Oxygen levels in Earth’s atmosphere were changing at a wild rate a billion years ago. Fluctuating oxygen levels may have accelerated the development of early animal life, according to new research.
Scientists believe that atmospheric oxygen developed in three stages, starting with the so-called Great Oxidation about two billion years ago, when oxygen first appeared in the atmosphere. The third stage, about 400 million years ago, resulted in the increase of atmospheric oxygen to the levels that exist today.
What is uncertain is what happened during the second stage, a time known as the Neoproterozoic Era, which began about a billion years ago and lasted about 500 million years, when in which early forms of animal life arose.
How were fluctuating oxygen levels discovered?
The question scientists have been trying to answer is: was there something extraordinary about the changes in oxygen levels in the Neoproterozoic era that could have played a key role in the early evolution of animals, did oxygen levels rise suddenly, or was there a gradual increase?
Fossilized traces of early animals, known as the Ediacaran biota, multicellular organisms that required oxygen, have been found in sedimentary rocks that are 541 to 635 million years old.
To try to answer the question, a research team from the University of Leeds, England, with support from the Universities of Lyon (France), Exeter (England) and UCL (England), used measurements of different forms of carbon, or isotopes of carbon, found in limestone rocks taken from shallow seas.
Based on the isotopic ratios of the different types of carbon found, the researchers were able to calculate the levels of photosynthesis that existed millions of years ago and deduce the levels of atmospheric oxygen, he writes EurekAlert.
How much oxygen was there in the early Earth’s atmosphere?
As a result of the calculations, they were able to produce a record of atmospheric oxygen levels over the past 1.5 billion years, which tells us how much oxygen would have reached the ocean to support early marine life.
Dr Alex Krause, a biogeochemical modeller who completed his PhD at Leeds School of Earth and Environment, said the findings provide new insight into fluctuating oxygen levels.
“Early Earth, for the first two billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then the oxygen level started to rise, which is known as the Great Oxidation Event,” explained the researcher.
“Until now, scientists believed that after the Great Oxidation Event oxygen levels were either low and then increased just before the first animals appeared, or that oxygen levels were high for many millions of years before they appeared. Dr. Krause continued.
“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 the appearance of early forms of animal life. We see periods when the oceanic environment where early animals lived had abundant oxygen and then periods when it didn’t,” he says.
What effects have fluctuating oxygen levels had on biodiversity?
“This periodic change in environmental conditions would have produced evolutionary pressures where some life forms could have died out and new ones could have emerged,” said Dr Benjamin Mills, who heads the Earth Evolution Modeling Group at Leeds .
Dr Mills said periods of oxygenation expanded what we know as “habitable spaces”, parts of the ocean where oxygen levels would have been high enough to support early forms of animal life.
“It has been proposed in ecological theory that when you have habitable space that expands and contracts, this can support rapid changes in the diversity of biological life,” Dr Mills said.
“When oxygen levels drop, there is severe environmental pressure on some organisms, which could lead to extinctions. And when the oxygen-rich waters expand, the new space allows the survivors to rise to ecological dominance,” he says.
“These extended habitable spaces would have lasted millions of years, giving ecosystems plenty of time to develop,” the researcher concluded.
The study was published in Science Advances.