Geoscientist Online

Subsequent investigation has pushed the evolution of animals further back in time. The earliest animal fossils, small shells from the Doushantuo Formation in China, are found in rocks that are 632 million years old. However, organic chemicals (biomarkers) thought to have been derived from ancient organisms occur in oils present in sandstones from Oman, suggesting that animals may have evolved even earlier, around 713 million years ago. “Molecular clock” estimates, which are derived by comparing the genomes of different living organisms, also suggest that the evolution of animals took place earlier in Earth's history - perhaps as far back as 800 million years ago. These earliest animal ancestors were very primitive, likely similar to sponges and, crucially, were microscopic in size.

Poulton told the British Science Association: “Although we have pushed back the known record of animals since Darwin's day, the principal questions as to why animals appeared so late in Earth's history, and in particular, why there was such an abrupt increase in animal size, have continued to perplex scientists”.

“It is now increasingly apparent that the timing of the evolution of large complex animals is intimately related to the preceding environmental conditions. In this context, the history of atmospheric oxygenation and how this impacted upon ocean chemistry (oxygenation of the ocean is ultimately linked to oxygenation of the atmosphere), is thought to have played a central role. Over the last decade, armed with new techniques and some compelling new lines of reasoning, this history has been rewritten, for the first time providing a framework which is consistent with previously disparate lines of evidence” said Poulton.

On the early Earth, the atmosphere and oceans were devoid of oxygen. It was only after the evolution of the first oxygen-producing microbes (cyanobacteria) that oxygen was able to begin to accumulate in the atmosphere. Oxygenation of the atmosphere appears to have occurred over two major steps, one around 2.3 billion years ago, with a second major rise more than 1.5 billion years later. The first is thought to have resulted in atmospheric oxygen levels that were around 1-5% of modern levels. Crucially, large animals have an oxygen threshold (below which they cannot function) of about 10% of modern levels.

For decades it has been assumed, says Poulton, that this initial rise in atmospheric oxygen ultimately led to oxygenation of the deep ocean about 1.8 billion years ago. The purported evidence for this came from the sudden disappearance of iron-enriched sediments, known as banded iron formations (BIFs), from the rock record. It is well-established that these rocks form as a result of oxygen-free conditions in the deep ocean, and so their disappearance was taken as evidence for ocean oxygenation. So why the delay before the emergence of complex life?

“Our work has set a new timeline for how the ocean responded to rising atmospheric oxygen early in Earth's history. The new model for the evolution of ocean chemistry that we have proposed and tested via the rock record, appears for the first time to be consistent with geological observations. Thus our model explains why atmospheric oxygen did not continue to rise steadily after the first major rise in oxygen, and can explain why biological evolution apparently proceeded rather slowly, until a second major rise in atmospheric oxygen led to oxygenation of the deep ocean, thus stimulating the evolution of large animals” says Poulton.

It appears, Poulton says, that models scientists have used for decades have been wrong about the impact of rising atmospheric oxygen on the oxygenation history of the oceans. “We have demonstrated that the oceans remained anoxic after the first rise of atmospheric oxygen 2.3 billion years ago, and that instead the oceans became rich in toxic hydrogen sulphide, similar to the modern day Black Sea. These conditions are envisaged to have lasted for about a billion years and severely restricted biological evolution. We have then demonstrated that it was only after a second rise in atmospheric oxygen, about 580 million years ago, that the deep ocean became oxygenated. This set the scene for the subsequent rapid evolution of the first large animals, the Ediacara Biota, which are found preserved in deep ocean sediments within 5 million years of deep ocean oxygenation.”

Although Poulton believes that is new biogeochemical model provides a broad understanding of the timing of rises in atmospheric oxygen and its implications for ocean chemistry and evolution, many details remain unclear. “The next step is to identify and geochemically examine key rock sections at high resolution in order to understand the detailed dynamics and consequences of rising atmospheric oxygen” he says. “ Both major rises in oxygen are associated with the most severe swings in climate to have affected our planet, from potential complete coverage by thick ice (the Snowball Earth) through to intense greenhouse conditions.”

So, were glaciations a consequence of rising atmospheric oxygen, or did they also stimulate oxygen production? “This will be a major line of research in the immediate future” said Poulton.