In an experiment designed to show how life may have begun on Earth, scientists demonstrated that an electrical spark -- or lightning -- could produce amino acids, the building blocks of Earth' s life  However, unless the spark occurs in a reducing atmosphere, that is, one rich in hydrogen and lean in oxygen, amino acids do not form readily and tend to break apart when they do form
Scientists now believe that Earth' s atmosphere was actually rich in oxygen and lean in nitrogen at the time life began
Earth's atmosphere has changed significantly since fife first began.