Scientists have revealed what they believe to be the oldest fossil ever discovered - belonging to foul-smelling, single-celled microbes that lived 3.5 billion years ago, and is a likely distant ancestor of human beings.

The minuscule remains were found inside a lump of sandstone rock in Western Australia by scientists from OId Dominium University in the US and are thought to be 300 million years older than the previous oldest example of life on Earth.

Experts believe this basic microbe existed in a kind of cluster, with each cell hosting individual life but effectively living, working, and communicating as a single entity. The bundle has been compared to a small human society.

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Robert Hazen, from Washington’s Carnegie Institution of Science, said the fossil – which is about one-third of an inch thick - was once a purple and green “mat” that is likely to have smelt strongly of rotting eggs.

The mineralogist – who co-authored the research on the remains that was published in the Astrobiology journal last week – added that the stench would have been caused by the microbes’ basic, solar-powered respiration process.

Dr Hazen said the microbe may have had the ability to transfer sunlight into energy, but probably emitted “horribly smelly” sulphur as a waste product.

He added that, if humans were able to go back in time 3.5 billion years and visit a typical beach in Australia, they would probably find this “slimy mass of purple or brown fibres emitting this stench of sulphur compounds but living very happily”.

The research has been questioned by Nasa astrobiologist Abigail Allwood who said her own experience of finding billion-year-old fossils would suggest it is very difficult to prove the remains ever contained life.

That argument was countered by Dr Hazen however, who argued that dozens of scientific rules and criteria were applied before researchers concluded the fossils were once alive.