Cyanobacteria and the Great Oxidation
Has Earth's air always been like this?
That time, that place
Until the 1960s people assumed Earth's air had always been much as it is. Then geologists reading old rock saw something odd.
Rock older than 2.5 billion years showed no rust. Younger rock held bands of red, layer upon layer. Iron needs oxygen to rust.
Why this question
The red bands were iron that had been dissolved in the sea, meeting oxygen and settling out. So at some point oxygen had appeared in the water.
Where from? Not volcanoes, not space. In shallow seas there were stony layered mounds holding the pressed mats of bacterial colonies — stromatolites. The same mounds still grow in one Australian bay today, and cyanobacteria live inside them.
What was found
Cyanobacteria used sunlight to split water for food and threw the oxygen away. For the first few hundred million years all of it went into rusting the iron in the sea — hence the red bands. Only when the iron was spent did oxygen begin to accumulate, spilling into the sky around 2.4 billion years ago.
To most life of the time, oxygen was poison. Vast numbers died. It is the largest pollution event in Earth's history. Yet that pollution made the ozone layer that blocks ultraviolet, and creatures appeared that used oxygen as fuel — the line that leads to us.
Earth's air was always much as it is now
Banded iron appearing around 2.4 billion years ago, and stromatolites older still
The event is called the Great Oxidation. Bacteria that burned oxygen moved into other cells and became mitochondria; the cyanobacteria themselves moved in and became the chloroplasts of plants