The discovery of layered rock structures in Western Australia is the earliest known evidence of life on Earth. The fossils in question are stromatolites, layered rocks formed from the excretion of photosynthetic microbes.
The oldest stromatolites that scientists agree on were made by living organisms 3.43 billion years old. But there are also older specimens. Within the Dresser Formation in Western Australia, stromatolites that are 3.48 billion years old have been found.
However, billions of years have erased traces of organic material in these older stromatolites, raising questions about whether they were actually formed by microbes or whether they may have been formed by other geological processes.
"We were able to find certain microstructures in certain layers of these rocks that strongly indicate biological processes," said Keyron Hickman-Lewis, a paleontologist at the Natural History Museum in London, England, who led the research.
Microbes
The find could have implications for the search for life on Mars, Hickman-Lewis said. Stromatolites in the Dresser Formation are coated with iron oxide from the reaction of iron with oxygen in the atmosphere.
This is the same as the surface of Mars which is equally oxidized, giving it a rusty orange color. Hickman-Lewis said the rock may be able to accommodate similar structures left by ancient Martian life.
Hickman-Lewis and his team examined West Australian stromatolites that were first discovered in 2000 by study co-author Frances Westall. They used a variety of high-resolution 2D and 3D imaging techniques to peer the stromatolite layers at fine scale.
What they saw hinted at biological growth in all its glory. The researchers observed uneven layers, including a small dome shape that suggests photosynthesis, because the microbes with the most access to the Sun would grow stronger than those less tall in structure. They also noticed a distinctive columnar structure in modern stromatolites, which are still found in several locations around the world.
"The assemblage of microbes gives a layer that is uneven in thickness and tends to wrinkle or heave at very small spatial scales. This brings together all the structural clues, and ends in the characteristics of the microbial mat," says Linda Kah, a sedimentologist and geochemist at the University of Tennessee who is not involved in the study. involved in this study.
Clues uncovering Mars?
Evidence that Dresser Formation stromatolites are signs of ancient life does not make them the oldest life on the planet.
The name is perhaps more appropriate for stromatolites found in rocks 3.7 billion years old in Greenland, or microfossils from Canada which may be 4.29 billion years old.
However, it is very difficult to distinguish biological life from non-organic processes in these very old rocks. Thus, these and other findings from the same time frame are controversial.
Based on the minerals in the stromatolites, the Western Australian microbial bed may have formed in shallow lagoons fed by hydrothermal vents also connected to the ocean.
"The techniques used to study West Australian stromatolites could be useful in the search for life on Mars. Especially if Martian samples can be returned to Earth," Hickman-Lewis said.
Scientists should consider some of the analysis here as a test run of the analysis we should be doing in about a decade when we have samples from Mars.