<aside> ◯
Comment to the reader.
In the previous entry I described what humanity at this time understands about the origin of the universe and the formation of the world on which I live. With this entry, I begin a series of logs whose collective purpose is to take any reader — in the present, or far in the future — through what we currently know about how human beings came to exist.
This series will be marked under a Series tag I am introducing here for the first time: How We Came About. It begins with the simplest plausible chemistry on a young planet and walks forward from there, entry by entry, until it arrives at the species writing these words.
I have made every effort to draw on the most current theories and explanations available, selecting those that appear, at present, to be the most robust and well-supported. At the same time, I recognise that scientific understanding is not static. Ideas are continually tested, refined, and sometimes replaced as new evidence emerges.
Where possible, I cite sources from which I made inferences so that future readers — should they recover the technical literature of this era — can trace specific claims back to their origins and assess them for themselves. References are listed, in numbered form, at the end of each entry.
Given the evolving nature of knowledge, and the limits of any single author, it is likely that some aspects of this account will require correction, refinement, or expansion in future Earth Logs.
</aside>
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.
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.
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.
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.