Multi-celled animals may have evolved thanks to “wildly fluctuating” oxygen levels a billion years ago

Oxygen levels in Earth’s atmosphere “fluctuated wildly” around one billion years ago, which might have accelerated the flourishing of early animal life.

How Earth went from a barren rock hurtling through space to the only place in the universe (that we know for sure) teeming with life is still something of a mystery. Our best theories suggest that the primordial soup of compounds that formed in our planet’s young oceans bore the chemical ingredients necessary for life to evolve about 3.7 billion years ago.

For billions of years, the only organisms on Earth were simple, single-celled microbes. Suddenly, 635 million years ago, complex organisms emerged in what has become known as the Ediacaran geological period of Earth’s history.

Soon after in geological time – about 543 million years ago in the ‘Cambrian explosion’ – came the first appearance of most of the major animal body plans.

What conditions catalysed this huge upswing in life’s complexity and abundance?

Well, new research suggests oxygen seems to be a big part of it. Whether underwater, in the air or on land, almost all animals require oxygen to breathe (the only exception we know of is a strange worm discovered in 2020).

But our planet’s atmosphere didn’t always have oxygen. Scientists believe that the oxygenation of our air happened in three distinct stages.

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Read more: First complex ecosystems formed in Ediacaran – millions of years earlier than thought

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First, about 2.7 billion years ago, with the emergence of cyanobacteria – the first organisms to generate energy by photosynthesis, of which oxygen is a by-product.

This precipitated the Great Oxidation Event 2.4–2.2 billion years ago. This is the first time oxygen appeared in our atmosphere.

About 470 million years ago the first land plants occurred, bringing, 400 million years ago, the Paleozoic Oxidation event, which saw oxygen rise to the stable level we enjoy today.

You will have noticed I skipped over the second stage. That’s because little is known about this period of atmospheric oxygenation, which occurred in a time known as the Neoproterozoic Era beginning one billion years ago.

The changes in oxygen levels during the Neoproterozoic may have played a pivotal role in the early evolution of animals. But was the rise in oxygen sudden or gradual?

This is the question that scientists have sought to answer in new research published in Science Advances.

The team of scientists from British and French universities measured the different ratios of carbon isotopes found in limestone rocks taken from shallow seas. From this, they were able to estimate how much photosynthesis was taking place hundreds of millions of years ago, and to infer atmospheric oxygen levels.

They came up with a record of oxygen levels over the last 1.5 billion years using computer simulations based on the levels of carbon isotopes they found.

“The early Earth, for the first two billion years of its existence, was anoxic, devoid of atmospheric oxygen,” says first author Dr Alex Krause, a biogeochemical modeller at the University of Leeds. “Then oxygen levels started to rise, which is known as the Great Oxidation Event.”

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Krause says 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 life emerged. We are seeing periods where the ocean environment, where early animals lived, would have had abundant oxygen – and then periods where it does not.”

“This periodic change in environmental conditions would have produced evolutionary pressures where some life forms may have become extinct and new ones could emerge,” says project supervisor and second author, Dr Benjamin Mills, also at Leeds.

Mills says the oxygenated periods expanded “habitable spaces” in the early oceans where oxygen levels would have been high enough to support early animals. 

“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,” Mills says. “When oxygen levels decline, there is severe environmental pressure on some organisms, which could drive extinctions. And 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.”