Scientists at the Scottish Universities Environmental Research Center in Glasgow say the "tipping point" for complex life, when there was enough oxygen in the atmosphere to support its evolution, happened about 1.2 billion years ago rather than the 800 million years suggested by previous studies, Britain's The Independent reported.

An analysis of the chemical signatures of bacteria found in rocks near Lochinver in the northwest Highlands of Scotland showed that oxygen levels must have increased 1.2 billion years ago.

"Investigations revealed that these bacteria, which on a basic level use sulphur to obtain energy, were also using oxygen in a much more complex and efficient chemical reaction in order to generate their energy and survive," Professor John Parnell of Aberdeen University, who led the research team, said.

Fossil evidence of life on Earth goes back 3.8 billion years, but for about 3 billion years of that time living organisms consisted of simple microbes.

It was only relatively recently -- less than 1 billion years ago -- that complex life began to evolve.

"Our geochemical analyses have provided a clear signal that levels of oxygen in the atmosphere had increased to levels critical to the evolution of complex life -- from which we ourselves emerge -- much earlier than has been previously proven to date," Adrian Boyce of the research center said.

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