Oxygen levels within the Earth’s atmosphere are more likely to have “fluctuated wildly” one billion years ago, creating conditions that would have accelerated the event of early animal life, in keeping with recent research. 

Scientists consider atmospheric oxygen developed in three stages, starting with what’s often known as the Great Oxidation Event around two billion years ago, when oxygen first appeared within the atmosphere. The third stage, around 400 million years ago, saw atmospheric oxygen rise to levels that exist today. 

What’s uncertain is what happened through the second stage, in a time often known as the Neoproterozoic Era, which began about one billion years ago and lasted for around 500 million years, during which period early types of animal life emerged. 

The query scientists have tried to reply is - was there anything extraordinary in regards to the changes to oxygen levels within the Neoproterozoic Era that could have played a pivotal role within the early evolution of animals – did oxygen levels suddenly rise or was there a gradual increase? 

Fossilized traces of early animals - often known as Ediacaran biota, multi-celled organisms that required oxygen - have been present in sedimentary rocks which are 541 to 635 million years old. 

To attempt to answer the query, a research team on the University of Leeds supported by the Universities of Lyon, Exeter and UCL, used measurements of the different types of carbon, or carbon isotopes, present in limestone rocks taken from shallow seas. Based on the isotope ratios of the different sorts of carbon found, the researchers were in a position to calculate photosynthesis levels that existed tens of millions of years ago and infer atmospheric oxygen levels. 

In consequence of the calculations, they’ve been in a position to produce a record of oxygen levels within the atmosphere over the past 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 modeler who accomplished his PhD within the School of Earth and Environment at Leeds and was the lead scientist on the project, said the findings give a recent perspective on the best way oxygen levels were changing on Earth. 

The early Earth, for the primary two billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then oxygen levels began to rise, which is often 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 primary animals evolve, or that oxygen levels were high for a lot of tens of millions of years before the animals got here along.

But our study shows oxygen levels were much more dynamic. There was an oscillation between high and low levels of oxygen for a very long time before early types of animal life emerged. We’re seeing periods where the ocean environment, where early animals lived, would have had abundant oxygen – after which periods where it doesn’t. 

Dr Alex Krause, Biogeochemical Modeler

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

Dr Mills said the oxygenated periods expanded what are often known as “habitable spaces” – parts of the ocean where oxygen levels would have been high enough to support early animal life forms. 

He said: “It has been proposed in ecological theory that when you’ve a habitable space that’s expanding and contracting, this will support rapid changes to the range of biological life. 

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

“These expanded habitable spaces would have lasted for tens of millions of years, giving loads of time for ecosystems to develop.”