Scientists say fossil microbes found in rocks in Western Australia may be the oldest direct evidence of life on Earth, from 3.4 billion years ago.

Researchers said the microbes existed at a time of extreme conditions on Earth, with temperatures in the oceans between 100 to 120 degrees Fahrenheit and very little oxygen, the BBC reported Sunday.

In fact, the fossil microbes were processing sulphur for energy and growth, not oxygen, the scientists said, giving a hint of conditions on the ancient Earth.

"At last we have good solid evidence for life over 3.4 billion years ago. It confirms there were bacteria at this time, living without oxygen," Martin Brasier of Britain's Oxford University said.

Researchers said the first microbes on Earth evolved to suit the extreme conditions.

"On the early Earth, where free oxygen was rare or absent, evolving life had to employ other means to survive," David Wacey from the University of Western Australia said.

"This ability to essentially 'breathe' sulphur compounds has long been thought to be one of the earliest stages in the transition from a non-biological to biological world."

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