BY MARK SHWARTZ
Complex life-forms -- from protozoa to people -- need oxygen to survive.
But the early atmosphere of Earth contained very little oxygen, so the first creatures to appear on our planet were probably simple bacteria that thrived on other chemicals abundant in the harsh, volcanic environment.
The question of when free oxygen began to accumulate in the atmosphere remains highly controversial among scientists.
Part of the answer may lie in ancient deposits of fool's gold that formed in the ocean, according to Adina Paytan, an assistant professor of geological and environmental sciences.
"Without molecular oxygen, life could not have evolved to its present-day level of complexity," writes Paytan in the April 28 issue of the journal Science.