Scientists shed light on braked evolution on Earth 

PARIS (AFP) - An international team of scientists said on Wednesday the evolution of life on Earth had been braked for nearly two billion years because of oxygen deficiency and a lack of the heavy metal molybdenum in the oceans.

Until now, the mainstream theory has been that the oceans became oxygenated shortly after a spurt in atmospheric oxygen, around 2.4 billion years ago.

This should have been the big kickstart for life. If so, why did it take aeons for multi-cellular life to show up? It eventually happened around 600 million years ago, or in just the last tenth or so in the world's history.

In a paper published in the weekly British journal Nature, geochemists believe they have found the answer in ancient layers of black shale, a sedimentary rock rich in organic matter found in the sea.

They found low levels of molybdenum -- a micronutrient for bacterial life -- in a long period of Earth's past.

Molybdenum enables bugs to convert nitrogen from the atmosphere from a raw form into a type useful for living things, a process known as "nitrogen fixation."

Deprived of molybdenum, bacteria cannot fix nitrogen efficiently -- and this in turn affects multi-cellular, or animal, life which depends on bacteria for their own nitrogen intake.

"These molybdenum depletions may have retarded the development of complex life such as animals for almost two billion years of Earth history," said Timothy Lyons, a professor at University of California Riverside (UCR).

"The amount of molybdenum in the ocean probably played a major role in the development of life."

Molybdenum levels are also a handy indicator of oxygen levels in ocean chemistry. The deficiency of molybdenum over this long period also mirrors a deficiency in oxygen.

The new scenario is this: the burst of atmospheric oxygen 2.4 billion years ago provided a gasp of fresh air for life in the ocean -- but only for oxygen-gobbling photosynthesizing bacteria at its surface. The ocean depths remained relatively oxygen-free.

The molybdenum record shows that the second step occurred around 600 million years ago, when the entire ocean became oxygenated, which enabled the rise of multi-cellular life called eukaryotes -- the category that includes plants, humans and other complex creatures.

Still unclear, though, is how this complete oxygenation happened.

轉貼自：

http://news.yahoo.com/s/afp/scienceevolution;_ylt=AvXqQ2lSQ6CnSmq1fgeTRU8br7sF

(03/26/08)