The oxygen you are breathing is a waste product. Earth’s early air had almost none of it, and the atmosphere we have now was built by living things over billions of years. Then that new atmosphere changed what kind of life was possible. It runs in both directions, and that is the whole idea of this section.
📚 What you need to know
Atmospheric composition has changed significantly over time, and the evolution of life has been largely responsible.
The pre-biotic (before life) atmosphere likely had high carbon dioxide and methane, little or no oxygen, and probably high nitrogen — with no ozone layer.
Around 2.5 billion years ago, cyanobacteria developed photosynthesis, absorbing carbon dioxide and releasing oxygen.
Rising oxygen allowed aerobic respiration, which is more energy-efficient and enabled larger, more complex organisms.
Oxygen also formed the ozone layer in the stratosphere, shielding the surface from UV and making life on land viable.
Oxygen reacting with dissolved iron produced banded iron formations — geological evidence of early oxygenation.
The pre-biotic atmosphere
Earth’s early air was nothing like today’s. It is likely to have contained:
High levels of carbon dioxide (CO2) and methane (CH4).
No oxygen (O2), or only trace amounts.
Nitrogen (N2) probably already in high concentrations, similar to today.
With no oxygen there was no ozone layer, so the surface was exposed to harmful UV radiation. That alone made land a hostile place for anything living.
The great change
Oxygen was toxic to much of the life that existed at the time. The organisms that could tolerate it, and then use it, became the ancestors of everything complex that followed.
Photosynthesis and oxygenation
Cyanobacteria absorbed CO2 and released O2.
Over enormous spans of time this produced a gradual decrease in carbon dioxide and increase in oxygen.
The rise in oxygen allowed aerobic life forms to develop.
Formation of the ozone layer
More oxygen in the atmosphere allowed ozone (O3) to form in the stratosphere.
Ozone absorbs harmful UV radiation, creating a protective layer around Earth.
Shielded from UV damage, more complex life could evolve on land rather than staying in water.
Before the ozone layer, water was the shield — a few metres of it absorbs UV perfectly well. That is why life stayed aquatic for so long. The ozone layer is what opened up the continents.
Oxidation and mineral formation
Oxygen released by photosynthetic organisms in ancient oceans reacted with dissolved iron in the seawater, forming iron oxides such as hematite and magnetite. These settled onto the ocean floor in distinct layers, creating banded iron formations (BIFs).
Why BIFs matter as evidence: they are physical, datable rock recording the moment free oxygen appeared in the oceans and atmosphere. The oxidation process also changed Earth’s surface chemistry and which minerals could form.
How the new atmosphere changed life
Atmospheric change
Consequence for life
Oxygen becomes available
Aerobic respiration evolves. It is far more energy-efficient than anaerobic processes, so it supports larger, more complex and more energy-demanding organisms
Ozone layer forms
UV radiation at the surface falls sharply, making life on land viable and allowing diverse terrestrial species to spread
Carbon dioxide falls
The greenhouse effect weakens over geological time, changing the planet’s temperature regime
Oxygen reacts with dissolved iron
Banded iron formations are deposited, leaving a geological record of oxygenation
EXAM PRACTICE
Explain how the evolution of photosynthesis changed the atmosphere and how those changes then influenced the evolution of life. [5]
Step 1: the biological change
Around 2.5 billion years ago cyanobacteria evolved photosynthesis, taking in carbon dioxide and releasing oxygen.
Step 2: the atmospheric effect
Over time this lowered carbon dioxide and raised oxygen concentrations.
Step 3: first consequence for life
Free oxygen allowed aerobic respiration, which releases more energy per unit of food than anaerobic processes, supporting larger and more complex organisms.
Step 4: second consequence
Oxygen in the stratosphere formed ozone, which absorbs UV radiation.
Step 5: the outcome
With UV screened out, organisms could survive on land, so terrestrial ecosystems developed.
Life altered the atmosphere, and the altered atmosphere then shaped which life could evolveThe mark scheme wants both directions of the relationship. Answering only “life released oxygen” gets you half of it.
💡 Exam tip
Emphasise the two-way relationship. Life shaped the atmosphere; the atmosphere shaped life.
Name cyanobacteria specifically rather than saying “plants” — plants came far later.
Give banded iron formations as the evidence if asked how we know oxygenation happened.
Say why aerobic respiration mattered: more energy released, so bigger and more complex organisms became possible.
You do not need to memorise the detailed chronology, but the 2.5 billion year figure and the order of events are worth knowing.
⚠️ Common mix-up
Saying plants oxygenated the atmosphere. It was cyanobacteria, long before plants existed.
Assuming oxygen was immediately beneficial. It was toxic to much of the existing anaerobic life.
Putting the ozone layer before the oxygen. Ozone is made from oxygen, so the order cannot be reversed.
Describing the change as fast. It unfolded over hundreds of millions of years, which is exactly the contrast with modern change.
Forgetting nitrogen. It was probably already abundant, so it is not part of the story of change.
Up next: Climate Change — Causes and Impacts, where the enhanced greenhouse effect, feedback loops and tipping points from this sub-topic are put to work on the modern problem.
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