A2.1 Origins of cells

The origin of cells links prebiotic environments, abiotic organic synthesis, membrane compartments, self-replicating RNA, radiometric evidence, and the shared ancestry of modern cellular life.

Learning objectives

A2.1.1Conditions on early Earth• Early Earth had volcanic gases, high CO₂, methane, water vapour, and little free oxygen• Lack of ozone allowed intense UV radiation at Earth's surface• Prebiotic chemistry may have formed amino acids, bases, sugars, fatty acids, and nucleotides A2.1.2Cells as smallest units of life• Cells are self-sustaining units with membranes, genetic material, and metabolism• Life requires heredity, variation, compartmentalization, and Darwinian evolution• Viruses contain genetic material but lack independent cellular metabolism A2.1.3Spontaneous origin of cells• First cells had to arise from non-living materials by self-assembly• Required steps include organic synthesis, polymerization, self-replication, and membranes• Competing hypotheses include protocell-first, gene-first, and metabolism-first models A2.1.4Evidence for origin of carbon compounds• Miller-Urey produced amino acids from methane, ammonia, hydrogen, and water vapour• Later experiments used different gases and energy sources, including UV and ionizing radiation• Results support abiotic synthesis of amino acids, fatty acids, sugars, bases, and simple polymers A2.1.5Spontaneous formation of vesicles• Amphipathic fatty acids can spontaneously form monolayers, bilayers, and vesicles• Microspheres could enclose polymers and monomers, creating internal chemistry• Fatty acids likely preceded more complex phospholipid membranes A2.1.6RNA as presumed first genetic material• RNA can store genetic information and fold into catalytic ribozymes• RNA world hypothesis proposes RNA acted before DNA and protein enzymes• Evidence includes ribosomal RNA catalysis, RNA primers, and ribose forming under prebiotic conditions A2.1.7Evidence for last universal common ancestor (LUCA)• LUCA links the abiotic phase of Earth history to all later cellular life• Universal genetic code, shared biochemistry, and conserved genes support common ancestry• Genomic studies found shared gene families likely inherited from LUCA A2.1.8Dating first cells and LUCA• Fossils and surrounding rocks can be dated by radiometric methods• Molecular clocks use DNA or protein changes to estimate divergence times• LUCA may have existed around 4 billion years ago A2.1.9Evolution near hydrothermal vents• Nuvvuagittuq haematite tubes provide fossil evidence from ancient hydrothermal vents• Conserved genes suggest LUCA was anaerobic, CO₂-fixing, H₂-dependent, N₂-fixing, and thermophilic• Hydrothermal vents supplied hydrogen, carbon dioxide, iron, heat, and chemosynthetic energy

Volcanic activity supplied water vapour, CO₂, nitrogen and smaller amounts of gases such as methane and ammonia.
Free O₂ was scarce, so little ozone formed and intense ultraviolet radiation reached the surface.
CO₂ and methane strengthened greenhouse warming while water provided a reaction medium.

Early Earth and modern Earth compared by atmospheric gases, ozone and ultraviolet radiation reaching the surface.

Water, simple reactants and energy from UV, heat or electrical discharges could produce carbon compounds abiotically. The exact atmosphere is uncertain, and high-energy radiation could both build and destroy molecules.

System	Contribution to a self-sustaining cell	Boundary case
Membrane compartment	Retains components and controls exchange	A vesicle has a boundary but is not automatically alive
Metabolism	Supplies matter and energy for maintenance and growth	A virus lacks independent cellular metabolism
Heredity with variation	Stores information and permits Darwinian evolution	Genetic material alone is insufficient

A cell is the smallest self-sustaining unit of life because these systems operate together. Viruses contain genetic material and evolve, but they depend on host-cell machinery for metabolism and replication.

Simple inorganic substances give rise to organic monomers such as amino acids, sugars, bases and fatty acids.

Monomers become concentrated and join into polymers; mineral surfaces such as clay may assist concentration and bond formation.

Some molecules store information, copy with heritable variation or catalyse reactions, allowing selection to begin.

A membrane encloses cooperating molecules so that internal metabolism can differ from the surroundings and be inherited together.

Hypothesis	Proposed first system	A testable expectation
Protocell-first	A growing, dividing compartment with simple metabolism	Cell-like units can arise and divide without a complete genetic system
Gene-first	Self-replicating RNA or related molecules	RNA systems can sustain replication, variation and evolution
Metabolism-first	Self-maintaining reaction networks on mineral surfaces	Fed chemical networks can become autocatalytic and produce cell components

All three models address real transitions, but no experiment can recreate the exact history. Evidence can support a mechanism without proving that it occurred first on Earth.

Water was heated; vapour circulated with CH₄, NH₃ and H₂; electrical sparks supplied energy; a condenser cooled the mixture; organic products including amino acids accumulated in the trap.

Miller–Urey apparatus circulates water vapour and simple gases past electrical sparks, then condenses and collects organic products.

Later experiments using different gases and energy sources have produced:

amino acids and fatty acids
sugars and nucleotide bases
in some conditions, simple polymers

Together, the results support abiotic synthesis under some conditions. They do not show that a cell formed or that any experimental gas mixture exactly matched early Earth.

In water, polar regions face the water while non-polar regions turn away. As concentration rises, amphipathic molecules can form layers that curve and close into vesicles without cellular machinery.

A closed vesicle retains monomers, polymers and catalysts, allowing a distinct internal chemistry. Simple fatty acids are plausible early membrane components because they are chemically simpler than modern phospholipids. A compartment alone still lacks dependable heredity and metabolism.

RNA base sequence can store information, while intramolecular base pairing lets the strand fold into a catalytic shape. A replicating RNA system could therefore connect heredity, variation and chemical function before DNA genomes and protein enzymes dominated.

An RNA strand shown as a base sequence that stores information and as a folded ribozyme that catalyses a reaction.

Modern cells retain several possible relics of an RNA world:

catalytic RNA molecules called ribozymes
rRNA catalysis of peptide-bond formation in ribosomes
RNA primers required to begin DNA replication
ribose formation under plausible prebiotic conditions

These observations support an RNA-world hypothesis, but they do not identify the first replicator or reconstruct the complete historical sequence.

LUCA is the inferred last ancestral population shared by all surviving cellular life. It links the earlier abiotic phase to the continuous lineages from which Bacteria, Archaea and Eukarya descended; it was not necessarily the first life or one individual cell.

Nearly universal genetic code and shared biochemistry → conserved ribosome and core gene families across distant lineages → common inheritance is more parsimonious than repeated independent origin.

Researchers compare modern genomes, identify widespread homologous genes and exclude likely horizontal transfers. The surviving genes reveal some LUCA traits, but extinct lineages and lost genes cannot be reconstructed directly.

Method	What is estimated	Essential condition or limit
Fossil plus radiometrically dated rock	Minimum age of the organism or lineage	Fossil must be biological and geological context secure
Isotopic or chemical signature	Minimum age of possible biological activity	Abiotic explanations must be excluded
Molecular clock	Time of lineage divergence	Rate must be calibrated and may vary among genes and lineages

A 3.77-billion-year-old fossil means life existed by then; it does not date the first cell. Fossils, rock dates and molecular clocks converge on very ancient life and a LUCA near 4 billion years ago, but they do not provide an exact birthday.

Two independent evidence lines converge:

Geology: ≥3.77-billion-year-old Nuvvuagittuq vent rocks contain haematite tubes and carbonaceous material compatible with microbial activity.
Genomics: conserved genes suggest LUCA was anaerobic, H₂-dependent, CO₂-fixing, N₂-fixing and thermophilic.

A hydrothermal vent supplying hydrogen, carbon dioxide, iron, heat and chemical gradients, alongside ancient haematite tubes and inferred LUCA genes.

Vents provide H₂, CO₂, iron-rich minerals, heat, pores and chemical gradients, allowing chemosynthetic energy transfer without sunlight. Tube shape alone is not proof of life, and gene-based physiology is a reconstruction; convergence makes the setting plausible rather than certain.

Early reactants + energy → organic monomers → polymers and catalysts → compartments → heredity, metabolism and selection. Miller–Urey, vesicle self-assembly and RNA catalysis show that individual transitions are chemically feasible; they do not reconstruct one proven route.

Shared molecular machinery → LUCA. Fossils, dated rocks and molecular clocks → minimum ages and divergence estimates. Vent fossils + inferred physiology + geochemistry → a plausible environment for early cellular evolution.

Ask for every origin claim	Example
What was observed?	Organic products, conserved genes or haematite tubes
What does it support?	A feasible mechanism, common ancestry or a vent setting
What remains uncertain?	Exact sequence, date, environment or extinct alternatives

3 marks

This investigation was performed in a chamber at $85^{\circ} \mathrm{C}$ leading to total evaporation of the salt water within one day. The atmosphere inside the chamber consisted of nitrogen, carbon dioxide and water vapour. Evaluate the experiment on the basis of similarity with conditions that existed on the prebiotic Earth.

1 mark

Which of the following characteristics found in a structure necessarily indicates that it is alive?

3 marks

Describe processes needed for spontaneous origin of life on Earth.

1 mark

The Miller-Urey experiment carried out in 1952 tested the hypothesis of the chemical origin of life. What were the key findings?

1 mark

State the name for primitive, phospholipid-enclosed structures that may have preceded cells.

5 marks

Describe the structure of RNA and the evidence that it was the first genetic material used to store information.

2 marks

Outline the use of two named radioisotopes for dating fossils.

2 marks

Outline how organic compounds may have been synthesized deep in the oceans.

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—A2.2 Cell structureRead

A2.1 Origins of cells

Learning objectives

Early Earth made different chemistry possible

A cell integrates the systems needed for life

Abiogenesis requires several linked transitions

Competing hypotheses change the order of events

Miller–Urey tested chemical feasibility—not the creation of life

Amphipathic fatty acids can create compartments

RNA can link heredity with catalysis

Shared molecular machinery points back to LUCA

Dating methods constrain different events

Independent evidence converges on hydrothermal vents

Summary: follow the evidence from chemistry to common ancestry

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