Research Stories

by Diane Boudreau

The Great Oxidation Event (GOE) marked a dramatic rise in O₂ in our planet's atmosphere around 2.45 billion years ago. But life on Earth–primarily bacteria–remained virtually unchanged for another billion years after that.

To understand the magnitude of that delay, think about all the changes that have happened in only half that time, over the last 500 million years. Fish, insects and amphibians all came into being. Rooted plants began covering the landscape. Dinosaurs ruled the Earth and then went extinct. Mammals and birds came onto the scene. Primates evolved and, eventually, human beings.

So why did it take so long for complex life to get revved up? Molybdenum may be an important part of the answer.

In early 2008, a group of researchers from around the world announced that a deficiency in O₂ and molybdenum in ancient oceans might have delayed the evolution of animal life on Earth. ASU biogeochemist Ariel Anbar was one of the authors of the study, which was led by Clint Scott of the University of California at Riverside. The research appeared in the March 27 issue of the journal Nature.

Although O₂ levels in Earth's atmosphere began rising around 2.45 billion years ago, they did not reach modern, "breathable" levels until about 500 million years ago. The Nature study shows that molybdenum in the ocean also did not reach its modern levels until more than a billion years after the GOE.

The researchers do not know why this happened. Still, it does hold important implications for the evolution of life. What does a heavy metal in the ocean have to do with living things? A whole lot, it turns out.

The biosphere needs molybdenum to convert nitrogen from a gas into a form useful for living things. This process is called "nitrogen fixation." Some kinds of bacteria can do it well. However, more complicated life forms–eukaryotes–can't fix nitrogen on their own.

"If molybdenum was scarce, bacteria would have had the upper hand," says Anbar. "Eukaryotes depend on bacteria having an easy enough time fixing nitrogen that there's enough to go around. So if bacteria were struggling to get enough molybdenum, there probably wouldn't have been enough fixed nitrogen for eukaryotes to flourish."

The finding helps explain an otherwise puzzling scarcity of eukaryote fossils before about a billion years ago.

This research was supported by the National Science Foundation and NASA. For more information, contact Ariel Anbar, Ph.D., School of Earth and Space Exploration, College of Liberal Arts and Sciences, 480.965.0767. Send email to anbar@asu.edu