SCIENTISTS HAVE a new understanding of the conditions that led to the appearance of the world’s first large animals almost 600 million years ago.
The rise of oxygen levels in the Earth’s atmosphere and oceans played a critical role.
Many scientists are concerned with how life evolved in response to the environment. Others, like Dr Simon Poulton, are interested in how life itself changed the environment for future life forms.
“I’m not a palaeontologist, I’m a geochemist – I’m the opposite, I collect rocks and destroy them,” he told a session of the British Science Festival yesterday.
The abrupt appearance of animals in the fossil record was an issue that perplexed Darwin and remains an outstanding scientific conundrum, explains Dr Poulton, reader in biogeochemistry at Newcastle University.
“It is now increasingly apparent that the timing of the evolution of large complex animals is intimately related to the preceding environmental conditions.”
Over the last several years Poulton has been rewriting the conventional understanding of how the levels of oxygen in the atmosphere and oceans have increased, thereby answering questions about “how the Earth got to its current habitable state, and how animals, in particular humans, gained such a strong foothold on the planet”.
Going back more than two billion years, the Earth’s atmosphere and oceans were completely devoid of oxygen, explains Poulton. “Oxygenation of the atmosphere appears to have occurred over two major steps, one centred around 2.3 billion years ago, with a second major rise more than a 1.5 billion years later.”
For decades conventional wisdom held that the rise in oxygen in the atmosphere led to the oxygenation of the deep ocean about 1.8 billion years ago, explains Poulton.
“The evidence for this comes from the disappearance of iron-enriched sediments, known as banded iron formations, 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 oxygenation of the ocean.”
However, Poulton’s work has shown that the oceans remained oxygen-free even after the first rise in atmospheric oxygen 2.3 billion years ago.
“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 one billion years and would have severely restricted biological evolution.
“We have demonstrated that it was only after a second rise in atmospheric oxygen about 580 million years ago that the deep ocean became oxygenated,” explains Poulton.
“This set the scene for the subsequent rapid evolution of the first large animals, which are found preserved in deep ocean sediments within five million years of deep ocean oxygenation.”
Poulton and his team are now attempting to “better understand the detailed dynamics and consequences of rising atmospheric oxygen.
For example, 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.
Jeremy O’Brien is based at the University of Bristol, and is on placement at The Irish Times as a British Science Association media fellow