The loss of large amounts of hydrogen gas during the early stages of our atmosphere's evolution may account for why Earth's air is so rich in life-supporting oxygen today. So say scientists from the NASA Ames Research Center in a report appearing in the current issue of the journal Science.

"Without oxygen, the most sophisticated life on the earth would have been green microbial scum," study author David Catling notes. "Fortunately, some bacteria in the early oceans were able to separate water into hydrogen and oxygen." Indeed, this separation still takes place in photosynthetic organisms today. Using the energy of the sun, they break the water molecules apart and use the hydrogen to create organic compounds like carbohydrates, leaving oxygen behind as a by-product.

Conventional wisdom holds that large quantities of such hydrogen-laden organic matter were eventually buried in the earth, allowing oxygen to accumulate in the atmosphere. But Catling's team proposes instead that the hydrogen escaped into space through a process called methane photolysis, in which the hydrogen-containing methane reacts with oxygen such that hydrogen atoms are freed.

If correct, Catling's theory could explain why the early earth stayed warm enough for life to thrive. "Three billion years ago, the sun was only four fifths as bright as it is now," he observes. "The earth should have frozen over." Methane, however, is a very effective greenhouse gas, and, according to Catling, its concentration in the atmosphere back then was 100 to 1,000 times what it is today.