Life › The microbial world › Great Oxidation Event
The ozone layer
after c. 2.4 billion years ago· Event· Earth, climate & extinctions
Strong evidenceGood evidence; dates or details still being refined.
A thin band of ozone gas high in the atmosphere that blocks most of the Sun's harmful ultraviolet light.
See it on the Life map →What happened
Ozone is a three-atom form of oxygen (O₃). It forms when ultraviolet (UV) sunlight splits ordinary oxygen molecules about 15–35 km up, and it absorbs the most damaging UV rays. Before the Great Oxidation Event there was too little oxygen for a shield, so surface life needed cover from water, sediment or mineral crusts. Models suggest a useful shield needs only a small fraction of today's oxygen.
Why it matters
It turned the sunlit surface from a sterilizing zone into livable space, a precondition for life later spreading into shallow water and onto land.
Think of it like
Like a roof membrane: a very thin layer, yet it keeps damaging weather off everything underneath.
The evidence (fossils, genes, rocks)
Mostly indirect: atmospheric chemistry models, plus sulfur isotope patterns in rocks that require strong UV in oxygen-free air and vanish around 2.4 billion years ago. The Curious Science map covers the physics.
Still debated
How thick the early ozone layer was, and whether it stayed strong through the low-oxygen 'Boring Billion', is still uncertain.
Connections (2)
▸Great Oxidation Event allowed the formation of The ozone layerWell established
The connection
Ozone is made from oxygen. Before the Great Oxidation Event the air held almost no free oxygen, so no shield could form. Once oxygen from cyanobacteria built up, ultraviolet light split some of it high in the atmosphere, forming ozone that absorbed the harshest radiation.
When and how
Globally, from roughly 2.4 billion years ago, as oxygen reached a small fraction of today's level.
The evidence
The chemistry is basic: ozone requires oxygen. The disappearance of UV-driven sulfur isotope signals in rocks about 2.4 billion years ago marks when oxygen rose.
Debates and open questions
How fast the shield strengthened, and how much it weakened when oxygen dipped during the Boring Billion, are uncertain.
▸The ozone layer made land safer for Plants colonize landStrong evidence
The connection
Life on bare ground faces full sunlight, including ultraviolet rays that damage DNA. The ozone layer, which built up after oxygen rose about 2.4 billion years ago, filtered most of that UV. Together with protective pigments and tough spore walls, it let plants survive in the open. It was a precondition, not a trigger.
When and how
The shield formed after about 2.4 billion years ago; plants reached land about 470 million years ago, so it was in place long before.
How sure are we?
The physics of UV absorption and models of the oxygen–ozone link; the role is accepted in principle but hard to measure for the Ordovician.
Debates and open questions
How strong the shield was before oxygen neared modern levels, and whether a later oxygen rise thickened it just before plants moved onto land.