Explosive growth of life on Earth fueled by early greening...( TEMPE Ariz. Earth's 4.5-billion-yea...)

TEMPE, Ariz. Earth's 4.5-billion-year history is filled with several turning points when temperatures changed dramatically, asteroids bombarded the planet and life forms came and disappeared. But one of the biggest moments in Earth's lifetime is the Cambrian explosion of life, roughly 540 million years ago, when complex, multi-cellular life burst out all over the planet.

While scientists can pinpoint this pivotal period as leading to life as we know it today, it is not completely understood what caused the Cambrian explosion of life. Now, researchers led by Arizona State University geologist L. Paul Knauth believe they have found the trigger for the Cambrian explosion.

It was a massive greening of the planet by non-vascular plants, or primitive ground huggers, as Knauth calls them. This period, roughly 700 million years ago virtually set the table for the later explosion of life through the development of early soil that sequestered carbon, led to the build up of oxygen and allowed higher life forms to evolve.

Knauth and co-author Martin Kennedy, of the University of California, Riverside, report their findings in the July 8 advanced on-line version of Nature (www.nature.com). Their paper, "The Precambrian greening of Earth," presents an alternative view of published data on thousands of analyses of carbon isotopes found in limestone that formed in the Neoproterozoic period, the time interval just prior to the Cambrian explosion.

"An explosive and previously unrecognized greening of the Earth occurred toward the end of the Precambrian and was an important trigger for the Cambrian explosion of life," said Knauth, a professor in Arizona State's School of Earth and Space Exploration.

"During this period, Earth became extensively occupied by photosynthesizing organisms," he added. "The greening was a key element in transforming the Precambrian world which featured low o
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