What Was the Oxygen Holocaust?

Imagine this: A time when Earth's atmosphere had no breathable oxygen. No animals. No plants. Just an Earth occupied by lots of microbes to which O2 was poison.

Then the Earth’s atmosphere changed. What was once an oxygen-poor world  became oxygen-rich, which opened to door to the evolution of high-energy lifeforms like animals - like us, in other words. But this was terrible news for all those microscopic organisms that had their way for so long on early Earth. This breath of fresh air meant their almost complete devastation: An “oxygen holocaust,” as some researchers refer to it.

All this happened 2.4 to 2.3 billion years ago, according to researchers  investigating some of the oldest rocks on Earth. By studying layers of rocks, including those drilled from deep in the Earth's crust, scientists have pinpointed when Earth's atmosphere went from being oxygen-free to having a measurable trace of the gas. This happened about 2.5 billion years ago, between 50 million and 100 million years before the oxygen holocaust really got started. It probably took about 300 million years for the oxidation event to run its course. 

Ironically, this atmospheric transformation was the product of several factors, including the rise of other organisms like blue-green algae. These algae use photosynthesis to convert the sun's energy to sugar in a way that produces oxygen as a byproduct. More of these organisms meant more oxygen.

But that's just one piece of the puzzle. Oxygen is an element that really likes to combine with other elements to make all sorts of more stable compounds. So it doesn't stick around in the atmosphere for long when there are things to react with.

“The things that sponge up that oxygen largely come from volcanoes and ‘outgassing’ of the environment,” says Dr. J. William Schopf, professor of paleobiology at the University of California at Los Angeles. “For example, if there is any hydrogen that comes out of a volcano, it reacts with oxygen to make water, H2O.”

This is especially the case with submarine volcanoes, he continues. These can release a lot of iron which dissolves in water and then is carried away by currents. When the iron-rich water reaches shallow seas where oxygen is being released by blue-green algae, there is an instant chemical reaction. The iron snaps up the oxygen and makes what we commonly call rust, ferric oxides and hematite. These newly made compounds then sink to the bottom of lagoons or shallow seas, building the layers of rocks that researchers are studying today.

“It makes a fine, rusty rain and takes the oxygen out of the water column and buries it,” Schopf says. “And that went on for quite a long time.”

It was only after the oxygen had reacted with all the available free iron in the oceans that it was able to escape the water and begin collecting in the atmosphere. From then on the only organisms that could thrive openly were those that could tolerate or needed oxygen. As for those ancient microbes who once ruled the world? They're still around, but they live literally under rocks and in other hidden places where oxygen levels are especially low.

Did You Know?
There are still organisms alive today that don’t need oxygen at all. They are called anaerobic organisms. The most infamous of which may be Clostridium, varieties of which can cause food poisoning, tetanus and botulism, among other maladies. Clostridium is also found in the cosmetic injection Botox.

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