silicon is the first entertaining possibility. It sits nestled directly below carbon on the periodic table so has a similar personality. It has the same four electrons in its outer shell, meaning that it has four electron spaces available, giving it the ability to make four single bonds with other atoms, just as carbon does. It can bind readily to itself to make Si-Si bonds much like carbon can to other carbons, and it also bonds easily to hydrogen and oxygen given the right conditions. On Earth, silicon is more abundant than carbon. It bonded with two oxygen atoms and formed SiO2 or quartz, the primary constituent of the rocks that make up the planet. The Earth is actually a silicon-rich, carbon-poor world with silicon unlikely ever to be in short supply.
So why on Earth did life choose carbon over silicon? One obvious answer is that outside the Earth and throughout the Universe there is much more carbon available than there is silicon, as fewer of the larger silicon atoms are formed within the cores of stars (we will explore how this happens in the next chapter). Silicon is used by life such as that found in the seashells abandoned along the beach but it is not the basis for any polymeric or metabolic chemistry. If complex silicon chemistry were possible on Earth, surely it ought to have resulted in life based on silicon, rather than its rarer chemical cousin, carbon. The answer may lie in the bonds that silicon makes with other elements, and how these may or may not be useful for life. For starters, it’s a larger atom so the bonds it makes with other atoms under the conditions found on Earth are weaker than those made by carbon. There is also a huge difference between what happens when silicon and carbon bond with all the oxygen floating around the planet. Under the conditions found on the Earth, the molecule carbon dioxide (one carbon and two oxygen atoms) is a gas at most temperatures, is very soluble in water (and is therefore available in liquid solutions for life), and can be broken down into its constituent elements of carbon and oxygen – both of which are incredibly useful for life. In contrast, silicon dioxide (one silicon and two oxygen atoms) does not exist as a gas, except at extremely high temperatures over 2,000°C (3,632°F). As can probably be anticipated by the fact that it is the constituent of many rocks on Earth, silicon dioxide is almost completely immune to being dissolved; it’s pretty solid. Finally, because silicon really loves to be bonded to oxygen, it is very difficult to break silicon dioxide into its constituent atoms. With respect to living organisms, silicon dioxide can be considered a very inert molecule and therefore somewhat useless for life processes. Consequently, carbon and carbon dioxide win the competition for being more useful to life, both as a molecule and split into individual elements.
Does this really matter when searching for alien life in the cosmos? Do the rules of chemistry work in the same way throughout the Universe? Would we observe silicon behaving differently on another planet if it had an environment unlike that of the Earth? Based on observations made by astronomers, the answer is probably no. Across the cosmic environment of the interstellar medium – interstellar clouds, meteorites, comets and stars – carbon molecules run rampant; not just simple ones, but also some of the more complex organic molecules as well. Oxidised silicon, such as silicon dioxide, is quite common in the cosmic environment although silicon molecules such as silane and silicones that we would consider as silicon-based life molecules are seldom identified.
Perhaps counter-intuitive elements such as arsenic might be capable of supporting life under the right conditions? On Earth, some marine algae incorporate arsenic into complex organic molecules, such as arsenosugars and arsenobetaines. Several other small life forms use arsenic to generate energy and facilitate growth. It has even been speculated that the earliest life forms on Earth may have used arsenic in place of phosphorus in the structure of DNA itself. Nonetheless, at no point has it been proposed as a possible replacement for carbon as the key to life. Titanium, aluminium, magnesium and iron are all more abundant in the rocks of the Earth’s crust than carbon; so metal-oxide-based life could even be a possibility under some very non-Earth-like conditions found on a different rocky world. Boranes may also be an option. They are dangerously explosive in Earth’s oxygen-rich atmosphere, but would be more stable in a reducing environment, one with little oxygen. However, boron’s low cosmic abundance in comparison to carbon makes it rather unlikely as a base for life. What about chlorine and sulphur? Although purely hypothetical, sulphur could replace carbon, as it is capable of forming long-chain molecules just as carbon does. Some terrestrial bacteria have already been discovered to survive on sulphur rather than oxygen but have not as yet been found to replace carbon. Nitrogen and phosphorus could also potentially form biochemical molecules since phosphorus behaves like carbon in that it can form long-chain molecules on its own and, when combined with nitrogen, can create quite a wide range of useful molecules. Thus far, with no examples of any of these alternative life forms currently in existence, we only have one form to study: that of carbon-based life. In our quiet corner of the Galaxy, the organic building blocks of life are rife and are thought to have rained down upon all primordial worlds. It seems, at least for now, that searching for life built from carbon is the only truly sensible way to go.
Living is Thirsty Work
Carl Sagan famously dubbed Earth the ‘pale blue dot’ for its ubiquitous liquid. Water occurs naturally across the Earth’s surface in all three phases – as a solid at the poles, a liquid in the oceans, and a gas in the atmosphere. Tasteless, odourless and virtually invisible as water vapour, it covers 70 per cent of our planet. The total liquid water on Earth is somewhere in the range of 1,260 million trillion litres (326 million trillion gallons), although 97 per cent of this is undrinkable salt water filling the oceans and seas of the planet. Only two-and-a-half per cent of all the water on Earth is fresh water and all life living on dry land is reliant on this tiny percentage. Human life can use only a fraction of this, less than one per cent, and of that, about 70–90 per cent is used for agriculture. That does not sound like a great deal is left for us to drink, does it? But actually it is! Given the enduring presence of water on Earth’s surface, it is not surprising that early life, and all subsequent life forms, were and are based upon and reliant on water. All life exists in an environment of water, whether it lives within it or uses it to form part of cell structures or as the main solvent in its metabolism.
It Came from Space … Or did it?
Where this life-giving fluid came from, however, is hotly debated. The infant terrestrial planets were completely devoid of both water and carbon; they were simply too hot, what with being newly formed and recently molten. This means that the water required to allow life must have risen to the surface of the Earth somehow or come from somewhere. We know it showed up after the Earth’s formation but probably only within the first billion years or so, either from deep within the cooling planet or from the reaches of space on board comets and water-rich meteoroids. Although the population of comets and asteroids passing through the inner Solar System is, thankfully for us, sparse today, it was a much busier time when the planets and Sun were young. Because our planet is in the Solar System’s Goldilocks Zone, a region encircling the Sun where water has the opportunity to remain stable as a liquid, once the water molecules had surfaced they remained, and quite possibly played a key role in the development of life.
Until recently, it was believed that collisions with icy bodies from the outer Solar System likely brought much of the Earth’s water. However, this theory was dealt a hard blow in 2014 as incredible results emerged from Europe’s Rosetta mission (of which we will learn more in Chapter 5). This groundbreaking venture made history by landing on Comet 67P/Churyumov-Gerasimenko in November 2014, and revealed that the water on the icy body is unlike any found on our planet. While the vast majority of water on our planet is made up of hydrogen and oxygen atoms, very occasionally we find a hydrogen atom has been replaced with a deuterium atom. Deuterium is an isotope of hydrogen, but holds two neutrons rather than just one in its nucleus, so is heavier. On Earth, for every 10,000 water molecules, three deuterium atoms can be found. This water has the same physical properties, but owing to the addition of deuterium is heavier. Comet 67P was found to contain water that was 3 times heavier than water currently present on the Earth, which means that this variety of comet could not have brought water to our planet. This discovery adds to other studies that have analysed water on different types of comet, such as those that originated in the Oort Cloud – a region of space that makes up the outer reaches of our Solar System – which also has a different signature to water found on Earth.
Many scientists now believe that Earth may have had water from the start, inheriting it directly from the swirling nebula that gave birth to the Solar System. The conventional story followed the journey of carbonaceous chondrites (water-rich varieties of asteroid) that would have delivered water during the late stages of Earth’s formation, possibly around 4.6 billion years ago, and meteorites do provide some of the answers we are looking for. Carbonaceous chondrite meteorites have been dated as some of the oldest rocks in the Solar System, formed around the same time as the Sun, before the first planets, and they have isotopic signatures of hydrogen similar to Earth’s seawater. However, it is now thought that the signatures of seawater have changed over geological time, gradually getting heavier. The original seawater on Earth does not match that found within asteroids but has a hydrogen isotopic ratio closer to that of Jupiter and the solar wind. These are both thought to preserve the original isotopic signature of the solar nebula. As such, water may have snuck into our own growing planet, despite its scorching temperatures, by sticking to dust particles. Some of it may well also have arrived from space, although only around 10 per cent appears to have originated from comets from the Kuiper Belt and the Uranus–Neptune region of the Solar System. From the perspective of life, however, the source of the water is relatively unimportant – that it is there is all that matters.
Although the exact mechanisms are poorly understood, it is clear that liquid water, was present on the surface of the Earth only a short time after its formation. Earth is not unique in containing liquid water, however. Jupiter’s moon Europa is covered with a sheet of ice that probably sits on top of a global salt-water ocean, and Saturn’s moon Enceladus shows evidence of sub-surface water as well. Mars, meanwhile, was once a relatively warm and wet world that apparently harboured large amounts of liquid water in the ancient past – it is the persistence of it in liquid form on the surface of the Earth that is unique and that allowed for the gradual evolution of life.
A Special Liquid
The Earth is a wet and watery world so it should not come as much of a shock to hear that life makes good use of this abundant liquid. All life exists in an environment of water and the earliest life on Earth is believed to have arisen in it. Despite its commonality, water is an extremely unusual molecule in its chemical and physical properties, and life has adapted to become entirely dependent on some of its unique characteristics.
Water as the molecule H2O is made up of two hydrogen atoms attached to one oxygen atom. Water molecules are greatly attracted to each other and this stickiness is what gives water its high surface tension (imagine insects walking across a lake on what looks like a film). Water is the universal solvent, a powerful medium that surrounds and dissolves more substances than any other liquid currently known. Salts, sugars, acids, alkalis and some gases – especially oxygen and carbon dioxide – are hydrophilic (water-loving) substances. This is a very useful quality for biological processes. All of the components in cells (proteins, DNA and polysaccharides) are found within water, although not actually dissolved, instead deriving their structure and activity from their interactions with it. Other substances, however, are hydrophobic (water-fearing), such as fats and oils, and so are immiscible in water and will not mix, instead forming individual layers. Water dissolves more substances in greater quantities than any other common liquid and allows them to interact with each other at speeds faster than those obtainable in a solid, and slower than in a gas. Chemical reactions can take place in these other phases as well, of course, but organic life is impossible as a solid or gas. Having molecules available in a dissolved liquid phase also helps cells in gathering essential nutrients and expelling waste products.
One of the incredible abilities of water is its response to changes in temperature. It remains liquid at a range of temperatures and pressures and is transparent in the visible electromagnetic spectrum. This matters because it allowed the rise of early photosynthetic bacterial and plant life, as sunlight was able to reach them through the overlying waters. Today, plants and bacteria have colonised bodies of water across the world, and in every environment. The boiling point of water is 100°C (212°F) and the freezing point 0°C (32°F). This is of utmost importance to the continuity and evolution of life as throughout the last 3.7 billion years of life’s history the temperature at the surface of the Earth has remained within this range at least somewhere on the planet. Water, as with all other liquids, boils into steam at different temperatures depending on the air pressure. For example, at the top of Mount Everest water boils at 68°C (154.4°F), compared to 100°C (212°F) at sea level, regardless of latitude. Conversely, water deep in the ocean near geothermal vents can reach temperatures of hundreds of degrees and still remain liquid owing to the overlying pressures created by such a huge body of water. Most known pure substances become heavier as they cool; water, however, has the anomalous property of becoming lighter when it cools to form ice. It expands to occupy a nine per cent greater volume, which is why ice floats on liquid water, as evidenced by icebergs, and as an additional bonus insulates the water beneath from freezing. Water acts as a good temperature buffer as it can absorb a great deal of heat energy without a big rise in its own temperature. This skill benefits all life on Earth – on a global scale by helping the planet to steady its climate through stabilising the temperature of Earth’s oceans, and at a cellular level by protecting an individual cell from wild temperature extremes that could disrupt and destroy metabolic enzymes.
Water is vital both as a solvent in which many substances within the body of an organism can dissolve and as an essential part of many metabolic processes; it is fundamental both to photosynthesis in plants and respiration in animals. Photosynthetic cells use the Sun’s energy to split off water’s hydrogen from its oxygen. Hydrogen is combined with carbon dioxide (absorbed from air or water) to form glucose (energy) and releases the oxygen. Many living cells use such fuels and oxidise the hydrogen and carbon to capture the Sun’s energy and reform water and carbon dioxide in the process (cellular respiration). Virtually every environment on Earth that has been examined seems to hold life that has evolved from the water-loving universal ancestor of all life on Earth. As long as water is available, life finds a way to exploit whatever thermodynamic disequilibrium exists.
Everything we know about life and its relationship with water suggests that Terran life (life on Earth) cannot exist without it. Yet, we can still ask the question as to whether water is specifically needed for life or if life is simply designed for a watery-type environment and any form of liquid solvent could be used? Perhaps terrestrial life evolved to exploit water simply because it was the only option to hand, so could life emerge in other, more widely available solvents on other worlds?
Ammonia, for example, shares many properties with water, and is actually quite analogous to it. It is liquid over a wide range of temperatures (–78°C to –33°C/–108.4°F to –27.4°F, at surface pressure on Earth) and an even greater range at higher pressures. It dissolves many organic compounds owing to the formation of hydrogen bonds, just as water does, and is abundant in the Solar System – it exists as liquid droplets in the clouds of Jupiter and within the dust of outer space. An ammonia or ammonia–water mixture stays liquid at much colder temperatures than plain water alone, so for the planetary bodies further away from their stars this could be a lucrative characteristic. Ammonia would not support the chemistry found in terrestrial life, however, although alternate biochemistries could be formed and may one day be found right here in our own Solar System, perhaps on Saturn’s largest moon Titan.
Another alternative to water is sulphuric acid. It’s seen in the cloud layers above Venus and there are those who think life is possible, floating within these acidic aerosols. Perhaps Formamide is a solvent life might be able to use. Formed by the reaction of hydrogen cyanide and water, it is liquid across a wide range of temperatures, dissolves salts, and persists in a relatively dry environment, such as a desert. Hydrogen fluoride has also been proposed, as in theory it is a good solvent for both inorganics and organics vital to carbon-based life and has a larger liquidity range than water. The major difficulty is its extreme cosmic scarcity; but this is not a deal-breaker. Liquid hydrogen cyanide is another possibility and, unlike hydrogen fluoride, has a reasonably high cosmic abundance.
If a high cosmic abundance of a solvent is an important factor for life, then the most abundant compound in the Solar System is surely worth considering: dihydrogen. It is the principal component (86 per cent) of the upper regions of the gas giants Jupiter, Saturn, Uranus and Neptune. So, is dihydrogen a liquid? Well, not exactly. Throughout most of the volume of gas giant planets where molecular dihydrogen is stable, it is a supercritical fluid – a substance that can effuse through solids like a gas (but isn’t one) and dissolves materials like a liquid (but also isn’t one). Little is known about the behaviour of organic molecules using supercritical dihydrogen as a solvent – one thing for certain is that the temperature at which dihydrogen goes supercritical is too high for stable organic molecules.
There is actually no need to focus strictly on polar solvents such as water when considering possible liquid habitats for life. Hydrocarbons such as methane, ethane, propane, butane, pentane and hexane are abundant throughout the Solar System and have boiling points up to 75.8°C (168.4°F) at standard pressures. Oceans of liquid ethane and methane have been observed to cover the surface of Titan. Perhaps if there were water droplets within hydrocarbon solvents on Titan, these bodies of liquid could be convenient cellular compartments for evolution. Pure hydrocarbon liquids may actually prove to be better than water for managing complex organic chemical reactivity. Methane could in theory support organic biochemistry although its low liquidity temperatures of –160°C (–256°F) may be too cold for biochemical reactions to run at the fast rates used by life as we know it to thrive. Perhaps life with slower metabolic processes could be possible?
All of these water replacements have pros and cons when considered in respect to our terrestrial environment. What needs to be considered is that with a radically different environment come radically different reactions. Life as we know it is built around a carbon scaffold using a water solvent; this has therefore become the standard chemical model for life. Weird extreme environments may contain weird extreme life forms, so in time we may find that water and carbon are not needed to support life in the far-flung corners of the Solar System, although it is incredibly hard to imagine and design experiments to test for this today.
Everything that goes into creating a life form must come from somewhere. The basic elements that form the backbone of everything around us were created many billions of years ago in the hearts of the first stars. Some people say we come from stardust – let’s see how true that really is.