Methane-forming microbes – possibly the oldest organisms of this kind – cavorted in hydrothermal systems under the ocean floor 3.42 billion years ago. This is proven by the exceptionally well-preserved microfossils in sedimentary rocks from South Africa, which bear witness to this early form of life. The discovery of these fossils not only demonstrates another habitable environment on early Earth, but also shows the conditions under which life could be possible on other planets.
How, when and where did the earliest life forms on earth evolve? According to theories, the first life could have been formed around four billion years ago – shortly after the earth was formed 4.57 billion years ago. The cradle of life was probably in hot springs, in whose cocktail of nutrients and chemicals the first bacteria developed. So-called microfossils give evidence of this – hair-like traces in rocks, the chemical signature of which indicates that they were once microorganisms. To what extent it is actually the remains of microbes and to what extent the traces can only be traced back to mineral deposits is, however, controversial in some cases.
Microfossils from underground cavities
A team led by Barbara Cavalazzi from the University of Bologna has now discovered microfossils that provide direct evidence of life 3.42 billion years ago: “We have found exceptionally well-preserved fossilized microbes that apparently thrived on the walls of cavities caused by warm water emerged from hydrothermal systems a few meters below the sea floor, ”reports Cavalazzi. “Subterranean habitats, heated by volcanic activity, likely hosted some of the earliest microbial ecosystems on earth, and this is the oldest example we’ve come across.”
The microfossils come from the Barberton Greenstone Belt in South Africa – a region that contains some of the oldest and best preserved sedimentary rocks on earth. The rock layers that Cavalazzi and her colleagues examined once contained underground passages in which volcanically heated water met cooler seawater. The interaction resulted in a unique mix of chemicals that formed a breeding ground for life to emerge.
Evidence of methane metabolism
In these rocks, the researchers discovered filaments as thin as a hair – some of them individually distributed over the walls of the former cavity, some of them tightly packed together in small depressions. Analyzes of the chemical structure showed: The microfossils had a carbon-rich outer shell and a chemically and structurally clearly delimited core. According to the researchers, their composition as well as their distribution clearly show that it was actually a question of life and that the filaments were not formed by deposits of minerals. The high content of nickel in the organic compounds within the filaments was noticeable. “Nickel enzymes are often found in organisms with anaerobic metabolism,” the researchers explain. “The measured nickel concentrations in the filaments are similar to those found in microbes living today that metabolize methane.”
The 3.42 billion year old microfossils could therefore be the oldest evidence of microbes with methane metabolism – possibly the first representatives of the archaea. “Although we know that archaea can be fossilized, we have very few direct examples,” says Cavalazzi. “Our results could extend the record of archaeal fossils for the first time into the era when life first arose on Earth.” According to the researchers, the find not only expands knowledge of early life and animate areas on early Earth: “Since we find similar environments on Mars, the study also has implications for astrobiology and the chances of finding life outside of Earth,” said Cavalazzi.
Source: Barbara Cavalazzi (University of Bologna, Italy) et al., Science Advances, doi: 10.1126 / sciadv.eabf3963