Four billion years ago, Earth was a hostile place with a thin atmosphere made mainly of carbon dioxide, volcanoes everywhere and oxygen levels too low to support air-breathing organisms. Nonetheless, a recent discovery of 4.28—3.75 billion-year-old microfossils suggests life existed under such conditions.
University College of London graduate student Matthew Dodd and his team believe they’ve found the oldest evidence of life on Earth in the form of fossilized bacterial tubes and filaments, preserved in a fragment of the Nuvvuagittuq Supracrustal Belt, also referred to as the NSB. While the NSB is now a rocky region in Quebec, Canada, it would have been part of the seafloor during the bacteria’s lifetime. The rock’s structure and chemical composition suggest the ancient bacteria thrived near hydrothermal vents, which are found near fissures in the seafloor that release lava. Such heat-resistant bacteria are called thermophiles.
Perhaps the most easily observable evidence that the NSB microfossils are of biological origin is their structural similarity to modern-day and other fossilized thermophiles. The newly discovered NSB microfossils, fossils of bacteria with similar composition to the NSB microfossils and some species of thermophiles living today all have a winding filamentous and tubular shape. Although it’s unlikely this type of structure could be created via abiotic processes, more evidence is needed to prove they are truly remnants of ancient bacteria.
All living organisms contain carbon, so fossils usually contain carbon. However, the NSB microfossils are composed of iron oxides. This is not a disqualifying factor though, since it’s possible its carbon components were replaced with iron compounds throughout time. Presence of carbonaceous material in the surrounding rock supports this hypothesis. However, experts in the field are far from convinced these microscopic fossils are truly four billion-year-old signs of life.
“Now, one of the interesting twists about these rocks from Nuvvuagittuq is that the age of those rocks is uncertain … the younger age [3.77 billion years] comes from dating a mineral called zircon, which is considered the gold standard for dating rocks that are this old …The 4.3 billion-year-age comes from what we call a ‘model age.’ It’s only correct if a certain set of assumptions hold true, and there’s no way to evaluate those assumptions,” said John Valley, UW-Madison Geoscience professor and discoverer of Earth’s oldest zircon, about the controversial aspects of the recent findings.
The NSB findings will surely be the subject of further research before any scientific consensus is reached, but it’s likely the younger age is more credible than the older.
Surprisingly, evidence of prehistoric life on Earth has consequences for the possibility of extraterrestrial life as well.
Valley explains life only needs three things to survive: liquid water, a source of carbon and some kind of energy. Around the same time these conditions were met on Earth—about 4.4 billion years before present—they were satisfied on Mars. If life could have evolved on Earth around that time period, it’s possible life existed on Mars, too. Although these are only speculations at the moment, the field of astrobiology may quickly advance as Martian samples become more accessible.
Astrobiologists may be in luck—NASA’s Mars 2020 mission plans to gather samples from the red planet to bring back to Earth. Valley is hopeful he will have the chance to study these samples, with the goal of identifying bio-signatures—chemical markers produced via biotic processes. Bio-signatures may lead the search for ancient life on both Earth and Mars, since intact fossils are often pulverized throughout a planet’s history of volcanism and tectonic movement.
As research on Earth’s early organisms continues, insight into the possibility of life on Mars will follow. If scientists can accurately date the most ancient of Earth’s bio-signatures, they can derive information about the environments capable of sustaining life. Pair that with knowledge about historical Martian environments, and scientists may have better luck in finding remnants of extraterrestrial life.
The research alluded to in this article was originally published in the journal Nature.