Microscopic remains discovered in 3.5 billion-year-old rocks constitute the oldest known fossils as well as the earliest direct evidence of life on Earth ever found . This has been confirmed by a team of researchers from the universities of Wisconsin – Madison and California, in Los Angeles (UCLA). In a study published in the journal Proceedings of the National Academy of Sciences , these scientists, coordinated by paleobiologist James William Schopf, of the latter American institution, and Professor of Geosciences John W. Valley, of the former, describe eleven microbial specimens belonging to five different taxa –in these are grouped organisms that are related to each other–.
According to these experts, it is possible to relate its morphological characteristics to the characteristic chemical traces of life. Although some specimens are, in essence, similar to some microbes that can still be found today, others are bacteria and archaea – a type of unicellular microorganisms – belonging to extinct species. In any case, they lived at a time when oxygen was not yet significantly in the atmosphere.
Safe from the elements
The rocks containing the fossils were taken in 1982 from the Apex flint deposit in Western Australia, one of the few sites where early rocky remains can still be found. These have been preserved because they have not been affected by the activity of the tectonic plates or other particularly aggressive geological processes. The organisms present in them were described for the first time in the journal Science , in 1993. Schopf, who led that initiative and today directs the UCLA Center for the Study of Evolution and the Origin of Life, indicates that they could be identified thanks to to its distinctive cylindrical and filamentous shape . However, his interpretation was questioned at first, and some critics of his work argued that, in reality, it was mineral substances whose appearance resembled that of some living beings.
Over the years, Schopf and his colleagues contributed new data to support their position, and they are now convinced that their early suspicions were true. To determine this, the group led by Valley used a secondary ion mass spectrometer. This device uses a low-energy ion beam to pluck atoms from the surface of the sample. Thus, they were able to separate the carbon in the fossils into its isotopes. They then used the mass spectrometer to measure their proportions – organic substances, whether found in rocks, microorganisms, or animals, contain characteristic proportions of their stable isotopes of carbon. “The differences that we have found in the proportions of carbon isotopes are correlated with their forms. If they were not remains of biological origin there would be no reason for that correlation to occur. On the contrary, these proportions are typical of biology and function. metabolic, ”Valley says in a note.
Life makes its way
From their analysis, the researchers were able to verify that among the microorganisms, each about 10 micrometers wide – a human hair is the same thickness as eight of them – there were phototrophic bacteria, which take advantage of solar radiation to generate energy , methane-producing archaea and gammaproteobacteria, which oxidize this gas, a compound that according to some theoretical models had an important presence in the primitive atmosphere.
” This type of study suggests that life could be a very common phenomenon in the universe, ” says Schopf. “But above all, the presence of these microbes on Earth 3.5 billion years ago indicates that they would have developed on our planet long before that date; although nobody knows how long before. Furthermore, it confirms that even the most primitive life can evolve and give rise, in this case, to more advanced microorganisms ”. Professor Valley himself who participated in this trial carried out a study in 2001 in which he proved that 4.3 billion years ago there were already oceans on our planet. “We have no evidence that there was life on Earth at that time, but that doesn’t mean it didn’t exist,” Valley concludes.
Reference: SIMS analyzes of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions . James William Schopf et al. PNAS . 12-18-2017. DOI: 10.1073 / pnas.1718063115
Images: Jeff Miller – James William Schopf / University of Wisconsin Madison
