In northern Canada, rock formations from the early days of our planet are exposed – they could be around four billion years old. Following some earlier finds of possible microfossils from that period, researchers have now uncovered more fossilized structures that may have come from early creatures. It is a stalk just under a centimeter long, from which parallel filaments radiate. Between them are hundreds of rounded or tubular structures. The arrangement and chemical composition of these complex structures makes it rather unlikely that they arose purely abiotically, according to the team. It could therefore be the oldest known traces of life.
So far it is unclear when life first appeared on our planet. The fragile cells of early organisms were not preserved, so definitive fossils are lacking. Researchers are therefore dependent on indirect clues – for example minerals and chemical compounds that typically only arise through the activity of living cells. Striking microstructures in the form of filamentous or rounded fossils could also be relics of early unicellular organisms or colonies of organisms. At the same time, however, geochemical processes can also produce very similar structures, which makes it difficult to distinguish such abiotic relics from real microfossils. This is one of the reasons why some previous finds of possible early signs of life are controversial. This includes tiny filaments of hematite discovered in 2017 in what is known as the Nuvvuagittuq Rock Belt in Quebec, Canada. This rock formation is dated to be 3.7 to 4.28 billion years old, so these structures may represent the oldest known signs of life.
Complex branched structures
Since then, however, other researchers have questioned whether these structures really come from living organisms. They argued that very similar abiotic structures, known as ‘chemical gardens’, can form in alkaline liquids. This is one of the reasons why the scientists led by Dominic Papineau from University College London have now examined further rock samples from the Nuvvuagittuq rock belt. With the help of microtomography and an ion beam analysis, they X-rayed a fist-sized rock sample and created thousands of images of its inner workings. From these scans, the team then created a three-dimensional, high-resolution model of the microfossils contained in the rock. In addition, they analyzed the chemical and mineral composition of 100 micrometer thin rock sections using various microscopic and spectroscopic methods.
The analyzes showed that, in addition to the previously identified filamentous filaments, there are other, more complex structures hidden in the rock. These consist of a stem almost one centimeter long, from which parallel branches branch off. These filamentous, coiled extensions average 16 micrometers thick and up to 100 micrometers long, Papineau and his colleagues report. Between these branched structures are numerous shorter threads and hundreds of oval structures around 120 micrometers long. “These irregular ellipsoids mostly occur in groups and are typically arranged linearly and parallel to the filaments,” the team writes. In addition, there are also somewhat larger rosette-shaped hematite structures and spherical forms with a filamentous inner structure. All structures contain iron oxide minerals, similar to those produced by iron-processing bacteria today, and carbon in the form of graphite.
Structures cannot be explained completely abiotically
According to the research team, the complex shape and composition of the newly discovered structures suggest that at least some of them may have been biologically formed. “Although abiotic experiments can explain some simpler features of filament bundles, none of these features are found in the samples we examined,” report Papineau and his colleagues. “The hematite filaments found in the Nuvvuagittuq rock do not match the observations of known chemical gardens.” For example, such abiotic processes cannot produce such complex, parallel branched structures. Conversely, such branched forms are known from many microbial fossils from recent Earth history. These include traces of iron-oxidizing bacteria on hydrothermal vents in submarine volcanic areas.
The scientists therefore consider it very likely that the microstructures in the ancient Nuvvuagittuq rocks actually come from early organisms. “Our study strongly suggests that different types of bacteria existed on Earth between 3.75 and 4.28 billion years ago,” says Papineau. “This means that life could have evolved as early as around 300 million years after the formation of the earth – by geological standards this is fast.” If this is confirmed, it would also have far-reaching implications for the search for extraterrestrial life. “If life can develop so quickly under the right conditions, then this increases the chance that there will also be life on other planets,” says Papineau.
Source: Dominic Papineau (University College London) et al., Science Advances, doi: 10.1126/sciadv.abm2296