Narrator:       This is Science Today. The "Great Oxidation Event" is an important milestone in Earth's history, when oxygen levels in the Earth's atmosphere rose sharply. One of the big questions about this event, which occurred about 2.4 billion years ago, has been establishing whether organisms that make oxygen evolved prior to this event.

Reinhard:       A lot of what this work is focused on is trying to nail that down a little bit more specifically, exactly how much oxygen was or wasn't around when this transition took place.

Narrator:       Chris Reinhard, a graduate student at the University of California, Riverside, was part of a team of geoscientists who led a national, multi-campus study that sought chemical clues about this event by looking at ancient rocks.

Reinhard:       What we're interested in is trying to figure out whether these organisms may have evolved prior to this rise in atmospheric oxygen and what we're starting to find is that they probably did and might have evolved some hundreds of millions of years before oxygen actually began to get going in the atmosphere. Then you have to question, well why did it take so long to begin to accumulate in the atmosphere, which is sort of the next frontier of this, I think.

Narrator:       For Science Today, I'm Larissa Branin.