Animal evolution may have been accelerated by Earth’s fluctuating oxygen levels

A research team at the University of Leeds have discovered that the process of animal evolution could have been greatly altered by ‘wild fluctuations’ in oxygen levels.

Around one billion years ago, oxygen levels in Earth’s atmosphere are likely to have greatly altered, which created conditions to accelerate early animal evolution.

The team at Leeds, supported by the Universities of Lyon, Exeter, and UCL, believe that atmospheric oxygen developed in three stages.

The three stages of changing oxygen levels

The team traced the first oxygen fluctuation to around two billion years ago, a period known as the Great Oxidation Event, where oxygen first appeared in Earth’s atmosphere. The third stage occurred around 400 million years ago, where levels rose to those that exist today.

However, scientists are uncertain what happened during the second stage, or the Neoproterozoic era. This rise occurred around one billion years ago, lasting for around 500m years. During this time, animal evolution began, which has left scientists questioning how this time played a pivotal role in early evolution.

The team tried to determine whether the Neoproterozoic era experienced sudden increases in oxygen levels, or if they gradually increased.

To do this, they used measurements of the different forms of carbon found in shallow sea limestone rocks. Fossilised traces of early animals have been found in sedimentary rocks that are 541-635m years old.

Based on the carbon isotope ratios of the different types of carbon found, they were able to calculate photosynthesis levels that existed millions of years ago, and infer atmospheric oxygen levels.

A new perspective

As a result of these calculations, the researchers were able to produce a record of oxygen levels in the atmosphere over the last 1.5bn years. These records can determine how much oxygen would have been diffusing into the ocean to support early animal evolution.

Dr Alex Krause, a biogeochemical modeller and lead scientist on the project, said the findings give a new perspective on the way oxygen levels were changing on Earth.

He said: “The early Earth, for the first two billion years of its existence, was anoxic and 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.

“But our study shows oxygen levels were far more dynamic. There was an oscillation between high and low levels of oxygen for a long time before early forms of animal evolution. We are seeing periods where the ocean environment, where early animals lived, would have had abundant oxygen and periods where it does not.”

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

Dr Mills explained that the oxygenated periods were known as ‘habitable spaces’. Oxygen levels in these parts of the ocean would have been high enough to support early animal evolution.

He said: “It has been proposed in ecological theory that when you have a habitable space that is expanding and contracting, this can support rapid changes to the diversity of biological life.

“When oxygen levels decline, there is severe environmental pressure on some organisms, which could drive extinctions. When 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 plenty of time for ecosystems to develop.”

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