A little oxygen is better than none.

A little oxygen is better than none

If the thin air atop a mountain peak makes you gasp for breath, consider how hard it would have been to breathe during the Archean era, the first 2 billion years of Earth's history. Scientists have long believed that the atmosphere back then held virtually no oxygen. But new calculations paint a considerably different picture of that distant period, suggesting that Archean air held a low but significant amount of oxygen.

Geologist Kenneth M. Towe of the Smithsonian Institution in Washington, D.C., estimates that oxygen made up 0.2 to 0.4 percent of the air at that time. While this concentration pales in comparison with the current level of nearly 21 percent, it is a trillion times greater than what many scientists thought was present in the Archean atmosphere. Towe discusses his work in the Nov. 1 NATURE.

Oxygen in the modern world comes from photosynthetic organisms, which produce the gas by breaking down water molecules. Such organisms existed during the Archean, but scientists believed that the oxygen produced by photosynthesis never had a chance to build up in the atmosphere. The prevailing theory holds that iron in the oceans absorbed the oxygen, keeping atmospheric levels of the gas at essentially zero. But Towe calculates that the iron in Archean rocks could not have absorbed all the oxygen produced by the photosynthetic organisms.

Noting that atmospheric oxygen concentrations did not rise to extremely high levels during the Archean, Towe reasons that some factor other than iron must have helped keep the levels down. He proposes aerobic respiration as that factor. In the modern world, organisms use the oxygen-absorbing aerobic process to metabolize food and gain energy. While most scientists believe aerobic organisms did not evolve until oxygen levels increased dramatically at least a billion years later, Towe suggests they emerged during the Archean, surviving on the small amounts of oxygen available at the time.