This pillow basalt in Australia lined the seafloor roughly 3.2 billion years ago when Earth was a 'waterworld'

• Earth's first microbial life inhabited a world with hardly any dry land, study says
• Discovery is based on study of rocks in Northwestern Australia's outback
• Scientists compare the ancient Earth with the future in 1995 film Waterworld

Earth was once a 'waterworld' just like the one depicted by Hollywood in the Kevin Costner blockbuster, according to new research.

A global ocean covered the planet 3.2 billion years ago, when the first primitive microbes were emerging.

There would have been hardly any dry land – mirroring the way it looked in Waterworld, the 1995 post-apocalyptic sci-fi movie.

US scientists studied the ancient rocky Australian outback for traces of oxygen to form a picture of the ocean water from billions of years ago.  

The finding could shed light on the evolution of life on Earth and explain how single-cell organisms first emerged.

'Kevin Costner, eat your heart out – you may want to start planning the prequel,' said the research team.  

'Early Earth, home to some of our planet's first lifeforms, may have been a real-life "waterworld" – without a continent in sight. 

'It may even have looked a bit like the post-apocalyptic, and land-free, future imagined in Costner's infamous film Waterworld.'

In the film, humanity struggles to survive after the ice caps melt and inundate the planet with water.

But, unlike in the movie, there were no fish – only tiny aquatic organisms called cyanobacteria.

A vista of the Panorama district looking down from the top of the ancient ocean crust to its base

The discovery, reported in the journal Nature Geoscience, is based on an analysis of rocks from Northwestern Australia's outback.

They date back to a period known geologically as the 'Paleoarchean' – spanning a period 3,600 million to 3,200 million years ago – when life consisted of nothing more complex than bacteria.

'The history of life on Earth tracks available niches,' said co-author Professor Boswell Wing, of the University of Colorado Boulder. 

'If you've got a waterworld, a world covered by ocean, then dry niches are just not going to be available.'

It was identified from the chemical signatures of an ocean in a chunk of crust that's been turned on its side in the Panorama desert in the Aussie outback. 

It is possible to walk across what used to be the hard, outer shell of the planet in the space of a day.

Benjamin Johnson inspects an outcrop in the Panorama district by what was once an ancient hydrothermal vent

It will take you from the base to spots where water once bubbled up through the seafloor via hydrothermal vents.

The researchers described it as a 'once-in-a-lifetime opportunity' to pick up clues about the ocean water from billions of years ago. 

'Today, there are these really scrubby and rolling hills that are cut through by dry river beds,' said lead author Dr Benjamin Johnson, now at at Iowa State University. 

'It's a crazy place. There are no samples of really ancient ocean water lying around, but we do have rocks that interacted with that seawater and remembered that interaction.'

He likened it to looking at coffee grounds to gather information about the water that poured through it.

US post-apocalyptic action film Waterworld depicts a future in 2500 where every continent on Earth is now underwater, due to a rise sea levels. This projected future could resemble an early Earth

The researchers analysed data from more than 100 rock samples from across the dry terrain.

They were looking, in particular, for two different flavours, or 'isotopes', of oxygen trapped in stone – Oxygen-18 and the slightly less heavy Oxygen-16. 

The ratio may have been a bit off in seawater 3.2 billion years ago – with just a tiny bit more Oxygen-18 than you'd see today.

Professor Wing said these are 'super sensitive' to the presence of continents.

Today's land masses are covered by clay-rich soils that disproportionately take up heavier oxygen isotopes from the water – like mineral vacuums for Oxygen-18, he explained.

Nostoc, a genus of cyanobacteria, under microscopic view. Cyanobacteria would have inhabited the early 'waterworld' of Earth 3.2 billion years ago

There simply weren't any soil-rich continents around to suck the isotopes up, but there could have been tiny spots of land dotted about.

'There's nothing in what we've done that says you can't have teeny, micro-continents sticking out of the oceans,' Professor Wing said. 

'We just don't think there were global-scale formation of continental soils like we have today.'

The researchers are now planning to scour other, younger rock formations at sites from Arizona to South Africa to see if they can find when the land masses we know today first roared onto the scene.

'Trying to fill that gap is really important,' said Dr Johnson said.