‘The early Earth was a fundamentally weird place,’ according to Dr Ashleigh Hood. Around 3-3.5 billion years ago, the orange sky was still, the barren landscape had no plants in sight, and the green ocean lacked any creature that we would recognise today.
Earth is teeming with life. It appears to be an oasis in our Solar System as we are yet to find any evidence of life elsewhere. Yet it was once very different. Dr Ashleigh Hood, recipient of the 2022 Phillip Law Postdoctoral Award, studies the story of life on Earth as it is told in sediment and the geological record.
After Ashleigh mapped a massive, “weird”, ancient reef the size of the Great Barrier Reef as a geology student, she was hooked. It was unlike anything she had seen in the modern world, and it was the beginning of many adventures that would take her around the globe. From the Flinders Ranges, where she was stalked by emus, to the US, Namibia, and Canada, she has trekked the world to understand what it was like hundreds of millions of years ago.
Scientists have a good understanding of Earth’s more “recent” history – the last billion years – however it becomes more enigmatic the further back in time they go.
The methane atmosphere and the iron- or hydrogen sulphide-rich seas were quite inhospitable. They were all devoid of oxygen. But then cyanobacteria appeared. These bacteria photosynthesise – they can convert sunlight into energy, producing oxygen in the process. Oxygen (O2) began to accumulate in the atmosphere, initiating the Great Oxidation Event 2.4 billion years ago. Atmospheric oxygen levels rose to 10% of their present levels by the end of the Great Oxidation Event – a rise that was only transient before dipping down again.
The following period is sometimes dubbed as “the Boring Billion”. It has long been considered a period when little happened on Earth in terms of biological evolution and changes in climate, the oceans, or the atmosphere. The first eukaryotes (cells with an advanced cell structure) had already evolved but the pace of evolution seemed to have stalled.
As time went on, marine life developed. It flourished in large reefs around 715 million years ago, quite different to the reefs of coral that we are used to. Ochre-rich red seabeds were littered with stromatolites, microbial reefs created by cyanobacteria. The reefs grew upwards, stretching towards the sunlight, their source of energy. Creatures unknown to Ashleigh and other scientists lurked below – but whatever was in the depths, they did not need light or much oxygen.
It did not help that, as things started to get going, Earth experienced two massive glaciation periods. For over 50 million years – which is most of the period since the extinction of dinosaurs up till now – the oceans virtually froze over all the way from the poles to the equator.
How did any life survive when the planet was encased in ice? Ashleigh and her team examined iron-rich rocks that were deposited around 700 million years ago, as the iron chemistry tells a story about oxygen dynamics during that time. In the absence of oxygen, iron was dissolved in seawater, but if present, oxygen would react with iron to form rocks that fell to the seafloor.
As it turns out, the team discovered that seawater closest to the ice-covered shoreline was oxygen-rich – the first direct evidence for any oxygen-rich marine environment during Snowball Earth. This provides a possible explanation for how marine life of the time may have survived and later evolved. Perhaps little pockets of oxygen were enough.
Oxygen levels continued to (unsteadily) climb at the start of the Cambrian period, 538.8 million years ago. Early fluctuations of oxygen levels provided life with new opportunities: aerobic metabolism is much more efficient than anaerobic. As the evolution of animal species took off, plants also appeared on the scene. However, early plants were small and restricted to coastal swamps, thereby having little impact on the biosphere.
Ashleigh found evidence for a second large jump in oxygen. She traces oxygen levels using cerium as a proxy, known as the cerium anomaly. The concentration of cerium (Ce) is sensitive to the presence of oxygen and is either depleted or enriched in a rock relative to other rare-earth elements. In oxygenated waters, it is oxidised to form insoluble Ce that accumulates and is left behind. It was not until 380 million years ago, in the Devonian, that the cerium anomaly indicated a spike in oxygen levels. This coincides with the evolution of trees and root systems.
‘Trees are the architects of the modern world,’ says Ashleigh. At the end of the Devonian, forests were emerging and becoming more widespread, providing a great source of oxygen via photosynthesis. While some scientists believed that the evolution of animals was the driver of Earth becoming closer to the familiar world we know, Ashleigh’s work suggests that it may not have happened the way it did without the oxygen that plants provided. Changes in oxygen caused a change in the trajectory of animal life. The world today is starkly different to the Devonian or Cambrian – or before both – as the creatures that lived then were adapted to very little or no oxygen. As Ashleigh says, ‘there were lots of whacky things’ back then.
Not only does Ashleigh’s work reveal Earth’s history, it also highlights what we might look for in other worlds in the universe that may indicate the presence of life (or at least, conditions amenable to life). The conditions that we are used to supporting life here on Earth only were created in the last 400 million years. With over 4,000 confirmed exoplanets, just give it time. But don’t hold your breath – there is now thankfully plenty of oxygen to go around.
Watch Dr Ashleigh Hood‘s presentation to the Royal Society of Victoria on 1 December, 2022 titled “Reefs, Revolutions and Redox at the Dawn of Animal Life.”
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