Look at any stagnant pond and you'll see a reflection of early Earth. Green scum. It's the way things were back before the planet had enough oxygen to allow life to evolve a little, to breathe, to get some legs under it and trot around.
Your scummy ancestors had the planet to themselves up to about 2.2 billion years ago. These single-celled organisms were the only things that could survive on a diet nearly bereft of oxygen.
But then things changed.
Something, someone, or some process suddenly pumped Earth's atmosphere full of oxygen. Life was good, and it flourished. Fossil records show that the first complicated, multi-celled organisms appeared 2.1 billion years ago.
Researchers have puzzled for decades over what caused the breath of fresh air.
A pair of new studies finds evidence that it was the scum of the Earth itself that made higher life forms possible.
"Bacteria in the early oceans were able to separate water into hydrogen and oxygen," said David Catling, a NASA Ames Research Center scientist who led one of the studies. The primitive microbes also produced methane gas, in which hydrogen atoms became trapped.
The methane, with its trapped hydrogen, was lost to space. Excess oxygen, left behind, built up in Earth's crust and then, for some unexplained reason, flooded the atmosphere.
The scenario is a theoretical speculation based on studies of modern-day microbes that scientists think behave similarly to their primitive counterparts. It is an effort to explain why oxygen suddenly flooded Earth's atmosphere, as other evidence suggests, even though microbes that would have produced oxygen were around for a half-billion years before the sudden change in the air.
Oxygen is a byproduct of photosynthesis: Plants and microbes use sunlight to make organic matter from carbon dioxide, releasing oxygen in the process.
But if oxygen mixes with organic matter or hydrogen, it reacts chemically and is converted to other substances. Scientists have previously thought that dead organic material, buried deep in the Earth, might have stayed hidden from oxygen in the atmosphere, allowing it to build up.
But in the new studies, the researchers took some modern scum -- interwoven "mats" of microbes which, after 2 billion years haven't evolved much -- and simulated the conditions of Earth's early atmosphere. The microbes were found to release large amounts of hydrogen during the night.
While some of the hydrogen might have escaped into space, the remainder could have provided an important food source for other microbes, such as those that produce methane.
"We found that the elevated levels of hydrogen within the mats favor the biological production and release of methane," said Tori Hoehler, who led the second study.
The research is discussed in two scientific papers, one published in the Aug. 3 issue of the journal Science, the other in the July 19 issue of Nature.
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