Most scientists believe that life is not unique to Earth, that it might even be common; however, we do not know this. In fact our understanding of how life arises is so limited, that we have no effective understanding just how unique – how precious – life on this planet really is.
The woods in Winter make one reflective. Life is subdued but persists; the trees promise to return to vibrant life in the Spring and moss and lichen and ivy speak to a persistence and resilience of life under the harsh conditions. Meanwhile, overhead, the stars are especially bright in the cold, crisp air and the long, dark nights. The life is so close, even at this time of year; in some beautiful, limestone hollows near my home, the moss is so think that you can plunge your entire hand in. The stars surround you, but even when they feel the closest they feel so very far away. And we know this to be true – the stars of the Big Dipper looming across the Northern Sky are 78 to 124 light years away, while those brilliant super-giants of Orion on the opposite horizon are 243 to 1,360 light-years away. The fastest object yet made by man, the Parker Solar Probe, would require 1700 years to traverse just one of those light years.
Those stars are far away. But they are also so very abundant. If you visit someplace with dark skies on a cloudless winter night with a new moon, the sky will be alive with 1000s of stars. But if you scan just a single part of that sky, you will see thousands more in that single frame. The Milky Way alone contains 100 to 400 billion stars, more than the number of humans who have ever lived.
And beyond the Milky Way, astrophysicists estimate that there are about 200 billion to 2 trillion galaxies in the observable universe.
Surely, we are not alone.
But we do not know that. Mentally and emotionally, we are presupposed to think that life is special but also inevitable and therefore common. It is related to the Anthropic Principle, of which there are a multitude of forms but most of which are a variation of: ‘The Universe must be suited for life, because if it was not we would not be here having this discussion.’ However, life could be a wildly uncommon accident and we are biased towards viewing it as common because we are the consequences of that accident. Could it even be a singly unique accident?
As an organic geochemist, I spend most of my life thinking about the chemical composition of life and how that has changed in the past, with implications for what is found in the fossil record. Sometimes, I ponder what life could look life on another planet, and what we would measure to detect either it or its fossil remains. In doing so, I (and experts in this field) reduce life and organic matter to its simplest necessities: a mechanism for storing the information that is passed on during replication (DNA and RNA); a mechanism for translating information into action (enzymes, i.e. proteins); and some sort of membrane that separates all of this from its environment but can allow the controlled entry of nutrients and removal of waste (membrane lipids). The simplest version of an abiotic protocell is often called a ‘chemoton‘ introduced by Hungarian theoretical biologist Tibor Gánti in 1952. Given the ephemeral nature of so many sources of energy, especially to a prebiotic cell with the most minimal enzymatic capacity, I suspect the simplest cell must also a mechanism for storing energy (sugars and lipids and ATP, with varying stabilities reflecting different roles in metabolism and long- or short-term energy mobilisation) – although perhaps other biomolecules could have served this capacity.
There are a multitude of competing and complementary ideas about the origin of life that could have preceded a protocell. Some have argued that the most critical and earliest step was the formation of RNA, perhaps the most elgant biomolecule that both holds information through its sequential ordering of four nucleotides and holds it in a format that can be used to form proteins in ribosomal factories. If RNA came first, one could imagine a pre-cellular, RNA world, with strands of RNA (and perhaps coenzymes that are chemically similar to RNA and ribozymes) dissolved in the ocean, becoming increasingly complex through interactions with primitive ribonucleoproteins.
But one of the most profound questions is how abiotic chemistry could have led towards meaningful increases in chemical complexity that ultimately results in life. The cosmos is governed by the laws of thermodynamics, including the 2nd law that dictates that entropy or disorder always increases. A rise in complexity – life itself, but also society, art, science – is fundamentally opposed to an increase in complexity unless there is an increase in entropy elsewhere. Fortunately, the Earth has a source of that – a star that is undergoing nuclear fusion, producing vast amounts of energy, dispersed according to thermodynamic principles into space, with a small amount landing on Earth and fueling life via photosynthesis. In the depths of the oceans and on an early Earth – before the very complex biochemical machinery of photosynthesis had evolved – chemical energy generated in hydrothermal systems could have produced the same thermodynamic tradeoffs. Arguably, the moment that life arose was when a single photon or chemical disequilibrium was captured and then transferred to drive an unrelated chemical reaction to create a more complex molecule than could have been formed abiotically.
But that energy only allows a localised increase in complexity; it does not explain how it is selected, the molecular equivalent of a pile of bricks randomly becoming a cathedral. Once life has evolved, we know that natural selection drives and shapes the evolution towards greater fitness, which allows the potential evolution towards greater complexity. But natural selection as we understand it biologically is driven by the composite behaviour of all of life’s components – its unified action of DNA, RNA, energy and enzymatic action. Mutations are ‘selected’ not through the DNA molecule itself but through their manifestation via new enzymes that can repair or build damaged cells, mediate new energetic pathways, create more viable membranes; and then eventually achieve multicellularity, and then digestive systems and nervous systems and eyes and ears; cellulose, chitin, lignin and collagen; flagella and fins and legs; and opposable thumbs and larger brains and standing upright.
The evolution of life did not need natural selection, at least not as we understand it for life once it had evolved; but it does need some selection mechanism to swim upstream against the flow of thermodynamics inevitability. Consequently, many of us view the protocell as the key to understanding life. A protocell is compartmentalisation. It encapsulates a system experiencing loss of entropy from an environment in which entropy is inexorably increasing. It encapsulates chemical complexity – all of the DNA and RNA and enzymes and coenzymes and sugars and ATP that represent the bare necessities of life. A protocell also creates a mechanism for thermodynamic selection, with genetic materials that are relatively unstable being preserved and rendered less reactive in a protocell environment. It also allows the development through proton gradients of chemiosmosis, one of the defining features of biological energy translation. Over hundreds of millions of years, could that allow the compartmentalised complexity to evolve to the point that enzymatic machinery could harvest energy from the environment for specific biochemical needs?
But envisioning a protocell is an astonishing chicken and the egg conundrum. What came first, the RNA or the membrane? The DNA or the enzymes?
Even the cell membrane itself is astonishingly complex from the perspective of abiotic formation. The cell membranes of all life on Earth today comprise a three carbon, three oxygen scaffold (a glycerol) to which are attached two long, hydrophobic fatty acyl tails – essentially long hydrocarbon chains linked to the glycerol through an ester bond – and one ‘polar head group’ similar to a sugar or phosphate. Abiotic formation of such a complex molecule is inconceivable abiotically, so we instead can consider simplified versions of it. Membranes (or micelles) can precipitate spontaneously from a solution of amphiphillic molecules – molecules that have both aqueous (water) and lipid (fat) loving components – with the lipids arranging themselves such that the hydrophilic component shields the hydrophobic one from its aqueous environment and the exact structure modulated by pH. A likely prebiotic membrane lipid, therefore, could have been fatty acids (analogous to the fatty acyl components of biological membranes). Fatty acids can form abiotically, but most mechanisms struggle to generate homologues with more than the 8 carbon atoms required for liposome formation, and even those that can yield longer chain homologues do so at concentrations several magnitudes too low to form a liposome. This is a challenge for other biochemical constituents, such that many researchers invoke the formation of protocells in hot springs that experienced periodic evaporation and associated concentration of critical components.
I focus on membranes because that is what my own expertise lies. We’ve not even touched on the minimum requirement of an enzyme – or more importantly, the minimum requirement for a suite of enzymatic biomachinery to mimic life and allow some degree of selection.
So here’s the rub: to the best of our understanding, the leap from simple abiotic compounds to even the crudest protocell remains beyond not just our practical but our intellectual grasp. And where we touch upon imagining it, we must invoke an astonishing range of consequences, from the required elements, chemistry, available energy and even environment.
Of course, that could be a failure of imagination and certainly my own imagination. And it is important to remember that even with the dates of life’s origins likely being relatively early in our planet’s history, these reactions still had hundreds of millions of years to occur and achieve some sort of selection that could incrementally allow a drift towards complexity. An equivalent span of time looking backwards from today extends nearly to the Ordovician, before mammals, before even dinosaurs and reptiles and amphibians and insects, back to the earliest fish. A lot can happen in 400 million years.
Humility demands that we not exclude what we have difficulty imagining. However, humility also demands that we not assume that for which we have no proof; that we not assume that biases derived from the experiences on a single planet, out on the edge of one of our galaxy’s arms, are at all representative of the wider cosmos. In The anthropic cosmological principle, John Barrow and Frank Tipler explore the many anthropic constraints and conclude that Homo Sapiens are probably the only intelligent life in the galaxy. I do not think that either life or intelligent life is unique to Earth, but it could be. And even if not, it could be incredibly uncommon in the cosmos.
And yet:
We exploit life as an inexhaustible planetary and cosmic resource, lacking proof of either. We treat this planet with such disdain; ignore potentially existential crises such as climate change and biodiversity loss with the ignorance of spoiled children; debate global geopolitics, build terrible nuclear and biological weapons, and wage cold and hot wars with existential implications with gobsmacking carelessness.
Sequel: We are alone, together. Even if there is other life, how much does it really matter if we can never meet them?