Beneath the rocks of Australia, microscopic fossils dating back 3.45 billion years reveal what the most primitive life might have looked like, in an environment devoid of oxygen or light. A study conducted by a team of scientists from the CNRS and the University of Newcastle in the UK now sheds light on the search for traces of primitive life not only on Earth, but perhaps also in similar environments elsewhere in the Solar System.
On Earth, life appeared very early, at a time when our planet was just a hot world, bombarded with ultraviolet radiation. These conditions, likely common to other rocky planets like Mars, might have favored the emergence of simple life forms: microbes feeding on and drawing their energy solely from the oxidation of mineral matter.
With this in mind, a team from the Centre de biophysique moléculaire in Orléans (CNRS), in collaboration with the University of Newcastle, revisited an iconic and very well-preserved site in northwestern Australia, the Kitty's Gap Chert, formed in 3.45-billion-year-old coastal volcanic sediments.
By analyzing these rocks, scientists identified tiny spherical structures, barely one micrometer in size, associated with organic molecules containing carbon, hydrogen, oxygen, nitrogen, and silicon. These chemical signatures and their organization around volcanic particles resemble those of colonies of lithotrophic microbes, capable of drawing necessary nutrients and their energy from the oxidation of mineral volcanic matter. Everything indicates that these fossilized cells constitute the oldest known microbial cells on Earth.
Since 2000, the team has been studying these fossil bacteria but had to wait for the development of a sufficiently sensitive instrument to be able, in collaboration with the company Ionoptika near Southampton, to analyze on-site the minimal and highly degraded quantities of organic matter directly linked to the fossil structures.
Small colony of fossilized cells in sediments from the Kitty's Gap Chert sampled in the Pilbara, Australia, dating back 3.45 billion years.
© Frances Westall
To achieve this, scientists combined scanning electron microscopy imaging with cluster secondary ion mass spectrometry (Cluster-SIMS), a method that detects traces of elements or molecules on the surface through ion bombardment. Molecular fragments containing all the elements C, H, N, and O, essential to life on Earth, and a repetition of carbon atom numbers were found, suggesting remnants of organic matter originating from living organisms.
Furthermore, the fact that some of these molecules are bound to silicon demonstrates that the biological structures were fossilized
in situ by silica (SiO
2), which was normal for life in those distant times and eliminates the possibility of more recent contamination. Thus, 25 years passed between the initial interpretation of these fossils and the definitive demonstration of their biogenicity.
Such an advance has implications far beyond our planet. If life could thrive in these primitive volcanic environments, it might also have appeared on Mars or the icy moons of Jupiter and Saturn. But detecting such discreet and buried life forms remains a challenge. The new study of Kitty's Gap thus provides a valuable guide for interpreting Martian samples that missions like Perseverance will one day return and, who knows, discovering that Life also emerged elsewhere from rock and water.
Editor: AVR