Life on Earth may well have flourished on Earth 3.43 billion years ago in an environment not too different to the warm little pond that Darwin imagined: a quiet shallow marine environment sandwiched in time between two active volcanic periods.
|Abigail Allwood in the Pilbara region of Western Australia, overlooking the area of an ancient reef system.
Credit: Australian Centre for Astrobiology.
In the June 8 edition of the journal Nature, Australian Centre for Astrobiology doctoral student Abigail Allwood reveals her discovery of a ten kilometer section of an ancient microbial reef system. She says the stromatolite shape associations and other evidence demonstrates no purely physical or chemical processes alone could have produced the entire ecosystem.
The ancient reef is cut off one end by a fault, and at the other as it disappears into what would have been deep water, not liked by the microbial communities that created the stromatolite structures, just like modern ones.
“If you start at the deep water end and trace it along the reef system, the numbers of stromatolite shapes increase and become more complex and varied, just as occurs in biological reef systems throughout the geologic record,” she says. “It is a classical biological response to the environment.”
Her other lines of evidence include the individual structures and the association of morphologies (shapes), the spatial distributions, and the way those relate to the palaeo-environment. Analysis of the rare earth element chemistry (with Balz Kamber, Laurentian University) confirms the deposition of the fine-grained sedimentary rocks known as chert and carbonate that make up the stromatolites happened in a marine environment.
“If you take a vertical section through time there is a brief change from the high temperature hydrothermal and volcanic deposition that dominated the Pilbara at the time to a shallow marine environment in which life flourishes virtually immediately, “ she remarks. “And then back again to another volcanic and hydrothermal episode, when the stromatolites disappear. This speaks volumes about the conditions that may have nurtured early life”
The Pilbara region of Western Australia contains ancient stromatolite structures up to almost 3.5 billion years old and is a key research area for the Australian Centre for Astrobiology. The stromatolites were first described almost three decades ago, and have been a source of spirited debate. Some think they were formed by primitive microbes, whereas others believe they were formed chemically near hydrothermal vents.
|Modern reef colonies, Sharks Bay, Western Australia
In an attempt to resolve the dispute, Abby and her colleagues studied the 10 kilometer stretch of stromatolite-rich rock, and identified seven differently shaped types of stromatolite, each in their own environmental niche.
Some look like upside-down ice cream cones, others like egg cartons. The complexity of the stromatolite system would require an unbelievable combination of purely chemical or physical processes to have formed without biological input. Viewed as a whole, the stromatolites resemble a reef formation, suggesting the presence of a complex ecosystem.
Abby describes how she made the discovery of the reef system over three field seasons between 2003 and 2005. In the second, in 2004, she and her field assistant Ian Burch, came back to their Pilbara camp one evening after surveying more of the stromatolites, and by the fire tried to figure out how the pieces of what they had seen fitted together.
Together they sketched models of what it would have looked like in the environment around them 3.5 billion years ago. “We suddenly realized this was a reef system,” she said. “So there was a hypothesis, and now it could be tested.”
“What I saw right at that moment just blew me away. It is realizing the rocks have a story to tell. You’ve got to give away any preconceived notions give up all those pressures and begin reading what the rocks are telling you.”
Understanding whether the structures were formed by microbial action tells us how soon life got going on Earth. Such knowledge is key to the puzzle of our own origins, but also vital in looking for past life on Mars and other solar system bodies.