This micrograph shows the bacteria (image by permission of scientists)
A bug which lives entirely on its own and survives without oxygen in complete darkness underground has been discovered in South Africa.
Desulforudis audaxviator, or bold traveller as it is known in English, relies on water, hydrogen and sulphate for its energy.
Because it gets by without oxygen, it could offer clues as to whether life exists on other planets.
The loneliest living species known to science was found inside a gold mine.
The rod-shaped bacterium was found 2.8km (1.74 miles) beneath the surface of the Earth in the Mponeng mine near Johannesburg, living in complete isolation, total darkness and 60C (140F) heat.
The find represents the first known ecosystem with a single biological species, scientists say.
It was identified in DNA extracted from water-filled cracks in the mine.
Dr Dylan Chivian of California's Lawrence Berkeley National Laboratory, who is part of the team that made the discovery, explained its significance.
"Early Earth and other planets didn't have a lot of oxygen on them, so life has evolved to use oxygen in order to get its energy," he told BBC Radio 5 Live.
"You know, if we ever discover life on other planets, it may very well be that they live without oxygen and instead potentially use chemicals like sulphate to get their energy."
The bug's name is partially based on a sentence in Latin from Jules Verne's novel Journey to the Centre of the Earth - "Descende, Audax viator, et terrestre centrum attinges," which translates as: "Descend, Bold Traveller, and attain the centre of the Earth."