Evolution is usually a gradual process, but about half a billion years ago it took off at a gallop in an event that's now known as the Cambrian Explosion. One of the leading theories is that this was thanks to a huge spike in oxygen levels, and now a team may have found where all that oxygen come from – plate tectonics.
For the first few billion years, life on Earth was happy to plod along as simple, mostly single-celled organisms. But the fossil record shows that roughly 541 million years ago, evolution absolutely cranked into overdrive, diversifying into all the broadest groups of life within about 25 million years – an extremely short amount of time, in the grand scheme of things.
"One of the great dilemmas originally recognized by Darwin is why complex life, in the form of fossil animals, appeared so abruptly in what is now known as the Cambrian Explosion," says Tim Lenton, co-author of the paper. "Many studies have suggested this was linked to a rise in oxygen levels – but without a clear cause for such a rise, or any attempt to quantify it."
The new study puts forward a story that begins long before the Cambrian Explosion. Ironically, it begins with a huge output of carbon dioxide, released from volcanic activity as the supercontinent Gondwana was forming between one billion and 542 million years ago.
This CO2-rich atmosphere warmed the planet, leading to faster weathering of continental rocks, which in turn released nutrients like phosphorus into the oceans. Photosynthesizing organisms loved this new combination of higher CO2 and more nutrients, so they thrived and cranked up their production of oxygen.
The researchers pieced this scenario together using a sophisticated biogeochemical model, which allowed them to simulate how much oxygen would have been released under these circumstances. They found that oxygen levels would have risen to about a quarter of what they are today – importantly, crossing a critical threshold that would have been necessary for the kinds of life that followed in the Explosion.
"What is particularly compelling about this research is that not only does the model predict a rise in oxygen to levels estimated to be necessary to support the large, mobile, predatory animal life of the Cambrian, but the model predictions also show strong agreement with existing geochemical evidence," says Josh Williams, lead author of the study.
The conclusion of the new study could fit in with other research about the state of the world around that time. Plate tectonics has also been suggested as being responsible for plunging the planet into a global deep freeze, a period sometimes known as Snowball Earth, about 700 million years ago.
This is within the window that Gondwana was forming, so perhaps the two stories are linked. Plate tectonics could have first caused the Snowball Earth state, then later released CO2 slowly until it warmed up, ending the ice age. Melting ice and weathering rock wash more nutrients into the oceans for photosynthesizing organisms, who are also enjoying the extra CO2. They give off more oxygen, which paves the way for an explosion of oxygen-requiring life to evolve.
Of course, there are many competing theories, so more direct evidence is always needed to support them.
The research was published in the journal Nature Communications.
Source: University of Exeter
