Origins of cells — IB Diploma Biology HL
How the first cells may have arisen: early Earth, Miller–Urey, fatty acid vesicles, the RNA world and evidence for LUCA near hydrothermal vents.
Revision notes
4 short notes, in the order of the specification. Each one in short:
On early Earth there was no free oxygen and therefore no ozone layer, so ultraviolet light reached the surface, and high concentrations of carbon dioxide and methane raised temperatures. Under these conditions carbon compounds may have formed spontaneously by chemical processes that no longer occur. The Miller–Urey experiment showed that amino acids can form this way from simple gases.
Cells are the smallest units of self-sustaining life: one cell can carry out all the functions of life. Viruses are considered non-living because they have no metabolism of their own and can only reproduce inside a host cell. Today cells only arise by division of pre-existing cells, so explaining how the first cells formed spontaneously is a major challenge.
Fatty acids coalesce spontaneously in water into spherical bilayers called vesicles, creating a membrane-bound compartment whose internal chemistry can differ from the outside. RNA is presumed to have been the first genetic material because it can be replicated and has some catalytic activity, so it could act as both genes and enzymes. Ribozymes in ribosomes still catalyse peptide bond formation.
The last universal common ancestor (LUCA) is the most recent organism from which all living things are descended. Evidence includes the universal genetic code and genes shared by all organisms. Dates are estimated from fossils, radiometric dating, carbon isotope ratios and molecular clocks, and fossil and genomic evidence suggests LUCA evolved near hydrothermal vents.
Exam questions
8 exam-style questions (26 marks), each with its mark scheme.
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17 cards: flip them, mark what you knew, and practise the rest.
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The whole of cells (unity and diversity) on one page, so you can see where this subtopic fits.
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Quick check questions
Why did ultraviolet light reach the surface of early Earth?
Show answer
There was no free oxygen and therefore no ozone layer to absorb it.
Name the four requirements for the evolution of the first cells.
Show answer
Catalysis, self-replication of molecules, self-assembly and compartmentalisation.
Which molecule in the ribosome catalyses peptide bond formation?
Show answer
rRNA, acting as a ribozyme.
Give two pieces of evidence for a last universal common ancestor.
Show answer
The universal genetic code and genes shared by all organisms.
Frequently asked questions
How did life begin on Earth?
No one knows for certain, but the leading hypothesis is that carbon compounds formed spontaneously on early Earth, RNA molecules began to replicate and catalyse reactions, and these became enclosed in fatty acid vesicles to form protocells. The first cells arose gradually as catalysis, self-replication, self-assembly and compartmentalisation emerged.
What did the Miller–Urey experiment show?
The Miller–Urey experiment showed that amino acids and other carbon compounds can form from inorganic gases such as methane, ammonia and hydrogen when energy from electric sparks is supplied. It supported the idea of pre-biotic synthesis, but it produced only monomers, and the early atmosphere may have been less reducing than the mixture used.
Why are viruses not considered living?
Viruses are considered non-living because they are not cells and cannot carry out the functions of life on their own. They have no cytoplasm, no metabolism and few or no enzymes, and they can only reproduce by using a host cell's ribosomes, enzymes and energy supply.
What is the RNA world hypothesis?
The RNA world hypothesis proposes that RNA was the first genetic material. RNA can store information in its base sequence, be replicated by complementary base pairing and act as a catalyst (a ribozyme), so it could have been both genes and enzymes in the earliest cells. Ribosomes still use rRNA to form peptide bonds.
What is LUCA in biology?
LUCA is the last universal common ancestor, the most recent organism from which all life on Earth is descended. Evidence for it includes the universal genetic code and genes shared by all organisms, and fossil and genomic evidence suggests it evolved near hydrothermal vents, probably around 4 billion years ago.
Written and checked against the IB Biology HL specification · Updated October 2026