I spent my whole professional life researching healthcare technologies, but I never studied current theories of abiogenesis, the natural process by which life arises from non-living matter, such as simple organic compounds.
But while reading “Chance and Necessity: Essay on the Natural Philosophy of Modern Biology” by Jacques Monod, I became curious. I read some stuff, bridged it with some personal narrative.
To be clear, we do not know how life appeared, and what follows is a mere narrative, with a limited scientific basis.
Let us imagine that a unicellular organism appears, but as a random event. It can duplicate every day and dies after seven days. After only 100 days, the population of organisms will be 10^30, which is roughly what the Earth's ecosphere can sustain. But of course, this assumes that there is no selection pressure, e.g., that all organisms die of old age. If we introduce a death rate of 50%, the population will never develop. But as soon as the death rate goes below 50%, the population growth explodes. With a death rate of just under 40%, we reach the ecosystem limit within one year.
My point is that once the first living organism appears (some authors call it LUCA, the Last Universal Common Ancestor), life is inevitable. But what is life? This is a particularly difficult question. Probably a decent tentative answer is that LUCA had a boundary (e.g., a membrane) that separated it from the rest of the environment; a metabolism, which would enable it to capture and convert energy in the environment, to sustain all its functions, and to keep its internal entropy low; and to replicate into another organism with low loss of its information content.
I picture life as the eternal fight between order and chaos, where chaos is the thermodynamic inevitability of entropic death, and order is the persistence of this self-replicating stuff that fights the good fight to prevent chaos from changing itself. And funny enough, life wins when it fails, mutations occur, and more resilient forms of life are pruned through natural selection. So, in a way, the best friends of life are death and chaos.
Several abiogenesis hypotheses propose that life emerged on Earth more than 3.5 billion years ago from prebiotic chemistry: a set of chemical processes occurring before the existence of living organisms. In some scenarios, these reactions occurred in environments such as shallow ponds, hydrothermal systems, mineral surfaces, volcanic regions, or other chemically active settings, where simple molecules were exposed to energy sources, including sunlight, geothermal heat, electrical discharges, and chemical gradients.
Among the many elements available on the early Earth, a small group became especially central to life: carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulphur, often abbreviated as CHNOPS. These elements form the backbone of most biological molecules. Other elements and ions, including calcium, magnesium, iron, nickel, zinc, and manganese, also became important in biological chemistry, especially as structural components or catalytic cofactors.
Experiments have shown that several key classes of biological molecules, or their precursors, can form under plausible prebiotic conditions. These include amino acids, sugars, lipid-like molecules, nucleobases, and nucleotide-related compounds. However, the transition from these simple compounds to self-sustaining, evolving chemical systems remains one of the most difficult questions in origin-of-life research.
One influential hypothesis is the RNA World. According to this idea, an early stage of life-like chemistry was dominated by RNA or RNA-like polymers. RNA is a polymer made of nucleotides. Each nucleotide contains a ribose sugar, a phosphate group, and one of four nitrogenous bases: adenine, uracil, cytosine, or guanine. RNA is especially interesting because it can both store information and catalyse chemical reactions. At some stage, RNA-like molecules may have become sufficiently abundant and stable for some folded forms to catalyse reactions, possibly including those that helped generate more RNA-like material.
At the same time, simple lipid-like molecules could have spontaneously assembled into vesicles: small membrane-bound compartments. If RNA-like molecules became trapped inside such vesicles, these structures would have resembled early protocells. These primitive compartments would not yet have been true modern cells, but they could have created local chemical environments in which useful molecules were retained, concentrated, and exposed to selection.
Over time, RNA molecules and short chains of amino acids may have begun to interact. These short amino-acid chains, called peptides, could have stabilised RNA structures and improved their catalytic efficiency. This stage is sometimes described as an RNA–peptide world. In such a system, molecular structures that were more stable, more efficient, or more easily replicated would have become more common. In this sense, a primitive form of natural selection could have acted before the existence of fully modern cells.
Eventually, longer peptides and then proteins appeared. Proteins are chemically more versatile than RNA and have become the dominant biological catalysts. Their emergence would have allowed increasingly reliable replication, more complex metabolism, and more efficient control of chemical reactions. The ancestors of ribosomes probably evolved gradually within this RNA–peptide context. Modern ribosomes still preserve a central catalytic role for RNA: the formation of peptide bonds is carried out by ribosomal RNA, not by protein. This strongly suggests that today’s translation machinery retains traces of an ancient RNA-based past.
DNA probably appeared later. Deoxyribonucleotides are structurally similar to ribonucleotides, but they contain deoxyribose instead of ribose, and DNA uses thymine instead of uracil. DNA is more chemically stable than RNA and is better suited to long-term information storage. Once protein enzymes had evolved, the conversion of ribonucleotides into deoxyribonucleotides could have enabled the emergence of DNA genomes. From that point onward, biological information could be stored more reliably in DNA, transcribed into RNA, and translated into proteins.
Long before the appearance of modern organisms, protocells therefore may have evolved into increasingly complex cellular systems containing genetic polymers, catalytic RNAs, proteins, primitive metabolic networks, and membrane-based energy-conservation mechanisms. ATP eventually became the main energy currency of cells, although the exact timing and nature of early ATP-producing systems, including ATP synthase, remain debated.
After a long period of early evolution, one lineage, or population of related lineages, gave rise to the Last Universal Common Ancestor, or LUCA. LUCA was not the first living organism. It was already a relatively advanced cellular system, with DNA, RNA, proteins, ribosomes, a genetic code, and complex metabolism. It is called “last universal” because all known organisms alive today — bacteria, archaea, and eukaryotes — descend from it. Life as we know it did not begin with LUCA. Rather, LUCA marks the deepest common root we can infer from living organisms today. The origin of life lies further back, in the long and still partly mysterious transition from prebiotic chemistry to evolving protocells.
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