Their results, which provide the best evidence to date of the age of the Calvin cycle - the photosynthetic cycle by which plants convert light energy and CO₂ into cellular tissue - will be published in the September 18 issue of the journal Nature.

The research was funded by the National Science Foundation (NSF), an independent federal agency that supports fundamental research and education across all fields of science and engineering, and by NASA.

"This research is solid indirect evidence of the very high level of atmospheric CO₂ in an ancient time period," says Enriqueta Barrera, program director in NSF's division of earth sciences.

Using samples taken from individual fossils of an ancient relative of algae, Kaufman and Xiao provide the first estimates of the concentration of CO₂ in the atmosphere some 1.4 billion years ago. Their study results show that the CO₂ concentration at that time was 10 to 200 times higher than today's levels. The gas therefore likely played a major role in keeping Earth warm, and probably dominated over another greenhouse gas, methane, after the atmosphere and oceans became oxygenated between 2 billion and 2.2 billion years ago.

"The sun was not as luminous then so it did not provide as much light and heat as it does now," said Kaufman. "Our new findings confirm models of how much greenhouse gas was required to keep Earth's temperature warm enough so the oceans didnít freeze during this time."

The Proterozoic period - the time period examined by Kaufman and Xiao--began 2.5 billion years ago and ended 543 million years ago. Scientists think many of the far-reaching events in the evolutionary history of our planet occurred during that period, including the appearance of abundant living organisms (probably early single- and multi-celled organisms) and significant oxygen in the atmosphere.

One of the ocean-dwelling organisms producing oxygen during the later Proterozoic period was Dictyosphaera delicata, a microscopic plant not much bigger than the dot in the letter i. To estimate ancient levels of atmospheric CO₂, Kaufman and Xiao measured ratios of two different forms, or isotopes, of carbon present in individual microfossils of this plant.

"It was a painstaking process to get individual organisms," Kaufman said. The scientists "were able to take a camel hair brush and, using one hair of the brush, pick up one of these microfossils, which had been removed from its substrate [rock] using hydrofluoric acid, which dissolves the inorganic minerals but not organic matter."

Numerous microscopic samples of fossilized cellular material were knocked out of each organism using high-energy beams of ions from an ion probe. The sample material was analyzed with a mass spectrometer to come up with the results reported.

Kaufman is known for his contributions to research indicating that Earth has been almost entirely covered in ice several times within the last billion years. Kaufman and other scientists believe that each of these "snowball earth" periods were ended by a warming of the Earth resulting from a buildup in the atmosphere of greenhouse gases, particularly carbon dioxide.