The rock contains the mortal remains of microbes that could use sunlight to photosynthesise like modern plants. Evolutionists said the discovery means that the very first lifeforms must have originated nearly 4 billion years ago.
William Schopf, a palaeobiologist at the University of California, discovered the tiny fossils during a microcopic analysis of rock taken from Western Australia.
'This is the oldest assemblage of microscopic organisms known,' he said. Their relative complexity at such an old age shows that life must have evolved rather rapidly once it began, he said.
'It also shows that it was probably not too difficult for life to get going, and that maybe similar conditions exist on many other planets.'
Geologists believe the Earth formed about 4.5 billion years ago. But massive meteorites bombarded the planet for several hundred million years after that, vapourising oceans and preventing life from evolving.
Professor Schopf, who is director of the Centre for the Study of Evolution and the Origin of Life, in Los Angeles, says in today's issue of the journal Science that the fossils are 1,300 million years older than any comparable fossils.
One of the most important aspects of the find is that the fossils are clearly related to the group of photosynthetic organisms, called cyanobacteria, that produced the oxygen on which animal life depends.
The rock is from Pilbara Block in the northwestern part of Western Australia. The area is rich in fossils because the rock - in places up to 30 kilometres thick - was not 'pressure cooked' like many ancient formations elsewhere in the world.
Volcanic lava was used to pinpoint the age of the organisms to within a couple of million years.
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