How fluctuating oxygen levels may have accelerated animal evolution

Oxygen levels in Earth’s atmosphere may have “fluctuated wildly” 1 billion years ago, creating conditions that could have accelerated the development of early animal life, new research finds.

Scientists believe that atmospheric oxygen evolved in three phases, beginning with the so-called Great Oxidation Event about 2 billion years ago, when oxygen first appeared in the atmosphere. In the third stage, around 400 million years ago, the oxygen in the air rose to where it is today.

What is uncertain is what happened during the second phase, a period known as the Neoproterozoic, which began about 1 billion years ago and lasted about 500 million years, during which early forms of animal life emerged.

The questions scientists have tried to answer are: Was there something extraordinary about the changes in oxygen levels in the Neoproterozoic that may have played a crucial role in early animal evolution? Did the oxygen level suddenly increase or was there a gradual increase?

Fossilized traces of early animals — known as Ediacaran biota, multicellular organisms that needed oxygen — have been found in sedimentary rocks that are 541 to 635 million years old.

To answer the questions, a University of Leeds research team supported by the Universities of Lyon, Exeter and UCL used measurements of the different forms of carbon, or carbon isotopes, found in limestone rocks from shallow seas. Using the isotope ratios of the different types of carbon found, the researchers were able to calculate the photosynthesis values ​​that existed millions of years ago and deduce the oxygen content of the air.

As a result of the calculations, they were able to create a record of atmospheric oxygen levels over the past 1.5 billion years, telling us how much oxygen would have diffused into the ocean to support early marine life.

dr Alex Krause, a biogeochemical modeler who received his Ph.D. at the School of Earth and Environment in Leeds and lead scientist on the project, said the findings offer a new perspective on the way oxygen levels are changing on Earth.

He added: “Early Earth was anoxic for the first 2 billion years of its existence, lacking atmospheric oxygen. Then oxygen levels began to rise in what is known as the Great Oxidation Event.

“Until now, scientists thought that after the Great Oxidation Event, oxygen levels were either low and then spiked just before we see the first animals evolve, or that oxygen levels were high for many millions of years before animals appeared.

“But our study shows that oxygen levels were much more dynamic. There was an oscillation between high and low oxygen levels for a long time before early forms of animal life appeared. We see periods when the marine environment in which early animals lived would have had ample oxygen — and then periods when it didn’t.

dr Benjamin Mills, who leads the Earth Evolution Modeling Group in Leeds and oversaw the project, said: “This periodic change in environmental conditions would have created evolutionary pressures where some life forms may have gone extinct and new ones could emerge.”

dr Mills said the oxygen-rich periods expanded so-called “habitable spaces” – parts of the ocean where oxygen levels would have been high enough to support early animal life.

He said: “In ecological theory, it has been proposed that an expanding and contracting habitat can support rapid changes in the diversity of biological life.

“When oxygen levels drop, some organisms are under severe environmental pressure that could lead to extinction. And as the oxygen-rich water expands, the new space allows survivors to ascend to ecological dominance.

“These extended habitable spaces would have lasted for millions of years, giving ecosystems plenty of time to evolve.”

The research is published in scientific advances.