A planet of chemistry, then a planet of cells. The first cells made themselves — and the rest is biology.
Every sub-topic below feeds at least one of these questions.
What plausible hypothesis could account for the origin of life?
What intermediate stages could there have been between non-living matter and the first living cells?
An extra 9 sub-topics for HL — same syllabus, deeper mechanism.
Conditions on early Earth and the pre-biotic formation of carbon compounds
Cells as the smallest units of self-sustaining life
Challenge of explaining the spontaneous origin of cells
Evidence for the origin of carbon compounds
Spontaneous formation of vesicles by coalescence of fatty acids into spherical bilayers
RNA can be replicated and has some catalytic activity so it may have acted initially as both the genetic material and the enzymes of the earliest cells.
Evidence for a last universal common ancestor
Approaches used to estimate dates of the first living cells and the last universal common ancestor
Evidence for the evolution of the last universal common ancestor in the vicinity of hydrothermal vents
Before life, Earth was a strange place by today's standards: no oxygen, no ozone, scorching ultraviolet, much more CO₂ and methane in the air.
Plus smaller amounts of methane and hydrogen. No free oxygen.
Free oxygen only entered the atmosphere later, as a by-product of photosynthesis.
No ozone layer (because no O₂) → intense UV reached the surface.
High CO₂ → strong greenhouse effect → much warmer than today.
In the 1920s Oparin and Haldane independently proposed that under these conditions, the available energy — UV, lightning, volcanic heat — could drive the spontaneous formation of organic compounds (carbon-containing molecules) from inorganic precursors. Over time, the organic molecules would grow more complex, eventually leading to self-replicating structures and the first cells.
Organic compounds are carbon-containing molecules (excluding simple oxides like CO₂ and carbonates). Everything else is inorganic. Pre-biotic chemistry needed inorganic gases to give rise to organic monomers.
The three-part cell theory: cells are the basic unit of life; all organisms are made of one or more cells; all cells come from pre-existing cells.
A cell can carry out, by itself, all seven characteristics of life — metabolism, homeostasis, response, reproduction, growth, development, genetic continuity. Subcellular components (ribosomes, mitochondria) can't. Viruses can't either — they need a host cell — which is the syllabus's exact reason for excluding them from the category "alive".
Viruses have genetic material (DNA or RNA) inside a protein coat — but no cells, no metabolism, no homeostasis, no response to stimuli, no independent growth, no independent reproduction. They are replicated by host cells. Genetic material alone is not enough.
Today, cells only come from other cells. Explaining how the very first one assembled itself is the central puzzle of origin-of-life science.
Four ingredients had to come together before the first cell could exist:
Some molecule had to speed up chemical reactions. Modern cells use protein enzymes — but proteins are themselves products of biology. Something simpler had to come first.
The molecule of inheritance had to be able to copy itself, so that information could be passed to descendants.
Membranes and compartments had to form from their components spontaneously, without anyone (or anything) building them.
The interior chemistry had to differ from the exterior. Without a boundary, no concentration gradients, no metabolism, no cell.
Scientific hypotheses must be testable. Origin-of-life hypotheses are unusually hard because the exact conditions on pre-biotic Earth can't be replicated, and protocells didn't fossilise. The best we can do is build plausible chemistry under conditions we believe are close, and see what forms.
Miller and Urey's classic experiment took the Oparin–Haldane hypothesis seriously and tested it in a flask. The result changed the field.
After one week the water had turned a brownish-black. Chemical analysis showed it contained amino acids and other complex organic molecules — built from the inorganic starting gases by nothing more than the apparatus.
Modelled the prebiotic atmosphere; demonstrated that organic molecules (including amino acids) can form spontaneously under abiotic conditions; the apparatus was simple, reproducible, and has been replicated many times by other groups.
The exact composition of the prebiotic atmosphere is still debated — Miller and Urey assumed a strongly reducing mix that may have been less hydrogen-rich in reality. The experiment did not produce all the organic molecules needed for life. It also couldn't simulate every condition (deep-sea vents, mineral surfaces, time-scales).
Modern cell membranes are phospholipid bilayers. It turns out that simpler amphipathic molecules — fatty acids — form spherical bilayers spontaneously when mixed with water. That's how the first compartments could have formed.
A phospholipid has a hydrophilic phosphate head and two hydrophobic fatty acid tails — it is amphipathic. In water, amphipathic molecules self-assemble: heads face the water, tails huddle together away from the water. Given the right concentration, the result is a closed spherical bilayer — a vesicle.
Fatty acids form spontaneously in Miller–Urey-style experiments. Once enough were around, they would have coalesced into vesicles automatically. If a self-replicating RNA molecule happened to be inside one of these vesicles, you'd have something that looked very much like a primitive cell — a protocell.
The biggest reason RNA is favoured over DNA as the original genetic material: RNA can both store information and catalyse chemistry. DNA can only do the first.
Inside every modern ribosome, the peptide bond that joins amino acids during translation is catalysed by RNA, not protein. The catalytic core of the ribosome is a ribozyme. That's a fossil from the RNA world, still doing its original job inside every living cell.
LUCA — the Last Universal Common Ancestor — is the most recent organism from which every cell now alive ultimately descends. Two strong lines of evidence point to its reality.
The same 64 codons specify the same 20 amino acids in bacteria, archaea, plants, fungi and animals. A code this complicated and arbitrary is wildly unlikely to have evolved independently in different lineages. Far simpler explanation: it was inherited from a common ancestor.
A handful of essential genes — for ribosomal RNA, key metabolic enzymes — are present in all three domains. Their sequences are similar enough across domains to be unmistakably homologous: shared from a common ancestor.
There may well have been other lineages of early life alongside LUCA's ancestors. If so, they were out-competed and went extinct. We see only LUCA's descendants because they were the line that survived.
Earth is about 4.6 Ga. LUCA appears to be around 4.0 Ga. Two main methods give us these dates.
The oldest known fossils — of cyanobacteria-like cells — are about 3.5 billion years old. Rocks around them are dated by measuring the decay of unstable radioactive isotopes (e.g. uranium → lead) which decay at known, constant rates.
The number of mutations accumulated in a gene since two species diverged is roughly proportional to the time elapsed. Calibrating mutation rates lets us estimate divergence dates — and projecting back to the deepest common ancestor of all life places LUCA ~4 Ga.
From radiometric dating of the oldest mineral grains.
Cyanobacteria-like microfossils.
From molecular clock analyses.
Of continuous evolution producing today's biodiversity.
The best current candidate location for the origin of LUCA is the vicinity of deep-sea hydrothermal vents. Two lines of evidence make the case.
Fossilised microbial communities (similar to modern cyanobacteria) have been recovered from ancient hydrothermal-vent precipitates. The chemistry around vents is rich in dissolved methane, CO₂, hydrogen and sulfide compounds — exactly the building blocks Miller–Urey-style experiments need.
Comparing genes shared across all three domains, biologists find a striking number that look like adaptations to high temperatures — enzymes that work best near 80 °C. This thermophilic signature in LUCA's gene set suggests LUCA itself was hot-adapted, consistent with a vent origin.
If you can't define one of these in a sentence, that's where to revise next.
“For what reasons is heredity an essential feature of living things?”
“What is needed for structures to be able to evolve by natural selection?”