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Fossil records of early animals from the Mistaken Point Ecological Reserve in Canada. CREDIT Dr. Emily. G. Mitchell – University of Cambridge
According to new research, oxygen levels in Earth’s atmosphere likely “fluctuated wildly” a billion years ago, creating conditions that could have accelerated the development of early animal life.
Scientists believe atmospheric oxygen developed in three stages, beginning with what is known as the Great Oxidation Event around two billion years ago, when oxygen first appeared times in the atmosphere. The third stage, around 400 million years ago, saw atmospheric oxygen rise to levels that exist today.
What is uncertain is what happened during the second stage, in a time known as the Neoproterozoic Era, which began about a billion years ago and lasted about 500 million years ago, during which time the first forms of animal life emerged.
The question scientists have tried to answer is: Was there anything extraordinary about changes in oxygen levels in the Neoproterozoic era that might have played a central role in the early evolution of animals? ? Did oxygen levels suddenly increase or was there a gradual increase?
Fossilized traces of primitive animals – known as Ediacaran biota, multicellular oxygen-requiring organisms – have been found in sedimentary rocks between 541 and 635 million years old.
In an attempt to answer the question, a research team from the University of Leeds supported by the universities of Lyon, Exeter and UCL, used measurements of the different forms of carbon, or carbon isotopes, found in limestone rocks taken from shallow seas. Based on the isotopic ratios of the different types of carbon found, the researchers were able to calculate the levels of photosynthesis that existed millions of years ago and deduce the levels of atmospheric oxygen.
Using the calculations, they were able to produce a record of oxygen levels in the atmosphere over the last 1.5 billion years, which tells us how much oxygen would have diffused into the ocean to sustain primitive sea 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 results give new insight into how oxygen levels change on Earth.
He added: “The early Earth, for the first two billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then oxygen levels began to rise, which is known as the Great Oxidation Event.
“Until now, scientists thought that after the Great Oxidation Event, oxygen levels were either low and then spiked just before we saw the first animals evolve, or oxygen levels were bred for several million years before the arrival of animals.
“But 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 the first forms of animal life emerged. We see periods where the ocean environment, where the first animals lived, would have had oxygen in abundance – and then times when it didn’t.
Dr Benjamin Mills, who heads the Earth Evolution Modeling Group in Leeds and oversaw the project, said: “This periodic change in environmental conditions would have produced evolutionary pressures where some life forms could have died out and new ones could have emerged.”
Dr Mills said oxygenated periods expanded what are known as “habitable spaces” – parts of the ocean where oxygen levels would have been high enough to support early forms of animal life.
He said: “It has been proposed in ecological theory that when you have habitable space that expands and contracts, it can support rapid changes in the diversity of biological life.
“When oxygen levels drop, there is strong environmental pressure on some organisms, which could lead to extinctions. And when oxygen-rich waters expand, the new space allows survivors to achieve ecological dominance.
“These expanded habitable spaces would have lasted for millions of years, giving ecosystems plenty of time to develop.”
Extreme variability in atmospheric oxygen levels at the end of the Precambrian, Science Advances
Astrobiology
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