You may be accustomed to seeing me rewind the clock only so far. We usually stop around 300,000 years ago to check in on early Homo sapiens. Occasionally, when we need to examine the ancestral foundations of the mammalian brain, we travel back roughly 200 million years. Today, we are going spectacularly overboard. We are setting the time machine to four billion years ago.

Our destination is the moment when ordinary, nonliving matter somehow decided to become alive. It remains one of the greatest mysteries science has ever attempted to solve.

It is also the earliest prequel to the self. Long before consciousness, memory, feeling, or thought could appear, chemistry had to acquire several remarkable capacities: preserving an organised pattern, making another version of it, and maintaining a distinction between itself and the surrounding world. Before there could be a self that knew itself, there had to be something capable of keeping itself going.

The real estate nightmare of early earth

To understand how life emerged, we have to look at the young Earth and the raw materials of reality. The universe is, at its most basic, an immense collection of particles. Left entirely to themselves in a closed system, physical processes tend toward equilibrium as energy disperses and the number of possible arrangements increases. In physics, this direction of travel is described through entropy.

If you have ever cleaned your house only to find it in disarray three days later, you are intimately acquainted with entropy’s general disposition.

Early Earth, however, was anything but closed or quiet. Sunlight, geothermal heat, electrical storms, volcanic activity, and chemical gradients continuously supplied energy. Under such conditions, local order could arise while the overall movement of entropy continued undisturbed. Modern origin-of-life research therefore studies chemistry operating far from thermodynamic equilibrium, where energy flows can sustain organised structures and reactions.

Chemistry also has rules. Some particles attract one another, others repel, and particular combinations settle into stable arrangements called molecules. The young Earth was rich in water, minerals, gases, and organic compounds produced on the planet or delivered by meteorites.

We do not know exactly where the decisive chemistry occurred. Proposed settings include hydrothermal vents, mineral surfaces, volcanic pools, shorelines, and bodies of water exposed to repeated wet–dry or hot–cold cycles. For the purpose of our journey, picture a chemically rich puddle on the young Earth, periodically heated, cooled, concentrated, and diluted. It contained countless molecular structures bumping into one another and doing nothing that a biologist would yet call alive.

Then some of those structures began to preserve patterns.

When chemistry acquired a memory

One leading account of life’s origin proposes that RNA-like molecules, or chemical predecessors with similar properties, formed chains capable of guiding the construction of complementary chains. RNA is especially interesting because it can carry information and participate in chemical reactions, two functions that modern cells distribute between DNA and proteins. This possibility lies at the centre of the RNA-world hypothesis.

Imagine one of these early chains floating in our puddle. Its exposed chemical edges acted like molecular Velcro, attracting complementary building blocks from the surrounding water. As the blocks lined up along the original chain, they linked to one another and formed a matching sequence.

A change in the environment—perhaps heating, drying, or another alteration in chemical conditions—separated the two chains. The original template and its new counterpart could then attract further building blocks. One became two. Two became four. Four became eight.

The molecules had acquired an extraordinary freak property: they could make copies of themselves.

The precise route remains unresolved, and no one has yet reconstructed the complete passage from simple prebiotic chemistry to the first living system. Yet template-directed replication gives us a plausible glimpse of the decisive transition. Matter could now preserve information beyond the temporary existence of one molecule. Chemistry had acquired a primitive memory.

The microscopic arms race

Richard Dawkins calls such self-copying molecules replicators. Once replicators exist, a particular logic takes over. Anything that makes enduring copies of itself will become more numerous than chemistry that produces no descendants.

The supply of raw materials was finite. As replicators multiplied, the chemical building blocks they required became scarcer. Replication became competition.

Copies were also imperfect. Some errors produced unstable or ineffective molecules. Others made little difference. Every so often, however, a variation gave its carrier an advantage. One replicator might remain intact for longer. Another might copy itself more rapidly or with greater accuracy. A third might appropriate fragments from rival molecules and use them as raw material.

Replicators carrying advantageous mutations multiplied more successfully. Their descendants inherited the improved feature, occasional copying errors introduced further variations, and selection continued. Chemistry had entered an arms race over persistence, fidelity, and access to resources.

Nothing needed to understand what it was doing. The logic followed automatically from replication with variation under finite conditions. Molecules that survived and copied themselves remained in the world; the others disappeared from the sequence.

The first inside and outside

Free-floating replicators had a problem. Their useful components dispersed into the environment, favourable reactions occurred only by chance, and anything they produced could drift away or be appropriated by competitors. A replicator required more than a good sequence. It needed somewhere for its chemistry to remain together.

Certain fatty molecules offered an answer. One end of such a molecule is attracted to water, while the other avoids it. When enough of these molecules encounter one another in water, they spontaneously arrange themselves into sheets and hollow spheres. No architect is required. Their chemical properties produce a membrane.

Some spheres would have enclosed replicators and other useful molecules by chance. These primitive compartments, commonly called protocells, created a separate chemical interior. Raw materials could enter, reactions could occur at higher concentrations, and their products could remain near one another. Researchers continue to investigate how RNA replication might have operated inside simple fatty-acid vesicles, whose membranes could grow and divide under plausible early-Earth conditions.

The membrane changed the unit upon which selection could act. Success now depended upon a whole collection of chemical processes remaining viable together. A protocell that acquired resources efficiently, maintained its internal conditions, and reproduced its contents could outnumber less capable compartments.

This protocell was still unimaginably distant from a psychological self. It nevertheless introduced the first material distinction upon which every later self would depend: an inside that had to be maintained against an outside.

Once an inside existed, its conditions required regulation. Useful substances had to be admitted, harmful conditions resisted, waste removed, damage repaired, and energy continually obtained. The earliest biological individuality arose from this sustained difference between a living unit and everything around it.

Every cell in your body still reenacts that ancient distinction across its membrane. Your psychological self appeared billions of years later, supported by an organism that had become extraordinarily accomplished at preserving itself, detecting changes, and responding to whatever crossed its boundaries.

And that’s why you have to pay taxes

Once replication, mutation, selection, and bounded individuality had entered the world, evolution had time to become truly excessive.

Protocells became cells. Some cells learned to exploit sunlight; others consumed chemical compounds or one another. Cells incorporated former rivals and turned them into indispensable internal partners. Separate cells began cooperating in larger bodies. Multicellular organisms developed specialised tissues, nervous systems, memory, emotion, consciousness, language, governments, and eventually tax authorities.

And there you have it. Life, from the earliest self-copying chemistry to the brain currently reading this sentence, is matter that learned to preserve and reproduce its organisation under conditions of relentless competition.

This is the ultimate prequel to everything we usually discuss. Long before thoughts, feelings, wishes, and motives appeared, your most distant ancestors faced the ancient requirements of life: remain intact, obtain energy, detect opportunity and danger, and carry their organisation into another generation. Each successful lineage added new ways of accomplishing those tasks. Nervous systems eventually brought the surrounding world into the organism as perception; affect gave biological conditions a felt significance; consciousness made a small part of this immense activity available to experience.

Your existence rests upon an uninterrupted sequence of successful replication extending across approximately four billion years. Every organism in that sequence preserved the conditions necessary for another generation to follow. The continuity reached your parents, then you, and now supports a mind capable of examining the process that produced it.

Long before a mind could ask who it was, life had to establish an inside, preserve it, and respond to whatever approached from outside. Four billion years later, one descendant of that first organisation can turn its attention toward itself and examine what the ancient project has become.