They typically have shells, but these organisms are soft because there is insufficient calcium carbonate at such depth to build hard parts.

Kaiko pulled the foraminifera out of the top centimetre of sediment at Challenger Deep, 10,896m (35,748ft) below the surface.

The team says the deepest trenches of the western Pacific were formed about six to nine million years ago.

They write in Science: "The lineage to which the new soft-walled foraminifera belong includes the only species to have invaded fresh water and land, and analysis of the new organisms' DNA suggests they represent a primitive form of organism dating back to Precambrian times from which more complex multi-chambered organisms evolved."

Similar, though not identical, groups have been found in other, slightly shallower, ocean trenches, they add.

The foraminifera probably ingest particles of organic matter that rain down from higher up in the water column or materials that are dissolved in the seawater.

Challenger Deep was discovered in 1951 by the Royal Navy ship Challenger 2 - hence the name.

Kaiko was lost on a mission to the Nankai Trough in 2003. There is currently no remotely operated vehicle in service that can reach the bottom of the Marianas Trench.

The Science team comprises members from Shizuoka University; the Japan Agency for Marine-Earth Science and Technology; Nagasaki University; and the Southampton Oceanography Centre.

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