Plants and Animals

The Impact of Fluctuating Oxygen Levels on Animal Evolution

The Impact of Fluctuating Oxygen Levels on Animal Evolution

According to the researchers, oxygen levels in the Earth’s atmosphere “fluctuated wildly” one billion years ago, creating conditions that could have accelerated the development of early animals.

Scientists believe that atmospheric oxygen evolved in three stages, beginning with what is known as the Great Oxidation Event, which occurred around two billion years ago and saw the first appearance of oxygen in the atmosphere. The third stage occurred around 400 million years ago, when atmospheric oxygen levels reached their current levels.

What is unknown is what happened during the second stage, known as the Neoproterozoic Era, which began approximately one billion years ago and lasted approximately 500 million years, during which time early forms of animal life appeared.

The question that scientists have attempted to answer is whether there was anything unusual about the changes in oxygen levels during the Neoproterozoic Era that may have played a pivotal role in the early evolution of animals – did oxygen levels rise suddenly or gradually? Early animal fossils, known as Ediacaran biota, multi-celled organisms that required oxygen, have been discovered in sedimentary rocks dating from 541 to 635 million years.

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. 

Dr. Alex Krause

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 the 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, the researchers were able to calculate photosynthesis levels that existed millions of years ago and infer atmospheric oxygen levels. 

As a result of the calculations, they have been 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 been diffusing into the ocean to support early marine life.

Dr Alex Krause, a biogeochemical modeller who completed his Ph.D. in the School of Earth and Environment at Leeds and was the lead scientist on the project, said the findings give a new perspective on the way oxygen levels were changing on Earth. 

How fluctuating oxygen levels may have accelerated animal evolution

He added: “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.”

“However, our research shows that oxygen levels were far more dynamic. For a long time before the emergence of animal life, there was an oscillation between high and low oxygen levels. We’re seeing periods when the ocean environment, where early animals lived, had plenty of oxygen, and then periods when it didn’t.”

“This periodic change in environmental conditions would have produced evolutionary pressures where some life forms may have become extinct and new ones could emerge,” said Dr Benjamin Mills, who leads the Earth Evolution Modelling Group at Leeds and supervised the project.

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 forms. 

“It has been proposed in ecological theory that when you have a habitable space that is expanding and contracting, this can support rapid changes in the diversity of biological life,” he said. When oxygen levels fall, some organisms face severe environmental pressure, which could lead to extinction. And as the oxygen-rich waters expand, the new space allows the survivors to rise to ecological dominance.”

These expanded habitable spaces would have lasted for millions of years, giving ecosystems plenty of time to develop.” The study, titled “Extreme variability in atmospheric oxygen levels in the late Precambrian,” was published in the journal Science Advances.