Fossils over 3.4 billion years old may be the remnants of archaeal microbes that lived and produced methane from hydrothermal vents at the bottom of the fossil sea.
The oldest rocks, preserved from the beginning of the formation of the continents, form greenstone belts on the surface, such as the Barberton belt in the north of South Africa. There, in the Makhonjwa Mountains, there are outcrops of volcanic and sedimentary minerals dating back more than 3.5 billion years. They were investigated by a team of scientists from South Africa, Italy and France, finding structures similar to early living organisms.
The authors identified them as representatives of the kingdom of the Archaea. About 3.42 billion years ago, they lived near hot, mineral-rich hydrothermal vents at the bottom of the fossil sea and produced methane as a byproduct of energy generation. If so, then the find became the oldest direct evidence of the existence of archaea. Scientists write about this in an article published in the journal Science Advances.
Archaea were once considered bacteria, but it has long been clear that these unicellular organisms are completely unlike them or eukaryotes. They do not have a nucleus or organelles surrounded by membranes, which, together with other features, makes them separate into a separate living domain. Archaea tolerate extreme conditions relatively easily and are considered one of the most ancient life forms, ancestral to all eukaryotes.
Barbara Cavalazzi and her co-authors believe that the microscopic filamentous fossils found in the minerals of the Barberton Belt are just such archaea. Their silicon shells are structurally different from the contents - this may indicate that silicon has replaced the former cell walls and membranes there. And inside, scientists found increased amounts of nickel - a metal that, in conjunction with the corresponding enzymes, is often used by methanogenic microbes, and in a suitable concentration.
Thus, the find may become the oldest known example of the existence of both archaea and methanogenesis - a primitive method of obtaining energy. In that distant epoch, when the Earth was not even a billion years old, the conditions on its surface were not at all like the present ones and they would remind us of some other world. Therefore, the identification of the organisms that existed there at that time can be of great importance for the search for them outside the Earth. Recall that some archaea can even be transported from planet to planet "aboard" meteorites.
However, such samples often force scientists to give "false positive" conclusions. It is not easy to identify the microfossils of individual cells that lived more than three billion years ago. They can be easily confused with structures of a geological nature, which are preserved much more often in the geological record. And nickel, as noted by Oxford geobiologist Julie Cosmidis, has a high affinity for organic substances and is able to accumulate in them by itself, without the participation of biological processes. Therefore, the find of Cavalazzi and her colleagues will undoubtedly require more careful verification.