Without microbial life, humanity would never have evolved. Just over 2 billion years ago, photosynthetic algae, thieving atmospheric carbon dioxide and switching it for oxygen, had succeeded in making our planet breathable. A new Nature Geoscience study reveals that they may have had a co-conspirator in this regard – the fiery mantle below.

It’s well-established that the mantle – the partly molten, partly solid segment of the world beneath the crust that makes up 84 percent of the planet’s entire volume – contains plenty of oxygen. Actually, it’s its most common element, constituting nearly 45 percent of the entire broiling mass.

A team of researchers from Yale, Arizona State University, and the Bayerisches Geoinstitut in Germany wanted to know if the concentrations of atmospheric oxygen on Earth billions of years ago was affected by the concentrations of oxygen in the mantle. In order to find out, they subjected synthetic samples of the lower mantle to various high-pressure oxygen environments.

“When there's less oxygen present in the rock, it's denser than when there is more oxygen present, even though the rest of the elements are the same,” principle investigator Kanani Lee, an associate professor of geology and geophysics at Yale, said in a statement. These denser rocks would have long ago dropped down to the boundary between the mantle and the outer core, and possibly turned into the types of colossal molten blobs found today beneath the African continent and the Pacific Ocean.

On the other hand, the lighter rocks would have ascended to the upper mantle. The researchers' model shows that, over time, through plate tectonics or volcanic activity, this material would have been forced up to the surface. Chemical reactions would have eventually liberated its mineralized oxygen, allowing it to be distributed in the atmosphere.

Ultimately, this study suggests that our planet’s primitive atmosphere was at least partly controlled by the turbulent terror of the superheated mantle. So is there any geological evidence demonstrating that this is exactly what happened?

^{Image in text: Simulated oxygen eruptions at Earth's surface. Yale University}

It’s agreed that thanks to photosynthetic bacteria, our world was inundated with free-floating oxygen 2.3 billion years ago during the so-called Great Oxygenation Event (GOE). Slowly converting atmospheric carbon dioxide to oxygen over time, free-floating oxygen only began to build up when it ran out of iron compounds to chemically react with.

Due to a series of biological and geological processes, the atmospheric concentration of oxygen has jumped up and down ever since. Notably, a recent study has revealed that a second spike around 815 million years ago coincided with the evolution and diversification of complex, multicellular life.

This new study estimates that the upper mantle’s oxidation would have happened far earlier, perhaps around 3.6 billion years ago. This roughly coincides with the earliest known appearance of colonies of photosynthetic bacteria, taking the form of layers of biological material known as stromatolites.

These were the pioneers of the GOE, the first organisms known to begin the epic atmospheric conversion process. This new study suggests that the oxygen build up that began around then – and culminated in the GOE 1.3 billion years later – was aided by the churning violence of the superheated mantle.

During the GOE, photosynthetic bacteria thrived, but a huge swath of microbes unable to use this newfound oxygen were poisoned by it and died. The GOE represented the earliest mass extinction known to science, and it is possible that the mantle and stromatolites worked together to ensure this zoological regime change took place.

^{Thanks to microbes and the mantle, our atmosphere became breathable. Adisa/Shutterstock}