Layered rocks in Western Australia are among the earliest known forms of life on Earth, according to a new study. The fossils in question are stromatolites, layered rocks formed by the secretions of photosynthetic microbes. The oldest stromatolites, which scientists believe were created by living organisms, date back 3.43 billion years, but there are older examples. Stromatolites dating back 3.48 billion years have been found in the Dresser Formation in Western Australia.
However, billions of years have erased traces of organic matter in these ancient stromatolites, calling into question whether they were actually formed by microbes or whether they could have been created by other geological processes.
“We were able to detect certain specific microstructures in individual layers of these rocks that clearly indicate biological processes,” said Keiron Hickman-Lewis, a palaeontologist at the Natural History Museum in London who led the study.
Hickman-Lewis and his team examined the Western Australian stromatolites, first discovered in 2000 by study co-author Frances Westall at the National Centre for Scientific Research (CNRS) in France. They used a variety of high-resolution 2D and 3D imaging techniques to peer into the stromatolite layers at a fine scale.
A sample of Dresser Formation stromatolite showing a complex layered structure of hematite, barite and quartz, and a domed upper surface. Keiron Hickman-Lewis
What they saw hinted at biological growth in all its messy glory. The researchers observed uneven layers, including small dome-shaped features that indicate photosynthesis, since microbes with the most access to the sun will grow more vigorously than those not as high up in the structure. They also saw columnar structures typical of modern stromatolites, which are still found in several locations around the world.
Microbial mats produce layers that are uneven in thickness and tend to be wrinkled or sinuous or rise and fall on very small spatial scales.
The evidence that the Dresser stromatolites are signs of ancient life does not make them the oldest life on the planet. That honor could go to stromatolites found in 3.7-billion-year-old rocks in Greenland, or perhaps microfossils from Canada that may be as old as 4.29 billion years. However, it is very difficult to distinguish biological life from inorganic processes in these very old rocks, so these and other finds from a similar time frame are controversial.
Researchers reported November 4 in the journal Geology that, based on minerals in the stromatolites, the Western Australian microbial mats likely formed in a shallow lagoon fed by hydrothermal vents that were also connected to the ocean.
Hickman-Lewis noted that the techniques used to study the Western Australian stromatolites could prove useful in the search for life on Mars, especially if samples from Mars can be returned to Earth. Scientists should “view some of the analyses presented here as a trial run for the analyses we’ll have to do in about a decade, when we have material from Mars.”