Mysterious rocks believed to be the oldest evidence of life on Earth have been unveiled
It's not easy to track down the oldest traces of life on Earth, but put a group of microbes between layers of rock and let them mature for billions of years. What you'll get looks more like rock than an ancient life form.
It takes a real eye to tell one from the other, and even then arguments are rarely settled.
Take a group of 3.48 billion year old rock formations from Western Australia for example. Thought to be the fossilized remains of microbial communities known as Stromatolites, ruling out the possibility that they are purely geological is easier said than done.
Now, a new analysis by an international team of researchers provides strong evidence that these formations are biological in origin, rather than the result of abiotic processes.
Paleontologist Kieron Hickman-Lewis of The Natural History Museum, UK, explains: “If an archaeologist had discovered the foundations of a ruined city, they would know it was the building of people because it would have all the hallmarks of a building by people – entrances, roads, bricks.
In much the same way, there are many structural elements that are integral to stromatolites that allow us to define their formation processes and decipher their origins. We can almost be archaeologists.”
Stromatolites dating back billions of years have been found scattered all over the world. It consists of microencapsulated or layered rocks that can be produced either by mineral layers of a microbe or by abiotic chemical reactions between the rock and its environment.
The paleontologist's job is to try to figure out what it is — not always easy, as we've seen in the 3.7-billion-year-old stromatolite-like layers in Greenland, which were first declared the world's oldest fossils and then turned out to be just ancient rock fossils.
Currently, 3.43-billion-year-old stromatolites from another site in Western Australia, Strelley Pool, are the oldest widely accepted traces of life on Earth. Heckman-Lewis and colleagues subjected the 3.48-billion-year-old stromatolites from the Dresser Formation of Western Australia to a new and rigorous study.
They used multiple techniques to examine the two- and three-dimensional microstructures found in Dresser's stromatolites, including optical microscopy and scanning electron microscopy.
None of these tests revealed microfossils or organic matter, but they showed structures and properties consistent with biological origin.
One day, the team concluded, the stromatolites were mats of photosynthetic microbial populations thriving on the floor of a shallow marine lagoon.
The team also observed column-like "barrier" formations, consistent with patterns in rocks known to have been formed by microbial growth.
These clues form strong evidence in favor of the biological origin of these ancient layers of rock, making them the oldest evidence of life on Earth - and having implications for the search for life elsewhere.
The research has been published in the journal Geology.
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