Hello? Earth Calling . . . PART 3

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Where Could Life Evolve?

Defining what is alive is very difficult but this question is of great importance to NASA and other space agencies around the world because they need to know what possible forms life can take in order to set up scanning parameters when they explore other planets and moons in our system as well as exoplanets. Often asked when a new exoplanet is discovered is whether the planet lies within its star's "Goldilocks zone." This is the distance from its star where the average atmospheric temperature (given sufficient pressure) should allow liquid water to exist on the surface.

The diagram above compares habitable zones based on star luminosity. Notice that Earth is right in the middle of our Sun's habitable zone. Mars would be in the habitable zone of a hotter, brighter star, and even Mercury would be in a habitable zone if it orbited a much dimmer red dwarf. This is not drawn to scale. Credit to Chewie;Wikipedia.

The Goldilocks zone is a rough guideline for where life could exist based on what was known about Earth life decades ago. Astrobiologists now speculate that life could have evolved in subsurface oceans of liquid water on moons in our solar system such as Titan, Enceladus, Europa and Ganymede.

Saturn's moon Titan, not in our Sun's habitable zone, nevertheless could potentially harbour exotic methane-based life on its surface as well as carbon/water-based life in its subsurface ocean, which is shown here as a blue layer in the cut-away image above left. Credit to Kelvinsong;Wikipedia

Discoveries of extremophilic organisms on Earth and the modeling of potential non-water biochemistries have stretched what we think of as necessary conditions for life. This progress has widened the potential zone of habitability and even makes it irrelevant. Additionally we are learning that life on Earth today is not what life was like on Earth billions of years ago. Life arose when conditions on our planet were nothing like what they are now. Should we be looking for planets that might provide those conditions as well? For now, liquid surface water and atmospheric oxygen, two features of Earth-based life, serve as logical starting points for the search for extraterrestrial life.

How and When Does Chemistry Become Life?: Definitions

NASA offers a working definition of life based on an entity's ability to take in energy from the environment and transform it for growth and reproduction. Scientists use this definition as a basic rationale to look for signs of possibly life-connected chemical transformations in the atmospheres of other moons and planets such as Titan and Mars. For example, trace amounts of methane found in the atmosphere of Mars between 2003 and 2006 by the European Space Agency's Mars Express Orbiter, and several ground-based observations, excited scientists looking for signs of life. Atmospheric methane is unstable. It quickly breaks down under the Sun's UV radiation and it reacts chemically with other gases in the Martian atmosphere. This means there must be an active source and it produces approximately 270 tons per year. Volcanism and meteorite strikes can theoretically produce this much methane but neither event has occurred recently enough. It is also possible that water-rock reactions and pyrite formation produce methane abiotically (via non-living chemistry). Organic compounds on meteorites could also be converted to methane through UV radiation. It is possible too (and this is what excites the researchers) that methanogenic microbes could be responsible. These microbes exist on Earth in oxygen-poor wetlands and inside our guts.

In the digestive tract of a termite's gut (termites shown left), protozoa break down cellulose in wood so the animals can digest it. That process releases hydrogen gas, which reduces energy available to the protozoa. Methanogenic archaea in the gut help the protozoa out by consuming the hydrogen, which is a great example of three-way mutualism between termites, archaea and protozoa. Photo is provided by Scott Bauer;Wikipedia

Extremophile methanogenic species live in hot springs and in hydrothermal vents. There they require no oxygen or organic nutrients. They use hydrogen as their energy source, which they could obtain on Mars just beneath the surface, perhaps where it is also warm enough for liquid water to exist and where it is protected from sublimation into the thin low-pressure Martian atmosphere. No direct evidence for any life has yet been found but the possibility has not been ruled out either.

If microbes do exist on Mars, they could have hitched a ride from ancient Earth. Tough spores of ancient Earth methanogens could have even been blown into space by meteoric impacts to eventually land on Mars and take up residence there (and vice versa) according to a hypothesis called panspermia. This is also one reason why probes sent to other planets and moons must be absolutely sterile, to avoid accidental contamination and a false positive result for life.

NASA's definition of life (the ability to take in energy from the environment and transform it for growth and reproduction) works quite well but there remains no universally accepted definition of life. Even NASA's working definition is imperfect because it excludes worker bees and mules, for example, as those individuals don't reproduce. However, life can be narrowed down to traits shared by all known living organisms, even the exotic extremophiles. Here on Earth two commonalities stand out - a carbon-based chemistry and a dependence on water. Most experts also require living organisms to have cells, while others do not. Viruses, plasmids and viroids are nothing more than fragments of RNA or DNA (two kinds of genetic material). In viruses the genetic material is contained in a protein sac or coat but it is not a proper cell because it contains no cytoplasm and no biomolecules are enclosed within a membrane.

This diagram (right) of a tobacco mosaic virus shows RNA (red) coiled within in a helix of external protein. Diagram by Thomas Spiettstoesser;Wikipedia.

Despite their simplicity, Viruses are capable of reproducing as a unit. In plasmids and viroids, the genetic material is loose. There is no enclosure at all. A plasmid (see below left) is a small DNA strand that exists inside a cell that is separate from its chromosomal DNA. Often found in simple unicellular organisms, plasmids perform functions useful to the cell. For example, a plasmid can become active in times of hardship to produce a protein toxin that it codes for. The toxin can afford the organism a temporary protective advantage.

In this not-to-scale diagram (left), simple circular double-stranded DNA strands of plasmids exist along with the host's DNA inside a bacterium. Work by User:Spaully;Wikipedia.

Viroids, on the other hand, are pathogens. Consisting of a single circular strand of RNA, they don't code for any protein. Instead, once inside a host they simply replicate using the host's cell replication enzymes. They infect the host's body and in doing so they are parasitic.

An additional infectious agent is the prion, which is composed entirely of protein. When it infects, usually neural, tissue in a host, it induces proteins to fold into the same misfolded shape as the prion. As this process continues, eventually amyloid folds form, consisting of tightly packed aggregates of misfolded proteins, which cause tissue damage and eventually death.

This micrograph of bovine brain tissue (right) is full of vacuoles or holes giving it the telltale spongy appearance of the prion-transmitted disease, spongiform encephalopathy. Credit to dr. Al Jenny;Wikipedia.

Viruses, plasmids, prions and viroids, by virtue of their simplicity, might represent relics from the time when life began to evolve on Earth. Viroids, which consist of short circular strands of RNA, may have been the first "proto-life" form to exist in a world before DNA or even proteins evolved. Now functioning as pathogens, these simple RNA strands may have once replicated in free form in ancient watery environments on Earth. By NASA's definition they would be considered alive, but most experts consider them, as well as plasmids, prions and viruses, to be nonliving because outside of their intracellular environment they cannot do anything. They rely on their host cell's replication machinery to reproduce, which leaves the question of how an ancient proto-life viroid could replicate outside of a cell. Instead of the cellular RNA polymerase used by a modern viroid, an ancient viroid might have taken advantage of some other organic molecule that catalyzed its replication reaction, a basic idea that we will expand on.

Potential Chemistries of Life

We know that carbon-based biochemistry in a water solvent worked here on Earth, but it might not be the only potential life chemistry. To explore what kinds of chemistry could work, we can start by asking what makes carbon and water work. What other chemical compounds could function in similar ways? As mentioned previously, a poly-lipid based biochemistry using methane as a solvent might work in an environment like Titan. In addition, chemists know several complex chemistries that use sulfuric acid as a solvent for their reactions. Ethane-ammonia is another potential biological solvent. These are just a few possibilities. Water works well as the solvent for carbon-based biochemistry. It is liquid over a wide range of temperatures. It has a high heat capacity so it can be used to regulate an organism's temperature. It can dissolve many compounds. It can act as an acid (H⁺) or a base (OH^-), which makes it a useful reactant and a product in many biochemical reactions. By itself water isn't a great biological buffer but when phosphates or carbonates are dissolved in water it becomes a great buffer, keeping the internal pH environments of cells constant. Planets and moons where a compound tends to be in a stable liquid state at that planet's/moon's average temperature and pressure can potentially use that compound as a biological solvent. A planet (or moon) could be very cold and use methane or ethane. A planet with extreme surface pressure could use liquid nitrogen or even supercritical hydrogen as a biological solvent. An extremely hot planet could use liquid sodium chloride as a solvent. From a potential solvent perspective, the Goldilocks range of exoplanets and moons is enormous. Could life using one of these exotic solvents evolve into complex organisms as well?

More than a solvent is required for life, at least on Earth. An entire suite of complex biochemistry is also essential. Carbon takes on the central role in biochemistry. It is abundant. It is small and light weight so it is easily manipulated by enzymes. Perhaps most usefully, it has four valence electrons to bind to other elements such as hydrogen, oxygen and nitrogen as well as with other carbon atoms, so it can form the long complex carbon chains of proteins and DNA. However, carbon is not the only element with these properties. Silicon also has four valence electrons and it can bind to itself. Silicon-based life, though, would have to be quite exotic. Unlike carbon, silicon tends to form crystal lattices rather than chains. The big downside of silicon is that its lattices don't break apart and reform easily into new compounds, so it lacks the chemical plasticity that makes carbon so useful for organism growth and decay. Silicon would not be able to cycle through living and non-living systems as carbon does, which ensures that carbon-rich nutrients are always available to organisms. Other elements besides carbon can form long chains, such as chlorine, sulphur, nitrogen and phosphorus. Phosphorus together with nitrogen can also form a wide variety of different molecules, including some that are ring shaped, similar to many complex carbon-based biomolecules. Several metal oxides can form a variety of complex structures as well and they offer the advantage of being more thermally stable at very high temperatures than carbon compounds.

Defining Life Is Tricky

While NASA defines life as an entity's ability to take in energy from the environment and transform it for growth and reproduction, there are other ways of approaching the mystery of life. A definition that defines life as any self-sustaining system that undergoes Darwinian-like evolution can be useful but it too has problems. "Self-sustaining" can be difficult to pin down. As mentioned, viruses, plasmids, prions and viroids are clearly not self-sustaining. They can't live or evolve outside of a living host. But even humans are not strictly self-sustaining. We need an ecosystem that includes other organisms to supply our oxygen and food and recycle our nutrients. Even our own bodies are more of an ecosystem than a single self-sustaining species. Our guts, for example, are complex communities rich with various bacteria, fungi and other simple organisms without which we could not digest food and absorb its nutrients properly, and we would eventually die. Evolution is problematic as well. How long of a time-scale does evolution require? It could take place so slowly that it would be virtually impossible to detect and measure.

The definition of life as any self-sustaining system that undergoes Darwinian-like evolution, though problematic, opens up some fascinating questions. The ultimate self-sustaining system that evolves over time is Earth's biosphere, which includes obviously living components but also non-living (abiotic) parts such as the atmosphere and the water cycle. Is it as a whole a living system? This proposition is called the Gaia hypothesis. According to this hypothesis, organisms co-evolve with their abiotic environment. A good example is Earth's stable oxygen-rich* atmosphere. The oxygen owes itself to the process of photosynthesis carried out by living organisms – plants, algae and cyanobacteria. Despite changing weather and a cycling climate, the oxygen level currently maintains a very consistent 21% of the total gas by volume. It wasn't always like this. The equilibrium state of atmospheric oxygen has shifted, or evolved, over time. Once oxygen began to accumulate on Earth, it reached an equilibrium state of around 15%, which it maintained for billions of years. 280 million years ago oxygen levels increased again and peaked at an equilibrium state of about 30%, due to a wet warm climatic period filled with lush oxygen-releasing vegetation. Our atmosphere is a self-regulating complex system with a stable but evolving equilibrium, which under this definition could be considered alive.

*An interesting side story here is that oxygen is no longer necessarily the clear biomarker of life on a planet that scientists once thought it was. Atmospheric oxygen is very reactive, much like Martian methane gas. It reacts with rocks to form oxides, taking it out of the atmosphere. It was thought that in order to maintain atmospheric oxygen on a planet, biochemistry (such as photosynthesis) carried out by some living organism must continuously replenish the atmospheric gas. Now there is evidence that a planet with just 0.05% titanium oxide on its surface (titanium oxide is known to be fairly abundant on the surfaces of rocky planets in our solar system as well as on the Moon) could produce as much atmospheric oxygen as Earth has, through an entirely abiotic photocatalytic reaction, meaning that oxygen in an exoplanet's atmosphere is not necessarily, but could be, a sign of life there.

Identifying a biochemical signature of life on another world can be very tricky even for NASA, and the exploration of Mars provides a good cautionary example. When the Viking Lander landed on Mars in 1976, it looked for signs of Earth-like metabolism as a possible indicator of life there.

An artist's concept of a Viking lander sampling Martin soil. Image from NASA

To test for this, it added radioactively labeled liquid nutrients to a Martian soil sample. The premise is that if the nutrients are consumed, a waste gas should be released and it should also be radioactively labeled and therefore detectable. This should indicate that some kind of metabolic reaction is consuming the nutrients, perhaps the work of a simple unicellular life form such as bacteria or archaea. The test was positive but after the initial euphoria died down a bit, subsequent tests revealed that the reaction was instead a result of Mars's unexpectedly unique soil chemistry.

An alternative definition of life is that it possesses some kind of embedded instructions, not necessarily DNA or RNA but something that works in a similar fashion. This definition covers all known living organisms and rules out cases, which we know are not alive, such as a wild fire, which grows, reproduces and uses energy and would be considered alive by NASA's definition. Certain chemicals can also act as if they are alive. Exposed to light and fed by chemicals, some compounds can form crystals that move, break apart and form again.

The Wikipedia entry on alternative life definitions describes and weighs several definitions that I don’t mention here, and it is well worth a read. I was fascinated reading it and finding that as those definitions evolved over time. There is gradual move away from Descartes-style reductionism (living organisms are composed of parts that function together like a machine) toward systems theory (the complexity of a living organism – and life itself – emerges from simpler organizations of non-living systems. This, as well as a move toward a multidisciplinary approach to problems, is a very current very big trend in science in general. As an example of this, last year NASA launched a multidisciplinary approach called The Nexus for Exoplanet System Science, or NExSS, to the search for extraterrestrial life, which will bring together experts in many fields from universities and institutes across the U.S. They will apply a systems analysis approach to existing and coming exoplanet data that will help them interpret observations.

Within any of these life definitions, grey areas exist but by trying to be specific, there is the danger that no single definition will be broad enough to cover the vast array of permutations in which life might exist in the universe. Science fiction writers are expert at testing the boundaries of what could be life. Could artificial intelligence be alive? What about non-physical sentience (see the mind-body problem)? While these authors clearly deal in fiction, those same questions are also seriously asked in the scientific community. The same kind of imagination sci-fi writers use is needed to figure out what kinds of biochemistry are possible in some of the extreme environments one might encounter on exoplanets. Honestly, what could be more fun than that job?

Even when life is present on a planet, its biosignature might not be readily detectable on its surface. To a distant alien observer, Earth during various periods of its evolution could have been mistaken for being completely lifeless. Life might have originated here well hidden in deep ocean hydrothermal vents on what once appeared on the surface as a forbiddingly hot, toxic and violent world. Later on, any biosignature would have been hidden under ice that was kilometres thick when Earth underwent planet-wide glaciation a few billion years after life first appeared, a period that could have lasted for millions of years.

Earth from afar then would look much like Europa does now. Under -160°C ice as thick as 30 km and as hard as granite, life could exist in Europa's dark deep subsurface ocean.

Natural-colour image (left) and enhanced-colour image (right) of Europa taken by the Galileo spacecraft in 1997 from 1.25 million km away. Credit to NASA

We could discover a similar exoplanet, with no observable biosignature, and dismiss it even though life may be abundant. This is one reason why astrobiologists do not discount the possibility of life hidden deep under the ice layers of frozen moons and planets in our own solar system, and why they try to keep an open mind with the discovery of each new exoplanet.

Next we will explore pathways from non-living organic chemistry to living biochemistry on early Earth, the only known planet or body where we know for certain this transition took place. Life from non-life is a huge mystery, one that determines how likely our universe is to have life on other planets.