Science

How plate tectonics may have changed the climate and paved the way for complex life to evolve

How plate tectonics may have c...
The trilobite is one of the most famous creatures to arise during the Cambrian Explosion – and a new study might explain how it came to be
The trilobite is one of the most famous creatures to arise during the Cambrian Explosion – and a new study might explain how it came to be
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The trilobite is one of the most famous creatures to arise during the Cambrian Explosion – and a new study might explain how it came to be
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The trilobite is one of the most famous creatures to arise during the Cambrian Explosion – and a new study might explain how it came to be

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

5 comments
Thinker
Could someone please explain the point of these sophisticated guesses put forward by this handful of scientists?
Nik
The Earth has had five Ice Ages, during the last 750 million years or so, at roughly 150 million year intervals. The 'Snowball Earth' event, and four others, including now! There is no known tectonic or other earth bound mechanism that can cause such regular events. So the cause must be extraterrestrial. There is such a cause. The Earth orbits the Sun, but the Sun orbits the centre of the galaxy, independently from the rotation of the galaxy, (which would tend to suggest that the Solar system was captured from elsewhere.) I have seen various figures quoted for the period of this orbit, but I prefer to accept that from an astronomical paper, which gave 600 million years. As the solar system orbits, it passes through the arms of the galaxy. During that passage interstellar dust attenuates the Suns energy reaching the Earth and causes an ice age, which can have a period of tens of millions of years. Other effects may include increased volcanic and earthquake activity, due to gravitational effects. In addition, the solar system follows a sinusoidal path through the plane of the galaxy, which has a period of 50-60 million years, and these passes may be accompanied by extinction events. The present ice age, (not to be confused with a 'glacial period' due to the 100,000 year Malenkovitch cycle,) has already lasted some 40+ million years, and may have at least that long to go before the Earths climate returns to normal, with no ice caps, and a very much hotter climate than present humanity has ever experienced. The Earth is not a closed system, but many specialists, in various fields, persistently overlook that fact when formulating solutions to the Earths past climate. So, prior to the Cambrian Explosion, it was too cold for much life to exist, except bacteria, and algae, but as the Earths climate warmed, with the increase of solar radiation reaching the Earth, as it exited the arm of the galaxy, life could return to normal, very rapidly!
Procyon Pizzavorus
It's a very interesting hypothesis. However, it's based on correlation, which does not imply causation. I read an article a week ago that suggested that snowball earth kicked plate tectonics in to high gear by "lubricating" the faults with finely ground dust. Another hypothesis suggests that the melting glaciers at the end of the snowball earth brought minerals (incl. Iron) in to the oceans, leading to an explosion in oxygen releasing organisms. These hypotheses, while interesting, need rigorous testing.
Procyon Pizzavorus
@Thinker: The point of all research is to know more about the universe we live in. The desire for exploration, learning, and understanding is what makes us human. And to your next question - "yes, but of what use is this knowledge?" - it was Faraday, wasting time and grant funds rubbing glass rods with silk cloth, that ultimately led you to posting this comment on the internet. @Nik: Your hypothesis about the dust on the galactic plane is interesting, but do you have any sources that cite that the dust densities are high enough to cause a significant reduction in sunlight reaching the earth? Wouldn't that level of density cause rapid mass accrual by the sun and the planets and cause major disruptions in orbit, not to mention disrupt the spiral arms themselves? As for the rotation of the galaxy - there is no such thing. The galaxy does not rotate, and the spiral arms are not fixed structures. The stars that comprise the galaxy orbit the center of mass, and the spiral arms are a result of the sum of stars traveling at similar velocities and gravitationally interfering with each other. There is no evidence to suggest that the solar system was captured from an extragalactic source. In fact, the composition of the solar system actually suggests it was born in the milky way (in contrast to the Gaia stream, for example).
Bob Stuart
There may have been a convergence of causes. Perhaps the background radiation had to decay before multi-cellular life could stabilize.