Tuesday, February 26, 2008

When Life Took Its First Breath

Pulsed oxidation and biological evolution in the Ediacaran Doushantuo Formation. 2008. K. A. McFadden et al. PNAS 105: 3197-3202 (the article is not up yet as of this posting)

The photo (field of view about 0.15 millimeter in width) is of an exceptionally preserved eukaryotic fossil from the Doushantuo Fm (635–551 million years old) in South China. Photo: Shuhai Xiao
From the press release:
The rise of oxygen and the oxidation of deep oceans between 635 and 551 million years ago may have had an impact on the increase and spread of the earliest complex life, including animals.
The atmosphere had almost no oxygen until 2.5 billion years ago, and it was not until about 600 million years ago when the atmospheric oxygen level rose to a fraction of modern levels. For a long time, geologists and evolutionary biologists have speculated that the rise of the breathing gas and subsequent oxygenation of the deep oceans are intimately tied to the evolution of modern biological systems.

To test the interaction between biological evolution and environmental change, an international team examined changes in the geochemistry and fossil distribution of 635- to 551-million-year old sediments preserved in the Doushantuo Formation in the Yangtze Gorges area of South China.

The stratigraphic pattern of carbon isotope abundances suggested to these researchers that the ocean, which largely lacked oxygen before animals arrived on the scene, was aerated by two discrete pulses of oxygen.

“The first pulse apparently had little impact on a large organic carbon reservoir in the deep ocean, but did spark changes in microscopic life forms,” McFadden said. The second event, which occurred around 550 million years ago, however, resulted in the reduction of the organic carbon reservoir, indicating that the ocean became fully oxidizing just before the evolution and diversification of many of Earth’s earliest animals,” she said.

Following this second oxidation event, between 550 and 542 million years ago, there was a worldwide increase of Ediacara organisms, complex macroscopic life forms, an event recently dubbed as the Avalon Explosion. “This was when we see the first burrowing animals and the first animals to form external skeletons, or shells.

The triggers for the oxidation events remain elusive, however. “These events recorded in the ocean were probably related to oxygen in the atmosphere reacting with sediments on land,” McFadden said. “Weathering of rocks and soils on the continents would result in the release of certain dissolved ions, such as sulfate, into rivers. These would then be transported to the sea where they might be used by bacteria to oxidize the organic carbon pool in the deep oceans,” she said.