There are few scientific questions so evocative, and few that attract such a polarised view, as the one that has challenged the human mind for centuries: “Are we alone in the universe?” The answer to the question is compelling either way. Aliens might await us in the interstellar void, or we are doomed for ever to a future of unreciprocated radio messages and perhaps, at most, a few Petri dishes of alien microbes. But would it matter if we found nothing?
Recently, evidence has been detected of a lake under the Martian polar ice. Sandwiched between thick layers of water ice known to be present on Mars and the ground underneath, the radar reflection, picked up by the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) instrument on the Mars Express spacecraft, suggests underground bodies of water, analogous to the lakes found buried deep in the Antarctic ice sheet. The media erupted into a frenzy. The Guardian ran with the headline that the underground lake “raises the prospects of life”. But does it?
Even though the discovery of liquid water, a known requirement for life, does and should make people excited, we don’t yet know the temperature or the salinity of the lake – and those factors have enormous implications for life. If the lake is full of perchlorates, another type of salt found on Mars, it might be toxic to life. Even in your own kitchen, they are watery substances that are devoid of life. Your honey never goes mouldy because the water activity, a measure of the amount of available water, is low. The same with many jams. It’s quite possible that the Martian water is sterile. On Earth, we have not found convincing evidence for life reproducing below −18°C. If the temperature of the lake is very much lower than that, its freezing point depressed by salts, then that would also preclude life. There is much that we need to know before we conclude that this is a habitat for life.
The reason why one should approach any evidence of conditions ripe for alien life with a degree of caution is twofold. First, at the moment, the only example of life that we have is that on Earth, and although it provides a useful basis to develop hypotheses and make predictions, it is necessarily limited. Making predictions about extraterrestrial life with a sample size of one should make any self-respecting scientist squirm. There are claims of alien abduction, UFO sightings and, in the more credible scientific literature, one paper in the journal Science, from 1996, claiming evidence for fossil life in a Martian meteorite. Yet there is not a single peer-reviewed paper that is widely accepted as providing unequivocal evidence for a single extraterrestrial protein, carbohydrate or even the smallest molecular fragment of alien life. Contrast this with our depth of knowledge about the Earth that is thought to host about a thousand billion billion billion microbes. Until we show that other worlds host life, we should be cautious about optimistic conclusions based on some environmental and chemical measurements, or, in the case of the Martian lake, some radar measurements.
Second, there is currently a more fundamental scientific limitation to our grasp on the likelihood of alien life – we don’t know how probable the origin of life is. The argument that extraterrestrial life is inevitable because, well, just look at the numbers of galaxies and stars in the universe, is logically flawed. If a single chemical reaction among the many between the first simple organic molecules and a self-replicating microbe that launched the great Darwinian experiment in evolutionary biology on Earth has an infinitesimally small probability of occurring, then when it is multiplied by the number of planets in the known universe the probability of life might remain itself infinitesimal. Extraterrestrial life might not exist, or it might be sufficiently rare and its distribution so diffuse that, to all intents and purposes, we are alone.
Over the past two or three decades, science has tended to tilt our view towards the idea that the origin of life is not contingent, but rather an inevitable product of simple physical and chemical processes. Remarkable experiments by David Deamer at the University of California, Santa Cruz, showed that if you extract carboxylic acids from meteorites and add them to water, they spontaneously form vesicles, membrane-like enclosed spheres that provide a compartment within which biochemistry could conceivably have been ensconced when life first emerged. Cellular structures, therefore, seem easy to make. In addition, the haul of simple organic molecules from meteoritic material that are the building blocks of life, such as amino acids that make proteins, sugars that make carbohydrates and nucleobases that fashion our genetic code, suggest that the raw materials from which life is constructed are common. Perhaps then, on any planet with some liquid water and a suitable cargo of organic molecules, life is inevitable.
However, science does not run on gut feelings, and until chemists can write down a series of reactions from the first pre-biological molecules to self-replicating life forms, we must, out of principle, concede the possibility that there is one reaction in that milieu that renders the origin of life improbable. Until then, we are left hanging between two equally plausible views of the universe: that life elsewhere is common and we will soon find it, or that life is extremely rare, perhaps unique to Earth. In light of this present-day impasse, claims from anyone that we can predict when we will find alien life or, worse, that we will find alien life in some stated period of time are empty.
Nevertheless, there are reasons to be optimistic that we can start to know something. The discovery of many of the basic requirements for life on the surface of Mars and a lake beneath the surface offers the possibility of answering the question of whether there was, or is, life in our own solar system. Other bodies, such as Jupiter’s moon Europa, which also has an ocean, add to the number of locations we might search for evidence of life. Of course, an ancillary question that dogs such a potential discovery is whether it is related to life on Earth. Even if we found life, perhaps other planets in our solar system all shared an ancient ecology that remains scattered, piecemeal, across the solar system to this day. Furthermore, our solar system emerged from one dusty disc. If we were to discover life on another planet that was of a completely independent origin, we still would not know whether some chance chemical process in our planet-forming disc was just right for lots of origins of life, but was repeated nowhere else.
To achieve true statistical independence, we need to look at planets elsewhere. The discoveries of Earth-sized planets around other stars open the prospect of examining these exoplanets for signs of gases, such as oxygen, in their atmospheres that could be indicative of life. Despite this thrilling vista, there remains the very real possibility that we are living in a biologically impoverished desert and that, over the coming decades, we discover that the worlds in our own solar system or orbiting our nearest stars have no obvious signs of life.
Astrobiologists, indeed scientists in general, are not on a quest to find alien life. They are on a quest to test a hypothesis, which is “there is detectable alien life in our galactic neighbourhood” or variations on that. The proof of this hypothesis would be momentous as it would constitute the discovery of alien life. The media would be wild with glee. But that outcome might not actually be extraordinary. The conclusion that given some carbon-containing molecules, some liquid water and basic physical principles such as thermodynamics, self-replicating cells inevitably emerge that go on to build radio telescopes I personally think would be rather dull.
Over the past few centuries, the exceptionalist view of our place in the universe has been overturned stone by stone. Copernicus’ insight that put the sun centre stage and consigned Earth to its place as a small body in its orbit was followed by Darwin’s that restricted humanity to a small branch of life attached to chimpanzees. (His insight was not new. Some ancient Greeks suspected a fishy origin for humanity, but he provided a compelling mechanism and a synthesis of data that cemented the theory of evolution.)
In light of this history, a much more exciting discovery would be endless worlds with potentially habitable conditions, but devoid of life. It would mean that the origin of life is exceptional and the Earth is something special after all. I am not an exceptionalist, and I desire no special place in the universe for our species. But the point is that the discovery of apparently no alien life in our solar system or elsewhere would entail a much more profound change to our view of our place in the universe than finding alien life plodding along in a Martian lake: a much more abrupt paradigm shift than merely continuing the litany of evidence of life’s banal place in the universe.
We still have no idea whether there is alien life out there. But when we do find out, by looking at the most plausible habitable environments, we will either discover a lot of microbes (some perhaps turned into scientists with whom we can have endlessly fascinating conversations about how unremarkable we all are) or we will find exceptionalism reborn, a new awakening of our special place in the universe. A failure to find extraterrestrial life might even constitute one of the most remarkable scientific and social developments of all time.
Charles Cockell is professor of astrobiology at the University of Edinburgh. His recent book, The Equations of Life: The Hidden Rules Shaping Evolution (Atlantic Books), explores the universality of biology.