“Oxygenation was waiting to happen.”
The early oceans and atmospheres of Earth had no free oxygen, although photosynthetic cyanobacteria produced it as a byproduct. Free oxygen isn't combined with other elements such as nitrogen or carbon, and aerobic organisms like humans need it to survive. About three billion years ago, small pockets of free oxygen started to appear in the oceans, and then about 2.4 billion years ago, a rapid increase in atmospheric oxygen took place. During this period of about 200 million years, the amount of free oxygen in the atmosphere suddenly jumped by about 10,000 times. This time is known as the Great Oxidation Event, and it transformed the Earth's surface chemical reactions entirely.
University British Columbia geologist Matthijs Smit and his colleague, University of Bern professor Klaus Mezger, knew that the Great Oxidation Event also transformed the composition of continents, so they began to study records of the geochemistry of igneous rocks and shales from all over the world to find a link — more than 48,000 rocks going back billions of years.
“It turned out that a staggering change occurred in the composition of continents at the same time free oxygen was starting to accumulate in the oceans,” Smit said in a press release. “Oxygenation was waiting to happen,” Smit added. “All it may have needed was for the continents to mature.”
The rock in modern Iceland and the Faroe Islands provides examples similar to what could be found in the continents before oxygenation: rocks rich in magnesium and low in silica. However, the rocks from the past contained the mineral olivine, which initiates oxygen-consuming chemical reactions when it comes into contact with water, locking up oxygen. That is probably what happened early in Earth's history when cyanobacteria produced oxygen.
As the continental crust evolved to become more like it is today, olivine virtually disappeared and the reaction it initiated stopped, allowing oxygen to accumulate. Once oceans became saturated with oxygen, the gas crossed into the atmosphere.
“It really appears to have been the starting point for life diversification as we know it,” Smit said in the release. “After that change, the Earth became much more habitable and suitable for the evolution of complex life, but that needed some trigger mechanism, and that's what we may have found.”
Although the cause of the change in the continents remains unknown, Smit notes that modern plate tectonics started at about that time, and many researchers theorize a connection between the events.
This isn't exactly about evolution, or abiogenesis, but by discovering how the most necessary substance for complex life became ubiquitous, these scientists may have solved a term in the equation for the origin Earth-based life. Such valuable knowledge could also be applied to our search for life beyond the solar system. We already suspect that the two innermost exoplanets of the TRAPPIST-1 system might have vast amounts of liquid water. If (or when) we discern the presence of oxygen, could we deduce the positions and composition(s) of exoplanets' continents, thus narrowing down a few terms on the far end of the Drake equation?
The answer awaits.
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