A recent study by the University of Leeds revealed that oxygen levels in the Earth's atmosphere fluctuated a billion years ago, leading to accelerated evolution of early mammals.
According to the general theory of scientists, when oxygen first entered the atmosphere about two million years ago, it evolved into three stages, starting with the Great Oxidation Event two billion years ago, when oxygen first appeared in the atmosphere, and about 400 million years ago, in the third phase, atmospheric oxygen reached current levels.
It is not known what happened during the second stage, the neoproterozosian era, which began about a billion years ago and lasted 500 million years.
Scientists have been trying to figure out whether there was anything exceptional to the changes in oxygen levels during the neoproterozo era, which has led to the early evolution of animals. Researchers have measured the different isotopes of carbon in the limestone species collected from the shallow oceans. They have calculated what photosynthesis was millions of years ago, and have calculated the levels of oxygen in the atmosphere based on the ratio of isotopes to various forms of carbon. As a result, they have re-created the levels of oxygen in the atmosphere over the past 1.5 billion years.
So far, scientists have thought that after the Great Oxidation Event, the oxygen level was either low and then jumped just before the evolution of the first animals, or that the oxygen level was high for many millions of years before the animals appeared.
The new study showed that the levels of oxygen were much more dynamic; the fluctuations between high and low levels existed well before the early life forms of animals appeared; it turned out that in the ocean environment, where the early animals lived, there was enough oxygen, and then there were periods when it did not exist; this periodic change in the environment caused evolutionary pressure, in which some forms of life could be destroyed and new forms of life emerge.