Microscopic structures in 3.46-billion-year-old rocks have been described as possibly the world's oldest fossils. However, a new study reports that at least one of these structures is not biological in origin, casting doubt on the prospects for the rest and potentially changing how we test for life on Mars.
The oldest rocks on Earth are a logical place to seek signs of early life, in the process providing us with hints about what we should be looking for on Mars. Such rocks are rare, so there was great excitement when tiny structures were found in samples from Western Australia's Apex formation that looked like they could be remnants of life.
The structures were found in chert, a silica-rich rock containing quartz crystals. Examples of what might have been 11 types of lifeforms were found in these rocks and assigned species names.
Whether these are actually the shadow of ancient lifeforms, however, has been a subject of much debate. In Astrobiology, a team led by Dr. Dina Bower of the Carnegie Institute claims to have “definitively” ruled out the possibility that part of the structure is a microbe. "The other microstructures in the primary rock should now be analyzed critically in order to prove that similar processes have not been responsible for the formation of those features," Bower said in a statement.
As with previous efforts, Bower shone lasers on the rocks and examined the scattered light to measure the interactions with molecular vibrations, a method known as Raman spectroscopy. By comparing the scattering observed with what others have collected on the impact of lasers on quartz from multiple angles, Bower was able to determine the size and orientation of the quartz grains in the rock with unprecedented precision.
Areas that had been identified as fossils turned out to be part of larger structures, distinguished by the presence of carbon. However, this carbon appears to come from at least two sources, rather than a single fossil, and to be younger than the rock itself.
"Based on our findings, we think that the Apex fossil that was designated as Eoleptonema apex in the originally described samples that we re-studied here was actually formed when a series of quartz grains cracked and was filled in with carbon-rich material to create a sheet-shaped structure within the larger crystal," said co-author Dr. Andrew Steele. Moreover, the paper reported, "The 'fossil' actually transects individual quartz grains."
The authors leave open the possibility that the carbon came from biological sources, but say there is no basis for the belief that the structure itself is the product of life.
"Studies have shown that 60 percent of the originally described alleged microfossils were found in material that is younger than its host rock, E apex being one such example," said Bower.
In this context, the authors consider the chances that the remaining supposed microfossils are actually signs of life to be poor.