Weighing The Other Evidence
Roughly a billion years after planet Earth's formation, complex microbial communities were already thriving along the windswept seashore, clinging to sediments and harnessing energy from sunlight instead of relying solely on chemical energy from rocks.
Although even earlier, more primitive bacteria known as chemolithotrophs—organisms that obtain energy by consuming minerals—likely evolved before these microbial mats, no direct evidence of Earth’s earliest life forms has been discovered yet. This is most likely because these simpler formed structures—such as simple biofilms or mats—are far less likely to be preserved in the geological record compared to the more complex and robust microbial mats that are the main feature of this article.
Although studies have pointed towards 4.1-billion-year-old carbon signatures in zircon crystals from Western Australia’s Jack Hills, 3.77-billion-year-old hydrothermal vent structures in Quebec, and 3.7-billion-year-old Greenland stromatolites (Isua Greenstone Belt), they are contested for a variety of reasons. The 3.5-billion-year-old Australian fossils (the main topic of this article) display distinct cellular structures and chemical signatures and continue to represent the strongest and most widely accepted evidence for the earliest life on Earth.
Microbial Mats In The Pilbara region of Western Australia
Let's delve deeper into these fossilized remains of these early microbial mat communities, which, due to their photosynthetic nature, were crucial in gradually oxygenating Earth’s atmosphere, fundamentally altering the planet’s surface environment and paving the way for new evolutionary pathways.
Scientists initially noticed web-like patterns and textures in the Pilbara sandstones, which they later identified as microbially induced sedimentary structures (MISS). These features were determined to have formed through the interaction of microbial mats with sediment, resembling processes observed in present-day environments.
Scientists did not initially set out specifically looking for microbially induced sedimentary structures (MISS) in the Pilbara sandstones; rather, they discovered them while studying the region’s ancient rocks for signs of early life. Prior to this discovery, researchers had already found stromatolites and microfossils in the Pilbara, but MISS of such great age had not been observed before.
The team included Dr. David Wacey from the University of Western Australia, along with US colleagues Nora Noffke and Daniel Christian of Old Dominion University, and Bob Hazen of the Carnegie Institution for Science in Washington. Professor Wacey was affiliated with the ARC Centre of Excellence for Core to Crust Fluid Systems, the Centre for Microscopy, Characterisation and Analysis, and the Centre for Exploration Targeting.
Comparisons of these ancient MISS with those found in younger rocks and modern environments (noting close similarities in form and preservation style) supported the interpretation that the structures were biogenic - produced by living organisms.
Deep Dive Into Their Carbon Chemistry & Dating Methods
Scientists measured the ratio of the stable isotopes carbon-13 (13C13C) to carbon-12 (12C12C) in the carbonaceous material embedded within the sedimentary rocks. In abiotic (non-living) carbon sources, the 13C/12C13C/12C ratio is typically close to the terrestrial standard, with 13C13C making up about 1% of the total carbon. However, biological processes—especially photosynthesis—preferentially incorporate the lighter 12C12C isotope, resulting in organic matter that is depleted in 13C13C relative to inorganic carbonates. The measured δ13Cδ13C values in the Pilbara samples were significantly negative, consistent with the isotopic signature of organic carbon produced by photosynthetic bacteria such as cyanobacteria.
The total organic carbon (TOC) content was also quantified, and the distribution of organic matter was mapped within the sedimentary matrix. The presence of organic carbon, especially when associated with sedimentary structures indicative of microbial mats, strengthens the case for a biogenic origin.
Despite the clear isotopic evidence, no preserved lipid biomarkers (such as hopanes or steranes), proteins, or microfossil cellular structures were found in these samples. This is not unexpected given the extreme age and geological history of the rocks, which would likely have destroyed or altered such fragile organic molecules.
There is no direct evidence that zircon crystals are present within the 3.49-billion-year-old sedimentary rocks that contain the microbial mats in the Pilbara region. Instead, the age of these rocks has been determined by dating zircon crystals found in nearby or interbedded volcanic ash layers or igneous intrusions. This is a common practice in Precambrian geology because sedimentary rocks themselves rarely contain minerals like zircon that can be directly dated. By analyzing the uranium-lead isotopic ratios in zircons from these associated volcanic rocks, scientists are able to establish maximum and minimum age limits for the surrounding sedimentary layers and the microbial structures they contain. Thus, the widely accepted age of 3.49 billion years for the Pilbara microbial mats is based on zircons from related volcanic material, not from the sedimentary rocks themselves.
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