Life existed in the oceans for hundreds of millions of years while the Earth's air was not fit to breathe, research suggested today.

Plant-like bacteria evolved at least 200 million years before oxygen began to build up in the atmosphere, a study has shown.

During this period in its history, known as the Archaean, the Earth was covered by a poisonous smog of methane, ammonia and other toxic gases.

Similar conditions exist today on Saturn's moon Titan. Life as we know it today could not have survived on the early Earth.

The new study involved an analysis of ancient preserved seabed rocks from South Africa dating back two to three billion years.

US scientists at Rutgers University in New Brunswick, New Jersey, found chemical evidence of nitrogen cycles that could not have taken place without the presence of free oxygen.

Nitrogen cycles relate to the way living things obtain and use nitrogen to produce complex organic molecules. Evidence of nitrogen cycles provides a 'fingerprint' of life.

The researchers, Dr Linda Godfrey and Dr Paul Falkowski, concluded that organisms which produced oxygen as a by-product of photosynthesis must have evolved by around 2.5 billion years ago.

Oxygen did not begin to enrich the atmosphere until at least 200 million years later.

The scientists wrote in the journal Nature Geoscience: 'Nitrogen is a relatively inert molecule and has an atmospheric lifetime of the order of around one billion years. In contrast, oxygen.. is highly reactive and must be produced continuously by oxygenic photosynthesis.

'It is unlikely that the gas was present above trace levels in the atmosphere of the Earth during the first two billion years of the planet's history, but when oxygenic photosynthesis first arose on the Earth is not known with certainty.'





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