Early Life on Earth

Fossilized cell remnants in 3.4 billion-year-old bedrock are from primitive microbes. 

Two rows of three squares each, the top left square pale gray with darker dots and the other five black with clusters of bright dots.

Paleoarchean filaments in top left photomicrograph, with mappings of concentrations of carbon, nitrile, and hydrogen.

Barbara Cavalazzi et al.

Where and how life on Earth first began remains a hotly debated area of science. Hydrothermal environments beneath the ocean are a top contender for early life, given their favorable mix of conditions, including chemistry, energy availability, and temperature. Now, an international team of researchers led by geobiologist Barbara Cavalazzi at the University of Bologna, Italy, has discovered evidence of fossilized microbes dating to roughly 3.42 billion years ago in a hydrothermal system just under the seafloor.

The suspected microfossils, in some cases measuring only tens of micrometers thick, were found in an outcrop of South Africa’s Barberton Greenstone Belt, an area known for its wellpreserved microbial fossils and ancient sedimentary rocks. The research team analyzed the microfossils across a range of scales, using microscopy, spectrometry, and X-ray imaging to illuminate their form and chemical composition.

Filamentous in shape, these prehistoric microbes appear to have formed two thin layers, with some arranged in clusters similar to a biofilm. Researchers uncovered them among the branches of a subseafloor hydrothermal vein that billions of years ago would have been filled with a rich mixture of fluids. Here, a few meters under the ocean floor, cool, low-oxygen seawater met volcanically warmed hydrothermal fluids, producing moderate temperatures and an abundance of nutrients essential to life.

These microfossils contain carbon, hydrogen, and nitrogen, as well as trace levels of sulfur and nickel. The presence of certain nickel compounds in a hydrothermal environment suggests that the putative microbes likely produced or used methane in their metabolism, as modern microbes do in similar anoxic environments today. Other features of the filaments also support the idea that they were once alive: a carbonaceous outer layer acted as a sheath around a distinct core—reminiscent of a cell wall or membrane surrounding cell cytoplasm. From the collective evidence, the researchers conclude that these microfossils represent the oldest methanecycling archaea microbes found in a subsurface environment.

According to Cavalazzi, these findings “are expanding the concept of habitability. Similar environments, and so forms of fossil life, could also possibly be detected on early Mars when the conditions for life were similar to those on early Earth.” And that could have implications in the search for life elsewhere in the Solar System, such as the watery moons of Jupiter and Saturn. (Science Advances)

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