How fluctuating oxygen levels may have accelerated animal evolution

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

Scientists believe that oxygen in the air evolved in three stages, starting with what is known as the Great Oxidation Event about 2 billion years ago, when oxygen first appeared in the atmosphere. The third stage, about 400 million years ago, saw oxygen in the air rise to levels that exist today.

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

The questions scientists have sought to answer are: Was there anything out of the ordinary about the changes in oxygen levels in the Neoproterozoic that may have played a critical role in early animal evolution? Do oxygen levels rise suddenly 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 dating back 541 to 635 million years.

To answer the questions, a research team from the University of Leeds, supported by the universities of Lyon, Exeter and UCL, used measurements of the various forms of carbon, or carbon isotopes, found in limestone rocks 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 infer the oxygen levels in the air.

As a result of the calculations, they have been able to produce a record of oxygen levels in the atmosphere over the past 1.5 billion years, which tells us how much oxygen would have diffused into the ocean to support early life in the sea.

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

He added: “The early Earth, for the first 2 billion years of its existence, was anoxic, devoid of atmospheric oxygen. Then oxygen levels started to rise, known as the Great Oxidation Event.

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

“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 emerged. We see periods when the ocean environment, where early animals lived, would have had a lot of oxygen — and than periods when this is not the case.

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 become extinct and new ones could emerge.”

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

He said: “It has been proposed in ecological theory that when you have a habitable space that expands and contracts, it can support rapid changes in the diversity of biological life.

“When the oxygen levels drop, there is a serious environmental pressure on some organisms that could cause extinction. And when the oxygen-rich water expands, the new space allows the survivors to gain ecological dominance.

“These extensive habitable spaces would have lasted millions of years, giving ecosystems plenty of time to develop.”

The research was published in Scientific progress.