BEIJING, March 27 (Xinhuanet) -- Scientists said oxygen deficiency and a lack of the heavy metal molybdenum in the oceans had braked the evolution of life on Earth for nearly two billion years, media reported Thursday.

    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.

    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. "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.

    (Agencies)