Little is known about the diversity and complexity of organisms, or the habitats in which they existed, on the early Earth. Understanding the organisms that lived in the distant past may provide clues to the origin of life on Earth and the prospects for finding life on other worlds. Fossils of unicellular life are key to this understanding. Fortunately, the Barberton Greenstone Belt, located in the Makhonjwa Mountains on the border of Eswatini and South Africa, contains rocks formed more than 3.4 billion years ago and offers well-preserved fossils of various organisms.
Barbara Cavalazzi, of the Università di Bologna and University of Johannesburg, and colleagues analyzed filamentous fossils embedded in chert in the Barberton rocks. Chert is an amorphous form of silicon dioxide that filled in fissures that were once located just beneath the floor of an ocean, creating hydrothermal veins. Hydrothermal veins form from water-based solutions filling in pre-existing fissures in hot environments, usually near volcanic activity. The filaments, which are about half a micrometer thick (35 times thinner than a typical human hair) and 100 micrometers long, were found in a biofilm layer less than one millimeter thick on the wall of the hydrothermal veins. Mapping of the chemical composition of the filaments was conducted using time-of-flight secondary ion mass spectrometry (ToF-SIMS), Raman spectroscopy, and X-ray spectroscopy. The elements carbon, hydrogen, nitrogen, and sulfur, all of which are key components of all known life, were found within the filaments. Nickel was also found in organic compounds within the filaments. Nickel has been found in fossils of similar age, and modern filamentous methane-producing and methane-consuming microbes contain nickel in amounts similar to those observed in the fossilized filaments. The analyzed fossils strongly suggest the presence of a methane-based ecosystem on the early Earth and hint at the possibility that similar ecosystems exist on other worlds. READ MORE