According to new research, oxygen levels in the Earth’s atmosphere likely “fluctuated wildly” a billion years ago, creating conditions that may have accelerated the development of early animal life. Scientists believe that atmospheric oxygen evolved in three stages, beginning with what is known as the Great Oxidation Event about two billion years ago, when oxygen first appeared in the atmosphere. The third phase, about 400 million years ago, saw the rise of atmospheric oxygen to the levels that exist today.
It is uncertain what happened during the second phase, a time known as the Neoproterozoic Era, which began about a billion years ago and lasted about 500 million years, during which early animal life forms appeared. The question the scientists tried to answer is was there something extraordinary about the changes in oxygen levels in the Neoproterozoic era that could have played a key role in the early evolution of animals – did oxygen levels rise suddenly or was there a gradual increase?
Fossilized traces of early animals known as Ediacaran biota, multicellular organisms that required oxygen were found in sedimentary rocks that are 541 to 635 million years old. To try to answer the question, a research team at the University of Leeds, supported by the Universities of Lyon, Exeter and UCL, used measurements of different forms of carbon, or carbon isotopes, found in limestone rocks taken from shallow seas. Based on the isotope ratios of the different types of carbon found, scientists were able to calculate the levels of photosynthesis that existed millions of years ago and infer atmospheric oxygen levels.
As a result of the calculations, they were able to create a record of atmospheric oxygen levels over the past 1.5 billion years, which tells us how much oxygen would have diffused into the ocean to support early marine life. Dr Alex Krause, a biogeochemical modeller who completed his PhD at the School of Earth and Environment in Leeds and was the project’s lead scientist, said the findings provide new insights into the way oxygen levels have changed on Earth.
He added: “For the first two billion years of its existence, the early Earth was anoxic, with no atmospheric oxygen. Then oxygen levels began to rise, known as the Great Oxidation Event.” “Until now, scientists thought that oxygen levels were either low after the Great Oxidation Event and then shot up just before we saw the first animals evolve, or that oxygen levels were high for many millions of years before animals appeared.
“However, our study shows that oxygen levels were much more dynamic. There was an oscillation between high and low oxygen levels for a long time before early animal life appeared. We see a period when the ocean environments where the early animals lived had plenty of oxygen – and then a period when it wasn’t.
Dr Benjamin Mills, who leads the Earth Evolution Modeling Group at Leeds and oversaw the project, said: “This periodic change in environmental conditions would create evolutionary pressures where some life forms could die out and new ones could emerge.” Dr Mills said the oxygenated periods expanded what are known as “habitable spaces” – parts of the ocean where oxygen levels would have been high enough to support early animal life. He said: “It has been suggested in ecological theory that when you have habitable space that expands and contracts, it can promote rapid changes in the diversity of biological life.
“When oxygen levels drop, there is a strong environmental pressure on some organisms that could lead to extinction. And when oxygen-rich waters expand, the new space allows survivors to achieve ecological dominance.” “These extended habitable spaces would last for millions of years, giving enough time for ecosystems to develop.”
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