In 2017, paleontologists found 3.75- to 4.28-billion-year-old microscopic filaments and tubes that appeared to have been made by iron-loving bacteria. However, not all scientists agreed that these structures - dating back about 300 million years before what is most commonly accepted as the first sign of ancient life - were biological in origin.
Now, after extensive analysis of these rocks, paleontologists have discovered a much larger and more complex structure - a stem with parallel branches on one side nearly 1 cm long - as well as hundreds of distorted spheres, or ellipsoids, alongside the tubes and filaments. While some of these structures may have been created through casual chemical reactions, the stem-liketree with parallel branches was probably biological in origin, as no structure created solely by chemistry has been found like it. The new findings, reported in the journal Science Advances, suggest that a variety of microbial life may have existed on the primordial Earth.
"Using many different lines of evidence, our study strongly suggests that several different types of bacteria existed on Earth between 3.75 and 4.28 billion years ago," said Dr. Dominic Papineau, a paleontologist at the University of China Geosciences.
"That means life may have started as little as 300 million years after the formation of Earth. In geological terms, that's fast - about one spin of the Sun around the Milky Way."
"These findings have implications for the possibility of extraterrestrial life," he added. "If life is relatively quick to emerge, given the right conditions, that increases the chance that life exists on other planets."
For the study, Dr. Papineau and his colleagues examined rocks from the Nuvvuagittuq Supracrustal Belt in Canada that they collected in 2008.
Once a piece of the seafloor, the Nuvvuagittuq Supracrustal Belt contains some of the oldest known sedimentary rocks on Earth, believed to have been deposited near a hydrothermal spring system.
Hematite tubes from hydrothermal vents in the Nuvvuagittuq Supracrustal Belt, Canada, found in 2017. Image: Matthew Dodd, University College London.
Paleontologists cut the rock into paper-thick (100-micron) sections to get a close look at the tiny, fossil-like structures made of hematite and encased in quartz.
These slices of rock, cut with a diamond-encrusted saw, were twice as thick as the previous sections the researchers had cut, allowing the team to see larger hematite structures in them. They compared the structures and compositions with more recent fossils, as well as with iron-oxidizing bacteria located near hydrothermal ventscurrent.
They found modern equivalents to the twisted filaments, parallel branching structures, and distorted spheres, for example, near the Loihi submarine volcano near Hawaii, as well as other vent systems in the Arctic and Indian oceans.
Using micro-CT and ion beam techniques, the scientists confirmed that the hematite filaments were wavy and twisted and contained organic carbon, which are features shared with modern iron-eating microbes.
In their analysis, they concluded that the hematite structures could not have been created through compression and heating of the rock (metamorphism) over billions of years, pointing out that the structures appeared to be better preserved in thinner quartz (less affected by metamorphism) than in thicker quartz (which has undergone more metamorphism).
The authors also analyzed the levels of rare earth elements in the fossil-laden rock, finding that they had the same levels as other ancient rock specimens.
This confirmed that the seafloor deposits were as old as the surrounding volcanic rocks, and not younger impostor seeps, as some have proposed.
"Our unprecedented findings contribute to the search for extraterrestrial life by demonstrating that multiple co-occurring biosignatures, including microfossils, dubiofossils, abiotic diagenetic microstructures, trace element and mineral compositions associated with expected products of diagenetically oxidized biomass can yield a well-founded interpretation for evolutionprimordial biological," the scientists said.
The discovery implies that only a few hundred million years are needed for life to evolve to an organized level on a habitable planet.