The atmosphere we breathe today is composed of about 20 percent oxygen, which is not just essential to many organisms -- it also provides protection from the sun's more dangerous rays. In the presence of sunlight, oxygen molecules in the atmosphere react to form ozone. Up in the stratosphere, the ozone layer absorbs harmful UV radiation coming from space -- protecting humans, animals and plants from the damage UV does. Three to four billion years ago, the atmosphere contained little oxygen and there was no ozone layer. "The earth's surface -- and areas of shallow water -- were subject to high levels of ultra-violet radiation," Andreas Kappler explains. "And yet, microbial life came to be. We wondered how that was possible."