The Origin of Life

The Question

How did living things arise on a planet that was, at first, not living?

This question has occupied human curiosity for as long as human curiosity has existed. For most of our history, the absence of a satisfying answer was filled by stories — creation myths, divine acts, sacred dramas. By the early twenty-first century of the Common Era, however, science had begun to assemble a different kind of answer: not a single moment of creation but a long, gradual emergence in which ordinary chemistry, given enough time and the right conditions, produced something extraordinary.

What follows is what we currently understand about how that happened. Some of these explanations may eventually prove incomplete or wrong. They are nevertheless the best account our species has been able to construct so far, and they have the advantage of being supported by evidence that anyone, in any era, can in principle re-examine.

The Earth Before Life

Earth had been forming for around a hundred and forty million years before its surface was even capable of supporting life — the difference between the planet's overall age (about 4.54 billion years ^[1]) and the earliest evidence of liquid water on its surface (about 4.4 billion years ^[2]). The young planet was extraordinarily violent: still being struck regularly by leftover debris from the formation of the Solar System, its surface molten in many places, its atmosphere likely dominated by carbon dioxide, nitrogen, and water vapour, with smaller amounts of methane, ammonia, hydrogen sulphide, and various other compounds delivered by volcanic outgassing and impacts. There was no free oxygen — the breathable atmosphere we now take for granted did not yet exist.

As the planet cooled, water vapour in the atmosphere condensed and fell as rain. Over extended periods — likely spanning many thousands to millions of years, repeatedly disrupted by continuing impacts and intense geothermal activity — rainfall accumulated in the basins of the cooling crust. Evidence preserved in ancient zircon crystals — the most durable surviving record of conditions on the very young Earth — indicates that liquid water was present on the surface as early as 4.4 billion years ago ^[2]. These early oceans were warm, salty, and saturated with dissolved minerals washed in from the rocks and delivered by impacting comets and meteorites.

This is the setting in which life began.

Why Atoms Organise Themselves

To understand how life could have started in such a place, we have to start with something more basic: the fact that atoms naturally arrange themselves into structures.

The biologist Richard Dawkins offers a useful framing of this idea in his book The Selfish Gene ^[3]. Soap bubbles, he points out, tend to be spherical because in a film of soap stretched around a pocket of gas, the spherical shape is the most stable arrangement under the forces involved. In the absence of gravity — for example, aboard a spacecraft — water gathers into spherical droplets for the same reason. On Earth, where gravity dominates, the stable surface for standing water is flat and horizontal. Salt forms cubic crystals because that is the most stable way for sodium and chloride ions to pack together. Inside the Sun, hydrogen atoms fuse into helium because, in the conditions that prevail there, the helium configuration is the more stable one.

What this means is that nature does not need a designer to organise matter. Wherever atoms find themselves, they tend toward whatever arrangement is most stable in the local conditions. Sand fills the gaps between rocks. Water finds its level. The early Earth — with its oceans, its volcanic gases, its lightning storms, and its mineral-rich shorelines — was a planet on which countless chemical arrangements were being tried and discarded constantly: every second, in every place.

Most of those arrangements went nowhere. Most of the molecules that briefly assembled were broken apart again by heat or radiation or collision. But every now and then, a molecule formed that held together. And every now and then, that molecule was joined by another, and another, until something more complicated existed.

The Building Blocks Of Life

In 1953, two scientists at the University of Chicago — a graduate student named Stanley Miller and his supervisor Harold Urey — set out to test whether the basic ingredients of life could form spontaneously under conditions that resembled those of the early Earth ^[4]. They sealed water, methane, ammonia, and hydrogen inside a sterile glass apparatus. They heated the water until it evaporated, fired electrical sparks through the gas mixture to simulate lightning, then cooled the gas and allowed it to condense and trickle back into the original flask. They left the apparatus running in a closed loop for one week.

When they analysed the contents at the end of that week, they found that around ten to fifteen per cent of the carbon in the system had become organic compounds. About two per cent had formed amino acids — the molecular building blocks of proteins, and one of the principal classes of biological molecules. (Re-analyses of Miller's preserved samples decades later, using more sensitive instruments, revealed an even greater diversity of organic compounds than the original analysis could detect.)

This was a remarkable result. Without any biological process, without any guiding intelligence, simply by exposing a few common gases to energy, they had produced the chemistry of life.

Subsequent experiments by other scientists, refining the conditions and varying the inputs, demonstrated that nucleobases — the molecular alphabet from which DNA and RNA are constructed — could form under similar conditions, and that, by separate routes, complete activated ribonucleotides could also be produced ^[5]. The full prebiotic pathway from simple molecules to working RNA has not yet been demonstrated end-to-end, but each major step has now been shown to be chemically possible. The broad conclusion that emerges is that the basic chemistry of living things is what one would expect to find on a planet of Earth's composition, given enough time.

There is more. In 1969, a meteorite fell near the town of Murchison, in Australia. When it was examined, it was found to contain over ninety different amino acids, including many of the same ones used by terrestrial life ^[6]. This meant that the chemistry which had occurred in Miller and Urey's flasks was also occurring naturally in space — within the small rocky and icy bodies of the early Solar System, around the same time Earth itself was forming. (Simpler organic molecules — formaldehyde, hydrogen cyanide, basic amino acid precursors — are observed to form in interstellar molecular clouds, and so this kind of chemistry has in fact been underway in the galaxy for far longer than any individual planet has existed.) The raw materials of life are, it appears, common throughout the universe.

Where Life May Have Begun

We do not know exactly where on Earth life first appeared. Several plausible candidates have been studied.

The earliest hypothesis, suggested by experiments like Miller and Urey's, was that life began in shallow tidal pools or coastal lagoons, where sunlight, lightning, and concentrated organic compounds could combine to produce ever-larger molecules. This idea is sometimes called the "warm little pond" model, after a phrase used by Charles Darwin in a letter written in 1871 ^[7] — long before the chemistry was known, but in which Darwin already imagined a place where the necessary ingredients might gather.

A second possibility was opened up in 1977, when researchers operating a deep-sea submersible discovered hydrothermal vents on the floor of the Pacific Ocean near the Galápagos Rift — fissures in the ocean crust where superheated, mineral-rich water erupts into the dark cold of the deep sea ^[8]. To the surprise of everyone, these vents supported entire ecosystems of strange creatures that lived without sunlight, drawing energy instead from the chemistry of the vent itself. This raised the possibility that life had not begun on the sunlit surface but in the dark depths, where mineral surfaces near hydrothermal vents could have served as natural catalysts for the chemistry of life. The chemist Günter Wächtershäuser and his colleague Claudia Huber demonstrated experimentally that key steps of life-relevant chemistry — first the activation of carbon compounds on iron-sulphur surfaces (1997), and then the formation of peptide bonds linking amino acids together (1998) — can in fact occur under such conditions ^[9].

A third possibility — explored more recently — is that life began deep underground, in porous rocks where chemistry could proceed slowly and in isolation, protected from the violent surface of the young Earth.

There is also a fourth possibility, sometimes called panspermia: that the first replicating chemistry did not arise on Earth at all but somewhere else — on Mars, on a moon, on a comet — and was delivered here by impact. This does not actually answer the question of how life began; it only relocates it. But it is consistent with what we know about chemistry happening in space.

It is possible that more than one of these settings was important — that different stages of prebiotic chemistry took place in different environments, and the precursors mixed before life as we know it consolidated. The honest current answer is that we are not yet certain.

What we are reasonably sure about is when. The earliest putative chemical traces of life — carbon inclusions whose isotopic signature is consistent with a biological source, though the interpretation remains contested in the geochemistry community — are preserved inside a zircon crystal dated to approximately 4.1 billion years ago ^[10]. Direct fossil evidence — microscopic structures interpreted as the remains of ancient bacterial mats, called stromatolites, though some examples are still debated as possibly abiotic in origin — appears around 3.5 billion years ago ^[11]. The consensus view is that by 3.5 billion years ago, and very possibly considerably earlier, life was already established, already cellular, and already widespread enough to leave traces we can read today.

The Replicator

The chemistry of life is interesting, but it is not the leap that matters most. The leap that matters most is the appearance of a particular kind of molecule: one that could make copies of itself.

Dawkins offers a useful description ^[3]:

"Think of the replicator as a mould or template. Imagine it as a large molecule consisting of a complex chain of various sorts of building block molecules. The small building blocks were abundantly available in the soup surrounding the replicator. Now suppose that each building block has an affinity for its own kind. Then whenever a building block from out in the soup lands up next to a part of the replicator for which it has an affinity, it will tend to stick there. The building blocks that attach themselves in this way will automatically be arranged in a sequence that mimics that of the replicator itself. It is easy then to think of them joining up to form a stable chain just as in the formation of the original replicator. This process could continue as a progressive stacking-up, layer upon layer. This is how crystals are formed. On the other hand, the two chains might split apart, in which case we have two replicators, each of which can go on to make further copies."

The first replicator, then, was something like a crystal that could break apart and grow again — but with one crucial property. When it copied itself, the copies were not always perfect. Sometimes a building block fell into the wrong slot. Sometimes a slightly different arrangement formed. Most of these mistakes produced replicators that were less stable than their parent, and they vanished. But occasionally a mistake produced a replicator that was more stable, or that copied itself faster, or that was better at scavenging building blocks from the surrounding soup.

These better replicators left more descendants than their less-effective relatives. Over time, the population of replicators in the early oceans was no longer a population of identical molecules. It was a population of slightly different variants, each one descended from the original, some more numerous than others depending on how well they performed.

This is the moment when chemistry crossed into biology. From the appearance of the first replicator forward, the future of the planet would be shaped not by accident alone but by something that resembled selection. Whatever copied itself well, persisted. Whatever did not, faded.

Every living thing on Earth today — every bacterium, every fungus, every plant, every animal — is descended from those first replicators. The DNA molecule inside every cell of your body is itself a replicator, a more sophisticated descendant of the original one. We are, in a precise sense, the long-term consequences of a chemical accident.

The RNA World

What was the first replicator actually made of? We do not know with certainty, but the leading hypothesis at present is that it was a molecule called ribonucleic acid, or RNA ^[12].

RNA is interesting because it can do two things at once. Like DNA, it can carry information — its sequence of building blocks can encode patterns. But unlike DNA, RNA can also fold into shapes that act as catalysts, speeding up chemical reactions in their vicinity. A molecule that can both store information and make things happen is exactly the kind of molecule that could, in principle, bootstrap itself into a self-replicating system without needing the help of separate enzymes.

The hypothesis that an early "RNA world" preceded the more complex DNA-and-protein world we live in now is currently the leading account of how the transition from chemistry to biology occurred, though it competes with other proposals — metabolism-first models, lipid-world models, and various hybrids — and the question is far from settled [12, 13]. In the RNA-world view, RNA replicators came first; later, some of them learned to use proteins to do work; later still, the more chemically stable DNA molecule was adopted as a long-term storage medium. Traces of this history are still visible in living cells today — every cell on Earth still uses RNA as the working translator between DNA and proteins.

Why This Happened At All

Some readers — in any era — may find it difficult to accept that something as elaborate as a self-copying molecule could have arisen by accident. The numbers seem too small, the leap too improbable.

It is worth thinking carefully about what "improbable" means at this scale.

The early oceans of Earth contained an enormous number of molecules — quintillions upon quintillions — interacting every second for hundreds of millions of years. Even an event with a vanishingly small probability per second, per molecule, becomes nearly inevitable when the number of opportunities is large enough. And Earth was only one of many places where this could have happened. In our galaxy alone, current estimates suggest there are between one hundred and four hundred billion stars, and a substantial fraction of them have planets [14, 15]. In the observable universe there are, on current estimates, somewhere between roughly two hundred billion and two trillion galaxies, the higher figure depending on how many faint, undetected galaxies one infers from the data ^[16]. The number of planet-years available for this kind of chemistry to be tried is, for practical purposes, beyond human comprehension.

There is also a subtler point that is worth holding in mind. We are asking how life began on Earth. We are asking this question, and we are alive to ask it. Whatever conditions were necessary for life to begin must therefore have been met at least once — because if they had not, no one would be here to wonder about it. We are observing the universe from the inside of a successful outcome, and we should not be surprised to find ourselves in conditions that permitted our existence. This selection effect, in physics and cosmology, is called the weak anthropic principle ^[17]. It is not an explanation of why life arose; it is a reminder of which questions a self-aware observer is in a position to ask.

This is not the same as saying the appearance of life was guaranteed, or that it was easy, or that it had to happen on Earth. It might well be that life is rare — that out of all the planets in all the galaxies, only a small fraction ever cross the threshold from chemistry into biology. But somewhere it was crossed, because we are here, and we are made of the consequences.

What Followed

Once a replicator existed, the rest of the story we will tell in subsequent entries was, in a certain sense, already underway. Replicators that were better at copying themselves would always outcompete those that were worse. Improvements would accumulate. Replicators would learn to surround themselves with protective membranes, then with whole cells, then with bodies built of many cells. Over the course of the next three and a half billion years, the descendants of the first replicator would diversify into every form of life we know — including us.

But that is for the entries to come. For now, it is enough to mark the moment.

Around four billion years ago, on a young planet near a young star, in conditions we still do not fully understand, ordinary chemistry produced something that could copy itself.

Everything that has happened on Earth since — every forest, every fish, every conversation, every war, every act of love, every reader of this log — is, ultimately, a consequence of that single chemical event.