Event 8

3 billion years ago

Cyanobacteria in stromatolites found in Australia
The first oxygen. This is a photo of stromatolites found in Australia’s Yalgorup National Park. Stromatolites were abundant during Precambrian times, and housed cyanobacteria, which are believed to be largely responsible for the increase in oxygen in early Earth’s atmosphere.
Credit: Photo by C Eeckhout /Wikipedia.

All over the world, starting about 3 billion and peaking about 2.3 billion years ago, something remarkable happened. The earth rusted. Amazingly, the tiny iron minerals in the once-stagnant oceans rusted or oxidized with the addition of oxygen to these seas, forming iron oxide. And that oxygen was apparently produced by lowly single-celled bacteria!

Blue Green Algae
Cyanobacteria are also known as blue-green algae.
Credit: Photo by Hans Paerl, University of North Carolina’s Endeavors magazine / NSF.

It appears that single-celled cyanobacteria colonies, represented in the fossil record by stromatolites, were using photosynthesis (taking the energy from sunlight and creating organic compounds from carbon dioxide and water.) The by-product of this process was the emission of oxygen. During this time oxygen in the Earth’s oceans and atmosphere rose from less than one percent to well over ten percent during this time (the modern oxygen value is about 20 percent).

This iron was deposited on ancient sea floors as small particles of rust that formed beautiful banded rocks called banded iron formations or BIF’s. They consist of alternate thin red layers and gray layers, thought to represent annual seasonal cycles of deposition. These banded iron formations document the Great Rust in the Earth’s history. Iron can make up over 30 percent of these rocks.

Approximately 1.3 billion tons of iron ore are mined on the Earth each year. This iron produces our cars, airplanes, ships, railroads and trains, construction material for buildings and bridges (e.g. I-beams, rebar, nails, screws, bolts), and machine tools, as well as our kitchen, farming, and gardening utensils. The vast majority of these deposits formed during the Great Rust some 2.5 to 2.2 billion years ago.


Rocks dated to between 2.5 and 2.2 million years ago contain minerals, especially iron oxides, which could only have formed under water in the presence of significant amounts of oxygen. Earlier rocks in the Earth’s history do not show this oxidation process, indicating that oxygen was present only in very low amounts.


Without the presence of some 20 percent oxygen in our atmosphere, you would die within a few minutes. Think about that. Thank prehistoric bacteria for each breath you take. As the earth’s atmosphere and seas became oxygen-rich from the photosynthesis of bacteria, new forms of life with complex metabolisms requiring oxygen we able to evolve and flourish. You are one of the many end products of evolution in this World of Oxygen. Furthermore, much of the technological world we have created and depend upon has been built with the iron produced during this period of earth’s evolution.

Banded Iron Formation
This is a banded iron formation at Rio Tinto, Spain.
Credit: Photo from Wikipedia/ NASA.


Evolution and Oxygen: https://www.youtube.com/watch?v=DE4CPmTH3xg&feature=related

Wikipedia’s overviews on stromatolites and on banded iron formations:

Explanation of the relationship between stromatolites, atmospheric changes on earth, and the formation of banded iron desposits:

An article on the connection between banded iron deposits and the oxygen record on earth:

A brief description of how Earth got its oxygen, telling how iron is retrieved and interpreted with regard to the formation of oxygen:

A short-but-sweet synopsis of a model (with a drawing) for the formation of banded iron deposits:

Another, somewhat more detailed, description of the formation of banded iron deposits:

Streaming video presentation at MIT by a Cal Tech Professor of Geobiology on the evolution of photosynthesis and its relationship to the formation of banded iron deposits:


Cockell, Charles et al. (Editors). An Introduction to the Earth-Life System. Cambridge: Cambridge University Press. 2008.

Hazen, Robert M. Genesis: The Scientific Quest for Life’s Origin. Washington, D.C.: George Henry Press. 2005.

Knoll, Andrew H. Life on a Young Planet: The First Three Billion Years of Evolution on Earth. Princeton: Princeton University Press. 2003.

Schopf, J. William. Cradle of Life: The Discovery of Earth’s Earliest Fossils. Princeton: Princeton University Press. 1999.

Schopf, J. William, (Editor). Earth’s Earliest Biosphere: Its Origin and Evolution. Princeton: Princeton University Press. 1983.

Schopf, J. William. Life’s Origin: The Beginnings of Biological Evolution. Berkeley: University of California Press. 2002.


Earth, The Biography: The Story of Our World.

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