About 2.5 billion years ago, our planet had virtually no oxygen, and lifeforms were primitive. Then, oxygen levels suddenly spiked, the entire landscape of the planet changed, and we were on our way to complex life. Now, at last, we know why.
Earth probably wouldn't have gotten much past simple multi-cellular organisms without the Great Oxidation Event, let alone give rise to intelligent life. Aerobic organisms are able to harness far more energy than their anaerobic counterparts, and that means much more complex lifeforms can evolve than would otherwise be possible.
But there's a mystery here. Before the Great Oxidation Event 2.4 billion years ago, all organisms were naturally anaerobic. That doesn't just mean they couldn't use oxygen — the gas was actually toxic to them. And yet the only way to generate oxygen on a planetary scale was for organisms to release it as part of photosynthesis. That means anaerobic organisms effectively committed mass suicide to pave the way for their aerobic successors. Indeed, scientists suspect the Great Oxidation Event kicked off the first and most massive extinction event in our planet's history.
And yet all that is exactly what cyanobacteria apparently did do, starting around 2.8 billion years ago. For the first 400 million years, all the oxygen they produced was captured by a mix of organic matter and dissolved iron. But at 2.4 billion years ago, these materials could no longer absorb oxygen, and so oxygen started accumulating in the atmosphere. The oxygen revolution had begun, and cyanobacteria had just signed their own death warrant.
Scientists have known the basics of this story for a while, but the mystery has been why cyanobacteria started producing oxygen. Now a team led by Gustavo Caetano-Anollés of the University of Illinois think they've found the culprit behind the annihilation of the anaerobic organisms and the rise of oxygen-loving life. The answer, it seems, is found in protein folds.
Folds are simply particular regions of a protein that serve specific functions. While the amino acids that make up these folds often mutate and transform, the folds themselves are extremely stable, retaining the same basic function for possibly billions of years. That makes protein folds one of our best shots at reconstructing the very earliest prehistory of the primordial Earth.
The researchers analyzed protein folds from about a thousand different organisms from across all the different domains of life. By cross-referencing all these folds from data taken from ancient microbial fossils, they were able to build up a timeline of protein history. They were then able to identify the most ancient form of aerobic reaction.
According to the timeline, protein folds existed 2.9 billion years ago that handled the synthesis of pyridoxal. While that may not ring any bells, this is simply an active form of vitamin B6, which helps the smooth function of a range of protein enzymes in our bodies. Manganese catalase, an oxygen-producing enzyme, also appeared around 2.9 billion years ago. With these structures on board, cyanobacteria would have begun producing lots of oxygen.
Still, that just tells us the mechanism that produced oxygen. It still doesn't explain what would have made this a good evolutionary decision for the cyanobacteria. But according to Caetano-Anollés, these specific protein folds point towards an answer.
These molecules would have been able to take hydrogen peroxide and split it into water and oxygen. Scientists have previously suggested hydrogen peroxide was unusually abundant in Earth's oceans about 2.9 billion years ago due to the effects of intense solar radiation on Earth's glaciers. For cyanobacteria struggling to deal with all this hydrogen peroxide, a molecule like manganese catalase that could degrade the peroxide into water was the perfect solution.
Of course, they couldn't have even begun to realize that the byproduct this reaction was creating would eventually spell their doom - and considering they still had another 500 million years before the extinction event began, they had a perfectly decent run. But this does mean two things: One, we have manganese catalase to thank for the rise of aerobic life on Earth. And two, rather surprisingly, oxygen is actually the original toxic waste.