Tiny bubble-like structures found in ancient rocks show life was thriving in the harshest of conditions billions of years ago – without sunlight or even oxygen.

Researchers have discovered fossils of ancient bacteria dating back 2.5 billion years, long before the planet’s atmosphere became rich in oxygen. They believe the microbes, which fed off sulphur, are the oldest of their kind ever discovered, hinting at diverse ecosystems in the planet’s past. Researchers from the University of Cincinnati (UC) in the US found fossilised bacteria in two separate locations in the Northern Cape Province of South Africa.

The 2.52 billion-year-old sulphur-oxidising bacteria are described by Czaja as exceptionally large, spherical-shaped, smooth-walled microscopic structures much larger than most modern bacteria, but similar to some modern single-celled organisms that live in deepwater sulphur-rich ocean settings today, where even now there are almost no traces of oxygen.

Researchers unveiled samples of bacteria that were abundant in deep water areas of the ocean in a geologic time known as the Neoarchean Eon (2.8 to 2.5 billion years ago). These fossils represent the oldest known organisms that lived in a very dark, deep-water environment. These bacteria existed two billion years before plants and trees, which evolved about 450 million years ago. With an atmosphere of much less than one per cent oxygen, scientists have presumed that there were things living in deep water in the mud that did not need sunlight or oxygen, but experts did not have any direct evidence for them until now.

Finding rocks this old is rare, so the understanding of the Neoarchean Eon are based on samples from only a handful of geographic areas, such as this region of South Africa and another in Western Australia, researchers said.

More information: Andrew D. Czaja et al, Sulfur-oxidizing bacteria prior to the Great Oxidation Event from the 2.52 Ga Gamohaan Formation of South Africa, Geology (2016). DOI: 10.1130/G38150.1