Animal evolution was accelerated by fluctuating oxygen levels

New research from the University of Leeds has revealed that oxygen levels in the Earth’s atmosphere “fluctuated wildly” one billion years ago, which could have accelerated the development of early animal life.

Atmospheric oxygen is believed to be developed in three stages, starting with the Great Oxidation Event around two billion years ago, when oxygen first appeared in the atmosphere. During the third stage, around 400 million years ago, atmospheric oxygen rose to levels that exist today.  

The second stage remains uncertain. This stage occurred during the Neoproterozoic Era, which started about one billion years ago and lasted for around 500 million years. This was when early forms of animal life emerged.   

Scientists have been trying to identify what led to the development of oxygen levels during this time that played a pivotal role in early evolution.  Did oxygen levels suddenly rise or was there a gradual increase?  

Early animals known as Ediacaran biota were multi-celled organisms that required oxygen. These organisms  have been found fossilized in sedimentary rocks that are 541 to 635 million years old.  

To investigate, the research team used measurements of carbon, or carbon isotopes, found in limestone rocks taken from shallow seas. Based on the different types of carbon found, the researchers calculated photosynthesis levels that existed millions of years ago to infer atmospheric oxygen levels.  

The experts used the data to produce a record of oxygen levels in the atmosphere over the last 1.5 billion years. This historical detail tells us how much oxygen would have been diffusing into the ocean to support early marine life. 

Dr. Alex Krause, lead scientist on the project, said the findings give a new perspective on how oxygen levels were changing on Earth. “The early Earth, for the first two billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then oxygen levels started to rise, which is known as the Great Oxidation Event.”  

“Up until now, scientists had thought that after the Great Oxidation Event, oxygen levels were either low and then shot up just before we see the first animals evolve, or that oxygen levels were high for many millions of years before the animals came along,” said Dr. Krause.

“But our study shows oxygen levels were far more dynamic. There was an oscillation between high and low levels of oxygen for a long time before early forms of animal life emerged. We are seeing periods where the ocean environment, where early animals lived, would have had abundant oxygen – and then periods where it does not.”  

This periodic change in environmental conditions would have produced evolutionary pressures where some life forms may have become extinct and new ones could emerge, explained Dr. Benjamin Mills, who supervised the project. He noted that the oxygenated periods expanded “habitable spaces” – parts of the ocean where oxygen levels would have been high enough to support early animal life forms.  

“When oxygen levels decline, there is severe environmental pressure on some organisms which could drive extinctions. And when the oxygen-rich waters expand, the new space allows the survivors to rise to ecological dominance.”

These habitable spaces lasted for millions of years, which provided enough time for early ecosystems to develop. 

The study is published in the journal Science Advances.

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By Katherine Bucko, Earth.com Staff Writer