IB Biology HLThe Origin of CellsPaper 1 & 2~11 min read
Formation of Carbon Compounds
Early Earth was hot, had no oxygen and was bathed in radiation that would kill most things alive today. Those same hostile conditions may be exactly what produced the first building blocks of life.
📘 What you need to know
Early Earth’s atmosphere had higher levels of carbon dioxide and methane. Both are greenhouse gases, so surface temperatures were much higher than today.
The atmosphere lacked free oxygen, so no ozone could form, so UV radiation reached the surface.
These conditions may have caused the spontaneous formation of carbon compounds by processes that do not happen today.
Adding energy (heat or UV) to the early gas mixture could produce amino acids, simple sugars, nucleotides and fatty acids.
Oparin and Haldane proposed this as the ‘primordial soup’ hypothesis.
UV radiation may then have catalysed the formation of larger polymers — proteins, complex sugars, mRNA and phospholipids.
Miller and Urey recreated these conditions in the lab and found traces of simple organic molecules, including amino acids, after a week.
You must be able to evaluate that experiment — methane levels, the energy source, water, and the failure to make nucleotides.
Conditions on early Earth
The conditions on early Earth were not able to support life. That sounds like a dead end, but those same conditions are believed to have been instrumental in producing the biological compounds that made life possible.
Higher atmospheric temperatures
The early atmosphere contained higher levels of carbon dioxide and methane than our present atmosphere.
Both are powerful greenhouse gases. They trap infrared radiation coming from the Earth’s surface and stop it escaping into space, which increases surface temperature. This is the greenhouse effect.
So surface temperatures on early Earth were much higher than they are today.
UV radiation
The early atmosphere lacked free oxygen. That was a problem for sustaining life, but it also prevented ozone from forming.
Ozone (O3) forms when UV radiation from the sun interacts with oxygen molecules (O2). No oxygen means no ozone.
Ozone absorbs damaging UV radiation and protects life on Earth. Without it, UV radiation penetrated all the way to the surface.
UV radiation causes DNA damage and increases the rate of mutation, which is harmful to living organisms.
Hold both ideas at once. UV radiation is damaging to life that already exists — but before life existed, that same energy was a way of driving chemical reactions that would not otherwise happen. The condition that is hostile to life is the one that may have started it.
From simple gases to the building blocks of life
These conditions may have resulted in the spontaneous formation of carbon compounds by chemical processes that do not currently occur.
Adding energy — heat or UV radiation — to the mixture of gases present in the early atmosphere could have led to the formation of organic molecules: amino acids, simple sugars, nucleotides and fatty acids. These would have formed the building blocks of early cells.
The scientists Alexander Oparin and JBS Haldane both proposed this idea as the ‘primordial soup’ hypothesis.
It is also possible that the high levels of UV radiation then catalysed the formation of larger polymers — proteins, complex sugars, mRNA and phospholipids — from these simpler molecules.
Each monomer on the left leads to its own polymer on the right — the same monomer-to-polymer relationship you meet throughout biochemistry.
Evidence: the Miller–Urey experiment
Miller and Urey recreated the conditions thought to have existed on Earth before life, using a specially built piece of apparatus. It let them do four things:
Step
What the apparatus did
What it represents on early Earth
1
Boiled water to produce steam
The early primordial ocean evaporating in the high temperatures
2
Mixed the steam with methane, hydrogen and ammonia
The early atmosphere
3
Added electrical discharges to the gases
Lightning — one of the energy sources available at the time
4
Cooled the mixture and collected the liquid
Water condensing in the atmosphere and falling as rain
After a week, Miller and Urey analysed the condensed mixture and found traces of simple organic molecules, including amino acids.
Steam rises from the heated flask, meets the gas mixture, is sparked, then cools and condenses into the trap — a complete water cycle in glass.
Evaluating the Miller–Urey experiment
This is where the marks are. The IB wants you to evaluate, not just describe, so learn these four criticisms:
Issue
The problem
Methane availability
At the time, it was believed the early atmosphere was accurately simulated by including high levels of methane. It is now believed that methane may have been in low supply on early Earth
The energy source
They used an electrical discharge instead of UV light. For the synthesis of organic molecules, carbon dioxide, nitrogen and water actually require nuclear and UV radiation along with electrical discharges
The presence of water
In a watery environment, amino acids tend to remain as monomers rather than joining to form proteins. Since water is needed to form the primordial soup, this contradicts the idea that complex molecules formed there
Nucleotides
Miller and Urey were unable to generate nucleotides. Nucleotides have since been made chemically, but using a different approach
Notice that none of these criticisms says the experiment was worthless. It showed that organic molecules can form from inorganic ones given energy — which was the real point. The criticisms are about whether the conditions matched early Earth accurately, which is a different question.
Worked examples
WE 1
Explain why UV radiation reached the surface of early Earth
Explain why levels of UV radiation reaching the Earth’s surface were higher on early Earth than they are today. (3 marks)
Point 1: the missing gas
The atmosphere of early Earth lacked free oxygen.
Point 2: what that prevented
Ozone (O3) forms when UV radiation interacts with oxygen molecules, so with no oxygen there was no ozone layer.
Point 3: the consequence
Ozone normally absorbs damaging UV radiation, so without it UV penetrated all the way to the surface.
No oxygen → no ozone → no UV shieldthree linked steps, three marks — do not jump straight from “no oxygen” to “more UV”
WE 2
Evaluate the Miller–Urey experiment
Outline what the Miller–Urey experiment showed, and evaluate its limitations as a model of conditions on early Earth. (4 marks)
Point 1: what it showed
Sparking a mixture of steam, methane, hydrogen and ammonia produced traces of simple organic molecules, including amino acids, supporting the primordial soup hypothesis.
Point 2: limitation — the gas mixture
High methane levels were assumed, but methane may actually have been in low supply on early Earth.
Point 3: limitation — the energy source
Electrical discharge alone was used, whereas synthesis from carbon dioxide, nitrogen and water needs nuclear and UV radiation too.
Point 4: limitation — the products
No nucleotides were generated, and in water amino acids tend to stay as monomers rather than forming proteins.
Real evidence that organic molecules can form — but an imperfect model of early Earth“evaluate” means give the value AND the limitations. Doing only one half caps your marks.
💡 Exam tips
Learn the gas list for the early atmosphere: water vapour, methane, ammonia, hydrogen and carbon dioxide.
Name Oparin and Haldane with the primordial soup hypothesis, and Miller and Urey with the experiment.
Say the products were traces of simple organic molecules — not “life” and not “cells”.
For evaluation questions, have at least two limitations ready plus one strength.
Link every step to energy: these reactions do not happen without heat, UV or electrical discharge.
Remember that these processes do not currently occur — today’s atmosphere has oxygen, and life would consume the products.
⚠ Common mistakes
Saying Miller and Urey created life. They produced simple organic molecules, nothing more.
Saying the early atmosphere had no gases. It had plenty — just no free oxygen.
Confusing ozone with oxygen. Ozone is O3 and forms from O2.
Writing that greenhouse gases trap UV. They trap infrared radiation.
Treating the primordial soup hypothesis as proven. It is a hypothesis with supporting evidence.
Forgetting the water criticism. It is the least obvious limitation and the one that earns marks.
Up next: The Evolution of Cells — how loose organic molecules could have become something enclosed, self-replicating and alive.
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