A recent study by researchers at the University of Leeds found that oxygen levels in
According to the general theory of scientists, when oxygenfirst entered the atmosphere about two million years ago, it evolved in three stages, starting with the Great Oxidation Event two billion years ago, when oxygen first appeared in the atmosphere. About 400 million years ago, during the third stage, atmospheric oxygen reached its current level.
It is unknown what happened during the secondstage, the Neoproterozoic era, which began about a billion years ago and lasted 500 million years. During this era, the earliest species of animals arose.
Scientists tried to find out whether there was somethingexceptional in the changes in oxygen levels during the Neoproterozoic era, which influenced the early evolution of animals. The researchers measured different carbon isotopes in limestone rocks collected from shallow oceans. They calculated what photosynthesis was like millions of years ago and calculated oxygen levels in the atmosphere based on the isotope ratios of different forms of carbon. As a result, they reconstructed oxygen levels in the atmosphere over the last 1.5 billion years. This is how scientists realized how much oxygen diffused into the ocean to support early marine life.
Until now, scientists thought that after the Greatoxidation events, either oxygen levels were low and then jumped up just before the evolution of the first animals, or that oxygen levels were high for many millions of years before animals appeared.
A new study finds that oxygen levelswere much more dynamic. Oscillations between high and low levels existed long before early animal life appeared. It turned out that in the ocean environment where early animals lived, there was enough oxygen, and then periods when there was no oxygen. This periodic change in environmental conditions created evolutionary pressures in which some life forms could become extinct and new ones emerge.
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