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Evolution may have been waiting for a decent breath of oxygen, said researcher Chris Reinhard. And that was hard to come by. His research team is tracking down O2 concentrations in oceans, where earliest animals evolved.

By doing so, they have jumped into the middle of a heated scientific debate on what rising oxygen did, if anything, to charge up evolutionary eras. Reinhard, a geochemist from the Georgia Institute of Technology, is shaking up conventional thinking with the help of computer modeling.

That thinking goes like this: “Atmospheric oxygen had a value of ‘x’ back then, and so we just assume that the whole ocean is a beaker that equilibrates with that value,” Reinhard said. Then all evolving animals everywhere had the selfsame concentration of oxygen to live on.

But oceans are expansive and asymmetrical; deep here, shallower there, frosty at the poles, soupy at the girth. Turbulences, streams and temperatures distribute sediment, algae, salt — and gases like oxygen — into lopsided stores.

Oceans leave some areas teeming and others vacuous. Then they reshuffle their loads. Even today, concentrations of dissolved oxygen vary widely from ocean region to ocean region.

Equating the global ocean to a placid lab beaker? “This is an okay thought experiment to start with, but I think everybody would acknowledge over a beer that it’s simplistic,” said Reinhard, an assistant professor at Georgia Tech’s School of Earth and Atmospheric Sciences.

So, he and his team modelled how oxygen entered oceans from the atmosphere and from aquatic sources, and how oceans might have shuffled it around during the mid to late Proterozoic Eon. That was 0.6 to 1.8 billion years ago, when the Earth’s atmosphere had only fraction of the breathable oxygen it does today.

In the model, the ocean didn’t share and share alike, but instead pushed dissolved O2 into areas of concentration that shifted starkly as corresponding concentrations in the atmosphere rose.

That has implications for the way scientists think about the timeframe for animal evolution on Earth and for future estimates for the probability of complex life on exoplanets.

The results and detailed modeling parameters were published on Monday, July 25, 2016, in the Proceedings of the National Academy of Sciences. The research was funded by the National Science Foundation and the NASA Astrobiology Institute.

Read more at the Georgia Tech website.