Like breathing? Thank water. And the continents.

One of the greatest mysteries of evolution involves oxygen levels in the atmosphere. At various points throughout 4.5 billion years of geological history, carbon levels have been 10X what they are today, yet life pushed on. Today, oxygen concentration is 21% of the atmosphere. One of the biggest puzzles in geochemistry is how it went from trace amounts to just the right one.

A research group has discovered one possible mechanism, relating to the way in which carbon dioxide is removed from the atmosphere over long timescales. 

"On the early Earth, CO2 levels were controlled by hydrothermal processes on the seafloor. As the Earth cooled, and the continents grew, chemical processes on the continents took over," says  Dr. Benjamin Mills of the University of Exeter.

Using computer models, their simulation led them to postulate that this switch may explain increasing oxygen concentration over Earth's middle age (the Proterozoic era), which ultimately led to conditions suitable for complex life. According to the authors, the oxygen rise is caused by a gradual increase in marine limiting nutrients, which are a product of chemical weathering of the continents.

"The more CO2 that is sequestered by continental weathering, the larger the phosphate source to the oceans. Phosphate availability controls the long term photosynthetic productivity, which leads to oxygen production.

"This is not the only reason oxygen rose to high levels, but it seems to be an important piece of the puzzle. Whilst the carbon cycle can function without large continents, it seems that their emergence was critical to our own evolution."