ife on Earth was born in the oceans, it is a quasi-certainty. It remains to be seen when and where. The discovery of tubular and filamentous structures in Canadian rocks old of at least 3.77 billion years ago, provides new clues to better understand the conditions of its occurrence. Described in the journal Nature on 2 March , these microscopic forms are presented by the international team which studied them as the oldest known microfossils.
“Our discovery reinforces the idea that life has emerged from hot hydrothermal sources in the oceans, shortly after the formation of the Earth [about 4.5 billion years ago]. This rapid occurrence is consistent with other evidence such as the recent discovery of 3.7 billion-year-old sedimentary formations that would have been created by microorganisms, ” says Matthew Dodd (University College London, UCL – London Center For Nanotechnology, LCN), the first signatory of the article in Nature . This same review, in September 2016, had announced the discovery in Greenland of these stromatolites, sedimentary structures attributed to the activity of “mattresses” of bacterial colonies.
The “greenstone belt of Nuvvuagittuq”
Stromatolites are not strictly speaking fossils: these calcareous formations result from the metabolic activity of bacteria, the direct trace of which has not subsisted. On the other hand, the new discovery concerns the remains of micro-organisms themselves, fossilized in the rock. The “Nuvvuagittuq Greenstone Belt”, located on the east coast of Hudson Bay, Quebec. In 2008, the oldest of these rocks were dated to 4.3 billion years (and at a minimum to 3.77 billion years), making them the oldest known and accessible. Jonathan O’Neil (University of Ottawa), co-signer of Nature ‘s article , helped to characterize them .
The newcomers come in several forms, tubes and filaments, made of hematite, an oxidized form of iron, but also rosettes produced by their activity. Here too, non-biological processes could have resulted in similar structures, but the authors claim to have ruled out this possibility. They consider that the forms taken by microfossils and other geochemical traces evoke those of the microorganisms that are still found today in the bottom of the oceans, near the “smokers”, these hydrothermal chimneys that exhale loaded waters In nutrients ensuring subsistence oases of underwater life. These tubes and filaments would be individual cell assemblies.
“These structures are composed of minerals that are expected to form by putrefaction and are well documented in the geological archives, from the origins to today ,” Dominic Papineau (UCL, LCN) said. The fact that we unearthed them in one of the oldest geological formations suggests that we have discovered one of the oldest forms of life. ”
An explosion of Life came very fast
The demonstration, however, does not convince Kevin Lepot (Université Lille -I), for whom the structures described could just as well be microlithes, resulting from a rapid cooling of magma. “I would favor a non-biological interpretation of the objects studied,” says the researcher, for whom the study of Nature does not make it possible to exclude their volcanic origin.
Pascal Philippot (Institute of Physics of the globe of Paris ) is more convinced by the new study than by the one published in September describing the stromatolites. “The Nuvvuagittuq system contains sedimentary rocks whose antiquity is not disputed and which are clearly better preserved than their Greenland equivalent ,” he recalls. It is always difficult to prove a biological origin with purely mineralogical criteria, but the authors present a number that go in the right direction. ”
As with every announcement of this type, controversies will not fail to arise , but it is part of a cluster of clues pointing “towards the possibility that the explosion of the living on Earth could be very fast,” recalls Pascal Philippot : Water has been there very early – the continents will only emerge three billion years ago; The bombardments of meteorites, known and dated thanks to the craters which constellate the Moon, were not necessarily sterilizing; A mild temperature appeared fairly quickly.
Will it be possible to go back in time? “It is not impossible that the melting of the ice associated with global warming releases older rocks, as was the case in Greenland,” imagines Pascal Philippot. Another track, the Martian rocks paced by robots, some of which are also four billion years old. The Red Planet was then also bathed by oceans. Could she see unborn life parallels that appeared on Earth?