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A2.1

Origins of cells

Theme A · Unity and diversity · Cells · Additional higher level only

This entire topic is additional higher level. SL candidates are not examined on any of it. It asks the hardest question in biology — how non-living chemistry became a living cell — and it is honest about the answer: nobody knows for certain, but there are testable hypotheses and real evidence for each step. What you are examined on is those steps, that evidence, and why the question is so hard to test.

🎯What you need to be able to do

  • Describe conditions on early Earth and explain why they allowed carbon compounds to form without life.
  • Distinguish living from non-living, and explain why cells are the smallest units of life while viruses are not considered alive.
  • Outline the four requirements for the first cells: catalysis, self-replication, self-assembly and compartmentalization — and explain why the origin of cells is hard to test.
  • Describe and evaluate the Miller–Urey experiment.
  • Explain how fatty acids can form vesicles, and why a membrane-bound compartment was needed.
  • Explain why RNA is thought to have been the first genetic material.
  • Outline the evidence for a last universal common ancestor (LUCA), how the dates of early life are estimated, and the evidence that LUCA lived near hydrothermal vents.

📚The biology

Conditions on early Earth

Earth formed about 4.5 billion years ago. The conditions on its surface for the first billion years were very different from today’s:

  • No free oxygen in the atmosphere. Oxygen gas only accumulated much later, as a waste product of photosynthesis.
  • Therefore no ozone layer, since ozone is formed from oxygen. So far more ultraviolet light reached the surface.
  • Higher concentrations of carbon dioxide and methane, both greenhouse gases, so surface temperatures were higher.
  • Frequent volcanic activity, lightning and meteorite impacts supplied large amounts of energy.

Under these conditions, carbon compounds such as amino acids, sugars and nucleotide bases could have formed spontaneously by chemical reactions that do not happen on Earth now. Two reasons they do not happen now: the energy sources and gas mixtures are different, and in today’s oxygen-rich world, with microbes everywhere, any such compounds would be quickly oxidized or eaten.

What counts as living

A cell is the smallest unit that can carry out all the functions of life on its own: it takes in nutrients and energy, carries out metabolism, grows, responds to its surroundings, keeps its internal conditions stable, excretes waste, and reproduces (A2.2). Nothing smaller than a cell is self-sustaining.

Viruses are not considered living because they cannot do any of this independently. Outside a host cell they have no metabolism, take in no energy, do not grow and cannot reproduce. They reproduce only by taking over the machinery of a living cell (A2.3). A virus is better thought of as a package of genetic information than as an organism.

Why the origin of cells is hard to explain

Today, cells only arise by the division of existing cells. A cell is so complex that it is hard to see how one could assemble from scratch. The problem is usually broken into four steps, each of which must have happened for the first cells to exist:

Catalysis
Reactions must be speeded up so that useful products form faster than they break down.
Self-replication
A molecule must be able to make copies of itself, so that information can be inherited.
Self-assembly
Molecules must spontaneously arrange themselves into larger structures, such as membranes.
Compartmentalization
A boundary must separate the inside from the outside so that internal chemistry can differ from the environment.

A scientific hypothesis must be testable, and this is a field where testing is genuinely difficult. The exact conditions on pre-biotic Earth are not known and cannot be recreated with certainty, and the first protocells did not fossilize, so there is no direct record of them. Scientists therefore test individual steps — can amino acids form from simple gases? can fatty acids form membranes? can RNA catalyse reactions? — rather than the whole sequence.

The Miller–Urey experiment

In 1952 Stanley Miller and Harold Urey tested whether the organic compounds needed for life could form from simple inorganic substances under conditions thought to resemble early Earth.

  • A flask of boiling water provided water vapour, representing the ocean.
  • The vapour passed into a flask containing methane, ammonia and hydrogen, the gases then thought to make up the early atmosphere, with no oxygen.
  • Electric sparks were passed through the gases, simulating lightning.
  • A condenser cooled the gases so that liquid collected and returned to the boiling flask, simulating rain.

After about a week the water had turned brown, and analysis showed it contained several amino acids and other carbon compounds. Re-analysis of Miller’s stored samples with modern techniques, decades later, found more than twenty different amino acids.

Evaluation. The experiment is important and its limitations are exactly what you will be asked about:

Strengths
It showed that organic molecules needed by life can form from inorganic ones without any living organism. It was controlled and repeatable, and it has been repeated many times. It turned a speculative idea into a testable one.
Limitations
The early atmosphere is now thought to have been less rich in methane, ammonia and hydrogen and to contain more CO2 and nitrogen, so the mixture may be unrealistic (though later experiments with such gases still yield some organic compounds). It made monomers, not polymers; nothing like a cell formed. The real conditions cannot be known exactly.

Vesicles: the first compartments

A cell needs a boundary so that its internal chemistry can become different from its surroundings — concentrating the molecules that react together, and keeping the products of useful reactions close by rather than letting them diffuse away.

Fatty acids are amphipathic: they have a hydrophilic head and a hydrophobic tail. In water they spontaneously arrange themselves so that the tails are shielded from water. At high enough concentrations they coalesce into a bilayer, and a bilayer sheet closes up into a hollow sphere, a vesicle, because a closed shape has no exposed edges where tails would touch water. This is self-assembly: no enzymes or energy input are needed.

Experiments show that such vesicles can grow by taking up more fatty acids and can divide when agitated. They are a plausible model for how the first compartments formed. Modern membranes are made mainly of phospholipids (B2.1), but simple fatty acids would have been more readily available on early Earth.

RNA as the first genetic material

Modern cells have a chicken-and-egg problem: DNA carries the information for making proteins, but proteins (enzymes) are needed to copy DNA and to make proteins. Which came first?

The RNA world hypothesis offers a way out. RNA can do both jobs:

  • RNA stores information in its base sequence, and it can be replicated by complementary base pairing.
  • Some RNA molecules have catalytic activity. These are ribozymes.

So a single kind of molecule could have acted as both the genes and the enzymes of the earliest cells. The strongest piece of evidence is still inside every cell today: in the ribosome, the formation of peptide bonds between amino acids is catalysed by rRNA, not by protein. The ribosome’s central reaction is carried out by a ribozyme — a molecular fossil of a time when RNA did the catalysis. DNA, which is chemically more stable, is thought to have taken over as the store of genetic information later.

The last universal common ancestor

The last universal common ancestor (LUCA) is the most recent population of organisms from which all life now on Earth is descended. It was not the first cell; it was the ancestor of every living lineage. The evidence that such an ancestor existed:

  • The genetic code is universal: all organisms use essentially the same codons for the same amino acids (A1.2).
  • Many genes are shared by all organisms, including genes for ribosomal RNA, ribosomal proteins and the enzymes that copy and read DNA. The same biochemistry — ATP, the same 20 amino acids, the same kind of membrane transport — is found in all three domains.

Other forms of life may well have evolved before or alongside LUCA. If so, they have left no descendants. The most likely reason is competition: once LUCA and its descendants had spread, they would have out-competed other early life for resources, driving it to extinction.

Dating the first cells and LUCA

Life has been evolving on Earth for an immense length of time — well over three and a half billion years. Several independent approaches are used to estimate dates:

  • Radiometric dating of rocks containing fossils or chemical traces of life, using the known decay rates of radioactive isotopes.
  • Fossil evidence such as stromatolites, layered mounds built by colonies of microorganisms, the oldest of which are about 3.5 billion years old.
  • Chemical traces of life, such as the ratio of carbon isotopes in ancient rocks, because living organisms take up the lighter isotope carbon-12 slightly more readily.
  • The molecular clock: differences in the base sequences of genes shared by all organisms accumulate at a roughly known rate, so the number of differences can be used to estimate how long ago lineages diverged (A3.2).

These methods carry large uncertainties, and estimates for LUCA vary, but most place it somewhere around 3.5 to 4.2 billion years ago — surprisingly soon after Earth became cool enough for liquid oceans.

LUCA and hydrothermal vents

Two lines of evidence suggest that LUCA lived in the vicinity of deep-sea hydrothermal vents, where hot, mineral-rich water emerges from the seafloor.

  • Fossil evidence. Tubes and filaments resembling the fossils of bacteria that live around modern vents have been found in ancient seafloor hydrothermal vent precipitates — mineral deposits that formed around vents — in rocks from Quebec, Canada, dated to at least 3.7 billion years ago.
  • Genomic evidence. Comparing the genomes of many modern bacteria and archaea, researchers identified gene families that are shared across both groups and were probably inherited from LUCA. These conserved genes suggest an organism that lived without oxygen, at high temperatures, and used hydrogen and carbon dioxide as sources of energy and carbon, with enzymes containing iron and sulfur — a close match to the chemistry of a hydrothermal vent.

Vents would also have provided a steady supply of chemical energy, mineral surfaces that could catalyse reactions, and protection from the intense ultraviolet light at the surface.

✏️Worked example

A student claims: “The Miller–Urey experiment proved that life began on Earth by natural chemical processes.”
Evaluate this claim. [6 marks]

An evaluation needs both sides and a judgement. A strong answer might read like this.

What the experiment did show. Miller and Urey passed electric sparks through a mixture of methane, ammonia, hydrogen and water vapour, with no oxygen, to simulate lightning in the early atmosphere. After a week, amino acids and other organic compounds had formed. This showed that the monomers of biological molecules can form from inorganic substances without living organisms, supporting the hypothesis that carbon compounds formed spontaneously on early Earth. The result has been repeated many times.

Why it does not prove the claim.

  • The gas mixture may not be realistic: the early atmosphere is now thought to have contained more carbon dioxide and nitrogen and less methane and ammonia.
  • It produced only monomers. Forming a cell also requires polymerization, catalysis, self-replication and a membrane-bound compartment, none of which the experiment addressed.
  • The real conditions on pre-biotic Earth cannot be known or replicated exactly, and protocells left no fossils, so the full sequence of events cannot be directly tested.
  • More generally, an experiment can support a hypothesis but cannot prove it; it shows that something could have happened, not that it did.

Judgement. The experiment provides evidence that one early step towards life was possible by natural chemistry, so it is consistent with the claim. But it does not prove that life began this way, and the word “proved” should be rejected.

Check it. Count the points against the mark allocation. A six-mark “evaluate” answer needs roughly two points in support, three or four limitations, and an explicit conclusion. An answer that describes the apparatus in detail but never weighs the evidence scores poorly however accurate it is.
Saying the experiment “created life”. It did not create life, cells, or even proteins — it created a mixture of small organic molecules. Examiners see this error every session. Be precise about what was produced: amino acids (monomers).

📝Practise

Work through these on paper, then reveal the answer. All are HL only.

1. Explain why ultraviolet light reached the surface of early Earth in much greater amounts than it does today.
There was no free oxygen in the early atmosphere, because oxygen gas is produced by photosynthesis, which had not yet evolved. Without oxygen there could be no ozone (O3), which forms from oxygen in the upper atmosphere. The ozone layer is what absorbs most ultraviolet radiation today, so without it UV light penetrated to the surface.
2. Outline the reasons that viruses are not regarded as living organisms.
Viruses are not cells: they have no cytoplasm and no plasma membrane of their own. Outside a host cell they carry out no metabolism, obtain no energy, do not grow, do not respond to stimuli and do not maintain homeostasis. They can only reproduce inside a host cell, using the host’s ribosomes, enzymes and ATP. Since the cell is the smallest unit capable of independently carrying out the functions of life, viruses fall outside the definition.
3. Explain why the spontaneous formation of vesicles was an important step in the origin of cells.
Fatty acids are amphipathic, so in water they spontaneously form bilayers that close into spherical vesicles — self-assembly with no enzymes or energy needed. A vesicle creates a compartment separated from the surroundings. Inside it, molecules can be kept concentrated so that reactions between them are more likely, and the products of reactions (and any replicating molecules) stay together rather than diffusing away. The internal chemistry can therefore become different from the outside, which is a requirement for anything that is to maintain itself and evolve as a unit.
4. State two properties of RNA that support the hypothesis that it was the first genetic material, and give one piece of evidence from modern cells.
(1) RNA can store genetic information in its base sequence and can be replicated by complementary base pairing. (2) RNA can act as a catalyst (ribozymes), so it could have served as both genes and enzymes. Evidence: in the ribosome, peptide bond formation during protein synthesis is catalysed by ribosomal RNA, not protein — a ribozyme still performing a central reaction in every living cell.
5. Explain why there is thought to be a single last universal common ancestor, and suggest why no other early life forms survive.
All organisms share the same genetic code and many of the same genes (for example for ribosomal RNA and proteins, and for DNA replication), as well as the same basic biochemistry. It is very unlikely that these would be shared by chance or evolve identically more than once, so they are best explained as having been inherited from one ancestral population. Other life forms may have existed, but none left descendants; a likely explanation is that they were out-competed for resources by LUCA and its descendants and became extinct.
6. Outline two types of evidence that LUCA lived near hydrothermal vents.
Fossil evidence: structures resembling fossilized microorganisms have been found in ancient seafloor hydrothermal vent precipitates (mineral deposits formed around vents), among the oldest evidence of life. Genomic evidence: gene families conserved across bacteria and archaea, and so probably present in LUCA, indicate an organism adapted to high temperatures, living without oxygen and using hydrogen and carbon dioxide, with iron–sulfur enzymes — the conditions found at hydrothermal vents.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

  • Exploring Origins (Harvard) — an animated, well-researched explanation of protocells, vesicles and the RNA world.
  • Understanding Evolution (University of California Museum of Paleontology) — accessible pages on the Miller–Urey experiment and on how scientists study the origin of life.
  • Natural History Museum, London — articles on the earliest fossil evidence of life and on LUCA.