IB Biology HLThe Origin of CellsPaper 1 & 2Nature of Science~13 min read
The Evolution of Cells
Cells only come from other cells dividing. So where did the first one come from? This note works through the four things that had to happen, and the three competing theories about the order they happened in.
📘 What you need to know
Cells are the smallest units of life. All cells have a plasma membrane and store genetic information in DNA.
Life is defined by metabolism, nutrition, excretion, reproduction, response to stimuli and growth. Viruses are non-living.
Four steps were needed for cells to arise: organic compounds from inorganic molecules; assembly into polymers; polymers that could self-replicate; and membranes forming compartments.
Three competing theories: protocell-first, gene-first and metabolism-first.
Fatty acids are amphipathic, so they spontaneously form monolayers, then bilayers, then vesicles (microspheres).
The RNA world hypothesis: RNA could both store information and catalyse reactions, so it may have come before DNA and enzymes.
Ribozymes are RNA molecules that catalyse reactions, including the formation of peptide bonds.
NOS: these theories are hard to test, because early Earth conditions cannot be exactly reproduced and no early cells fossilised.
Cells as the smallest units of life
All cells share certain features:
They are enclosed by a plasma membrane (cell surface membrane), which separates the cell contents from the outside.
They store genetic information in DNA, which is expressed during protein synthesis.
They contain cytoplasm and ribosomes.
What counts as alive?
Feature of life
Example
Metabolic reactions
Respiration
Nutrition
Taking in the materials needed to live
Excretion
Removing metabolic waste
Reproduction
Passing genetic information to offspring — this allows evolution by natural selection
Response to stimuli
Receiving and responding to the external and internal environment
Growth
Increasing in size
Viruses are considered non-living. They lack a cell structure and organelles, so they cannot carry out metabolic reactions or take in nutrition, and they are unable to replicate independently — they must rely on the cellular components of the host cells they infect.
The problem of the first cells
Cells are complex structures that can only form from the division of pre-existing cells. So here is the puzzle: how did the first cells come into existence when there were no pre-existing cells to divide?
Assuming the first cells did not arrive from somewhere else, they must have originated from the non-living components that made up the primordial atmosphere. That would have required four steps:
Every cell alive today still does all four of these things — this is a list of what had to be invented, not just a historical sequence.
Three theories about the origin of cells
The four steps above have to happen, but in what order? Several theories exist, and each starts with a different step.
Theory
What came first
How the rest follows
Protocell-first
A cell-like compartment capable of some basic metabolic functions arose spontaneously. These are called protocells
Initially they lacked genetic material, but could grow and divide into daughter “cells”. Eventually they acquired genetic material, most likely RNA at first
Gene-first
A nucleic acid, most likely RNA, spontaneously developed and had the ability to replicate itself
Evolution by natural selection then produced genetic variants that developed a cell membrane and basic metabolic processes
Metabolism-first
Life originated as a system of chemical reactions capable of sustaining itself
The system eventually evolved to form cells and genetic material. Favoured by many scientists, since most life processes require energy released by metabolic reactions
🧠
Memory trick: name tells you the answer
Each theory is named after whichever component came first. Protocell-first → the bag came first. Gene-first → the genetic material came first. Metabolism-first → the reactions came first. If you remember the name, you already know the theory.
Nature of Science: can these theories be tested?
One of the cornerstones of the scientific method is formulating hypotheses and theories that are testable. To test theories about the origin of cells, scientists would have to run experiments on the underlying mechanisms, which means replicating early Earth conditions in a laboratory.
Two problems make this very difficult:
It is not possible to replicate the conditions on early Earth exactly as they might have been.
It is impossible to know the exact nature of the first cells, because none of these early cells fossilised.
So the origin of cells remains a highly debated topic. The hypotheses that underpin the theories are hard to test, which is why several competing theories still coexist.
The spontaneous formation of vesicles
Step 4 — getting a membrane — turns out to be the easiest to explain, because it happens by itself.
Membranes matter because they separate the genetic material and biochemical processes inside a cell from the outside environment. This is called compartmentalisation, and membrane formation would have been a crucial step in the origin of cells.
The membranes of the first cells were likely made of fatty acids, because these molecules are amphipathic — one end is attracted to water and the other repels it.
Amphipathic
A polar (hydrophilic) head that mixes with water
and a non-polar (hydrophobic) tail that does not
That single property produces the whole structure, with no help required:
A few lipid molecules in water naturally form a monolayer on the surface — polar parts in the water, non-polar parts sticking out of it.
More lipid molecules form bilayers, with the polar parts facing outwards towards the water and the non-polar parts pointing towards each other.
These bilayers spontaneously form microspheres, or small vesicles, which could have formed the membranes of early cells.
The yellow circles are the polar heads; the lines and the pale band are the non-polar tails hiding from the water.
From fatty acids to modern membranes
These early membranes would have separated the internal chemistry of the cells from their outside environment. Two further chemical steps then produced the membranes we see today:
Fatty acids could have combined with glycerol during condensation reactions to form triglycerides.
These triglycerides could then have undergone phosphorylation to form simple phospholipids, which are the main component of modern cell membranes.
Later, eukaryotic cells evolved to contain multiple internal compartments, allowing a further division of activity within the cell.
The RNA world hypothesis
For early life to evolve, two abilities had to emerge:
A system capable of replicating itself
An ability to catalyse chemical reactions
In modern cells these jobs are split — DNA stores information and enzymes catalyse reactions. But neither would have been present in the pre-biotic world.
Scientists believe RNA may have performed both functions in early cells, since it can store genetic information and has enzymatic properties. This is the RNA world hypothesis. As life evolved, DNA took over as the genetic storage molecule and proteins became the biological catalysts.
Evidence type
The evidence
RNA can do both jobs
RNA can assemble spontaneously from nucleotides, can replicate itself, and can control the rate of chemical reactions — modern cells contain ribozymes that catalyse the formation of peptide bonds
RNA came before DNA
Ribose can be formed from methanal, one of the main products of the Miller–Urey experiment
RNA came before DNA
Deoxyribose in DNA is produced from ribose in an enzyme-catalysed reaction — so ribose must come first
RNA came before proteins
Ribozymes can join amino acids together to form proteins from an RNA template
Remember the four abilities that had to appear for early cells to evolve: catalysis, self-replication, self-assembly and compartmentalisation. If a question asks what was necessary for the evolution of early cells, that list is your answer.
Worked examples
WE 1
Explain how vesicles could have formed spontaneously
Explain how membranes could have formed spontaneously in the early oceans. (4 marks)
Point 1: the property that matters
Fatty acids are amphipathic — they have a polar hydrophilic head and a non-polar hydrophobic tail.
Point 2: a few molecules
A small number of lipids in water form a monolayer at the surface, with polar parts in the water and non-polar parts out of it.
Point 3: more molecules
With more lipids, bilayers form, with the polar parts facing outwards to the water and the non-polar tails facing each other.
Point 4: the result
These bilayers spontaneously close into microspheres or vesicles, creating a compartment with an internal chemistry different from the surroundings.
Amphipathic molecules in water arrange themselves — no machinery requiredthe word “spontaneously” is doing real work here: say clearly that nothing had to assemble it
WE 2
Explain the RNA world hypothesis
Explain why RNA is thought to have been the first genetic material, rather than DNA. (4 marks)
Point 1: two abilities were needed
Early life needed both a molecule that could replicate itself and something that could catalyse reactions.
Point 2: RNA does bothRNA can store genetic information and also has enzymatic properties, whereas DNA can only store information.
Point 3: chemical evidence
Ribose can form from methanal, a product of the Miller–Urey experiment, and deoxyribose is made from ribose — so ribose must have existed first.
Point 4: catalytic evidence
Modern ribozymes catalyse the formation of peptide bonds and can join amino acids from an RNA template.
One molecule doing both jobs is simpler than two appearing at oncethe deoxyribose-from-ribose point is the strongest evidence and the one most often left out
💡 Exam tips
Learn the four steps in order: synthesis → assembly → replication → membrane.
Know all three theories by name and be able to say what came first in each.
Use the word amphipathic, and explain what it means, when discussing membranes.
For the RNA world, the key phrase is that RNA can both store information and catalyse reactions.
Know what a ribozyme is — an RNA molecule that acts as a catalyst.
For NOS questions, give both problems: conditions cannot be exactly replicated, and early cells did not fossilise.
⚠ Common mistakes
Saying protocells were alive. They were cell-like compartments with some metabolic function, initially with no genetic material.
Confusing the three theories. The name always tells you which component came first.
Saying hydrophobic tails are “attracted to each other”. They are pushed together because they are excluded by water.
Writing that RNA replaced DNA. It is the other way round — DNA took over from RNA.
Calling viruses simple cells. They have no cell structure at all and are non-living.
Saying we cannot test these theories at all. We can test the mechanisms; what we cannot do is reproduce early Earth exactly.
Up next: Evidence for the Evolution of Life — LUCA, the tree of life, and how we put dates on events that happened billions of years ago.
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