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Early Earth, home to some of our planet's first lifeforms, may have been a real-life "waterworld" without a continent in sight- a discovery that can scientists to better understand how and where single-cell organisms first emerged on Earth.
The researchers took advantage of a quirk of hydrothermal chemistry to suggest that the surface of Earth was likely covered by a global ocean 3.2 billion years ago.
It may even have looked a bit like the post-apocalyptic, and land-free, future imagined in Kevin Costner's infamous film 'Waterworld'.
"The history of life on Earth tracks available niches. If you've got a waterworld, a world covered by ocean, then dry niches are just not going to be available," said Boswell Wing, an associate professor in the Department of Geological Sciences at the University of Colorado Boulder.
Lead author Benjamin Johnson and Wing analysed a geologic site called the Panorama district located deep in Northwestern Australia's outback.
It's also the resting spot for a 3.2 billion-year-old chunk of ocean crust that's been turned on its side.
The team looked, in particular, for two different isotopes of oxygen trapped in stone: a slightly heavier atom called Oxygen-18 and a lighter one called Oxygen-16.
The duo discovered that the ratio of those two isotopes of oxygen may have been a bit off in seawater 3.2 billion years ago -- with just a smidge more Oxygen-18 atoms than you'd see today.
"Though these mass differences seem small, they are super sensitive," Wing said.
He explained that 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.
The team theorized that the most likely explanation for that excess Oxygen-18 in the ancient oceans was that there simply weren't any soil-rich continents around to suck the isotopes up.
That doesn't mean, however, that there weren't any spots of dry land around.
"There's nothing in what we've done that says you can't have teeny, micro-continents sticking out of the oceans," Wing said. "We just don't think that there were global-scale formation of continental soils like we have today."
The researchers saw it as one-of-a-kind opportunity to pick up clues about the chemistry of ocean water from billions of years ago.
"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," Johnson said. The study appeared in the journal Nature Geoscience.
The study also feeds into an ongoing debate over what ancient Earth may have looked like: Was the planet much hotter than it is today?
"There was seemingly no way forward on that debate. We thought that trying something different might be a good idea," said Johnson.
