'Oxygen fluctuations stalled life on earth'

Know why life on earth stalled for more than a billion years after making the first appearance 2.3 billion years ago? It was because of massive oxygen fluctuations that brought a billion-year hiatus called "boring billion" in evolution.

A study suggests oxygen levels dropped soon after the first peak 2.3 billion years ago, which saw concentrations of the gas jump by at least 1,000-fold.

Earth's early atmosphere has seen two major spikes in oxygen concentration - one roughly 2.3 billion years ago, dubbed the "great oxygenation event", and a second 800 million years ago.

The evolution of complex life only took off after this second peak.

"Oxygen levels were, in fact, very dynamic, going up and down until they passed a threshold and pushed the planet to a different state," said Noah Planavsky, a biogeochemist at Yale University in New Haven, Connecticut.

Planavsky and his team harnessed an approach based on tracing the movement of chromium isotopes from land to ocean, which is highly sensitive to atmospheric oxygen levels.

By studying ancient ironstones, researchers found that oxygen levels were very low at various stages of the "boring billion" - below 0.1 percent of present atmospheric levels.

The earliest sign of significant chromium oxidation - suggesting high atmospheric oxygen levels - occurred around 800 million years ago.

"It suggests that atmospheric oxygen levels were a key environmental constraint on animal evolution," Malcolm Walter, a geobiologist at University of New South Wales in Sydney told the scientific journal Nature.

Planavsky presented the work at the third international conference on geobiology in Wuhan, China, recently.