Key Milestone in Evolution of Life Pinpointed at 2.5 Billion Years Ago

COLLEGE PARK , Md. -- A team of geologists has found evidence that single-celled microscopic organisms completed their evolution by 2.5 billion years ago, more than a billion years after life appeared on Earth.Their geochemical evidence, discovered in ancient rocks, suggests the sudden appearance a modern style nitrogen cycle.

"Nitrogen, which is needed for the production of amino acids and proteins, is essential for life as we now know it, but was not so available early in Earth's history," said Jay Kaufman, a University of Maryland scientist and co-author of a paper published in the February 20 issue of Science. "Only single-celled organisms, and of only a few types, are able to use nitrogen gas from the atmosphere and, in so doing, convert it to ammonia," he said.

According to Kaufman, the new evidence he and fellow geologists from the University of Washington and the University of Arizona uncovered suggests that 2.5 billion years ago bacteria evolved -in response to a rise of oxygen in shallow ocean water -that were able to further convert nitrogen into more biologically useable forms. "Only a relatively few organisms can assimilate ammonia; others, like plants and animals, need the oxygenated form, nitrate," Kaufman said. "The rise of microorganisms making nitrogen more available in the form of nitrate, released more energy to biological systems, which likely kick started the evolution of higher life forms."

The bacteria responsible for the modern nitrogen to nitrate cycle sit on the outermost branches of the microbial tree of life, thus their discovery indicates that microbial evolution and diversification was complete billions of years before animals took the world stage.

A geological rock hammer used as scale against a sample of the organic-rich 2.5 billion year old Mt. McRae Shale with pyrite nodules. Photo by A. Jay Kaufman.

Kaufman, and his colleagues analyzed the geochemical composition of a half-mile-deep core from the Mt. McRae Shale, ancient rock formed from ocean silt laid down 2.5 billion years ago and now preserved as land in Western Australia. Using detailed techniques for measuring small quantities of nitrogen isotopes, they found the oldest evidence for a metabolism that involved the combination of ammonia and molecular oxygen to form nitrate.

This study corroborates recent findings by Kaufman and colleagues of the accumulation of oxygen in surface oceans around this time, some 200 million years before oxygen accumulated in significant quantities in Earth's atmosphere.

"In two earlier studies, we looked at sulfur isotopes and the abundances of oxygen-sensitive elements," said Kaufman. "These geochemical indicators coupled with our new evidence of an aerobic [oxygenated] nitrogen cycle all point toward the same rise of oxygen in the surface ocean around 2.5 billion years ago."

This rise in oxygen came from photosynthetic organisms that split water with light and gave off oxygen. When the surface ocean became oxygenated, new oxygen-using chemical processes arose, including the modern nitrogen cycle.

The lead author of this study was Jessica Garvin from the University of Washington , Seattle. Other authors include Roger Buick from the University of Washington, Ariel Anbar and Gail Arnold from the University of Arizona , and Alan J. Kaufman from the University of Maryland . The study was funded by NASA and the National Science Foundation.

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