It's life, but not as we know it.

The oldest fossil ever discovered on Earth shows that organisms were thriving 4.2 billion years ago, hundreds of millions of years earlier than previously thought.

The microscopic bacteria, which were smaller than the width of a human hair, were found in rock formations in Quebec, Canada, but would have lived in hot vents in the 60C oceans which covered the early planet.

The discovery is the strongest evidence yet that similar organisms could also have evolved on Mars, which at the time still had oceans and an atmosphere, and was being bombarded by comets which probably brought the building blocks of life to Earth.

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The team who made the finding at University College London believe that looking for similar fossils on the Red Planet is the best chance of finding evidence of alien life.

"Early Mars and early Earth are very similar places, so we may expect to find life on both planets at this time," said doctoral student Matthew Dodd, the lead author of the study which was co-funded by Nasa.

"We know that life managed to get a foothold and evolve rapidly on Earth. So if we have life evolving in hydrothermal vent systems maybe even 4.2 billion years ago when both planets had liquid water on their surface, then we would expect both planets to develop early life.

"If we do future sample returns from Mars and look at similarly old rocks and we don't find evidence of life then this certainly may point to the fact that Earth might have been a very special exception, and life may just have arisen on Earth."

Prior to this discovery, the oldest microfossils reported were found in Western Australia and dated at 3.4 billion years ago, leading scientists to speculate that life probably started around 3.7 billion years.

But the new finding suggests life could have formed as early as 4.5 billion years, just one hundred million years after Earth formed.

The tiny lifeforms were discovered in the Nuvvuagittuq Supracrustal Belt in Quebec, which contains some of the oldest sedimentary rocks in the world, dating back to 4.3 billion years ago, when the area was an iron-rich ocean.

The organisms would have resembled small tubes, with a ball-like base which stuck to the ocean rocks, and a stalk suspended in the water to collect iron, on which they fed.

They are similar to iron-oxidising bacteria found near other hydrothermal vents today.

"We found the filaments and tubes inside centimetre-sized structures called concretions or nodules," said Dr Dominic Papineau (UCL Earth Sciences and the London Centre for Nanotechnology).

"The fact we unearthed them from one of the oldest known rock formations, suggests we've found direct evidence of one of Earth's oldest life forms.

"This discovery helps us piece together the history of our planet and the remarkable life on it, and will help to identify traces of life elsewhere in the universe." If similar life were found earlier on Mars, it could even indicate that life may have had a Martian origin, a concept known as panspermia.

This microscope image made available by Matthew Dodd shows a filament attached to a clump of iron at lower right in rock found in Quebec, Canada. Photo / AP
This microscope image made available by Matthew Dodd shows a filament attached to a clump of iron at lower right in rock found in Quebec, Canada. Photo / AP

Astronomer Royal Sir Martin Rees said it was possible that life had evolved on both planets at the same time.

"It's indeed possible that life started on Mars as well as the Earth, but then fizzled out - maybe leaving some traces that we will discover from future probes," he said.

"It's unlikely, I think, that we are 'Martians' in the sense that life started only on Mars and then moved here via a meteorite - as some people have claimed in the past."

Space expert Dr Dan Brown of Nottingham Trent University added: "The discovery is exciting since it demonstrates how quickly life can form if the conditions are right on a planet or moon.

"This makes it clear to me that as soon as we find conditions on an exoplanet that would favour life as we know it, the probability of finding some form of life on that planet is very high. However, we are not talking about little green aliens but about microorganisms.

"The environment in which these ancient microorganisms thrived is quite telling. These hot environments are similar to hydro thermal vents also thought to exist beneath the thick ice of Jupiter's moon Europa. Again, pointing us to another place to hunt for life.

"It also becomes ever more likely, that such life must surely have had existed on early Mars. If life then spread to Earth cannot be argued with these findings which state more about how quickly life can form on a planets, just millions of years after its formation."

The research was published in the journal Nature.