BioEd Online

A sharp increase in the amount of oxygen in the air may have sparked the evolution of complex animal life. Chemical analysis of 580-million-year-old rock sediment shows oxygen levels in the deep ocean surged upwards just before large creatures appeared on the sea floor.

Researchers have long thought that the emergence of complex life forms strange creatures called Ediacarans was likely sparked by an increase in oxygen that such animals would need to live. But proof of this theory has been scant.

Two papers, published by Nature and Science this week, both conclude that oxygen levels at this time did indeed shoot upwards in the deep seas. The air at that time, it now seems, probably had about 15% of today's oxygen levels.

All around the world

Don Canfield, ecologist at the Nordic Centre for Earth Evolution in Odense, Denmark, and his colleagues examined rocks from the Avalon Peninsula in Newfoundland, which hold fossils of Ediacaran creatures.

The team looked at the different types of iron in the rock to unpick what was happening with oxygen at the time. Waters without oxygen tend to accumulate a relatively small proportion of reactive iron such as iron oxide, carbonate and sulfide minerals in the sediments. The results, published online in Science¹, indicate that the ocean was largely oxygen-free during the Gaskiers glaciation, but then oxygen appeared after the ice melted.

A blast of mineral-rich water from this melting ice could have fed plant life and boosted oxygen levels that filtered down to the deep seas, the researchers speculate.

In a different approach, David Fike, of the Massachusetts Institute of Technology in Cambridge, the United States, and colleagues looked at the carbon and sulphur content in rocks from Oman, which were formed 635-548 million years ago. Isotopic data helps to reveal how much sulphate was around at the time, which in turn relates to oxygen concentrations. They too see a dramatic increase after the Gaskiers glaciation, they report in Nature².

Breathe easy

Oxygen first appeared on the planet some 2.3 billion years ago, but in levels too low to support complex animal life.

After the leap some 580 million years ago, it still took some time for complex animals to gain a stronghold. It was around 35 million years after the end of the Gaskiers glaciation that the Cambrian explosion introduced a diversity of skeletal life-forms to the planet. It is hard to say, however, exactly when the very first complex animals really first appeared, as their fossils may not have been preserved.

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