Early Earth 'was covered in water'

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A new model of the early Earth suggests that until around 2.5 billion years ago oceans covered almost the whole of the planet. Just 2% to 3% of the Earth's surface would have been dry land, compared with 28% today.

The Earth at that time may have resembled the way it looked in Waterworld, the 1995 post-apocalyptic sci-fi movie starring Kevin Costner. In the film, humanity struggles to survive after the ice caps melt and inundate the planet with water.

However, unlike in the movie, the oceans 2.5 billion years ago would have been devoid of fish, which had not yet evolved. Back then life consisted of nothing more complex than algae and bacteria.

The Australian scientists who produced the new computer simulation believe that billions of years ago the Earth's deep mantle was 200C hotter than it is today. A hotter mantle would have thickened and buoyed up the Earth's crust beneath the oceans, creating shallower basins and leading to the flooding of what is now land. The continental crust would also have spread, making it lower and flatter and more vulnerable to floods.

New Scientist magazine reported: "As the mantle cooled, land would have gradually appeared as the oceans became deeper and regions of high relief on the continental crust formed."

The transition may help explain why oxygen levels in the atmosphere rose at this time in the Earth's history, say the researchers led by Dr Nicolas Flament from the University of Sydney.

During the waterworld era, any oxygen produced by photosynthesising bacteria would have been quickly used up through reactions with decaying organic matter in the oceans.

When the newly emerged land eroded, it produced sediment that washed into the oceans and buried the organic matter, preventing further reactions with oxygen, the scientists believe. As a result oxygen was allowed to build up in the atmosphere and enable oxygen-breathing life to evolve and flourish.

The eroded sediment would also have fertilised the oceans with phosphorous, an important nutrient for living things.