Our planet's fossil record goes back an astounding 3.5 billion years. The most ancient signs of life on earth are stromatolites, iconic columns of layered sediment formed by mats of bacteria. But these formations aren't just features of the distant past – although the microbes themselves have evolved over the eons, rare modern species still form those same structures today, and now they've been discovered living in a new unexpected place: the swamps of Tasmania.

It was during a survey of the wetlands in the Tasmanian Wilderness World Heritage Area that researchers came across the conspicuous yellow blobs that turned out to be active stromatolite-forming microbe colonies, some as large as ten centimetres (four inches) across.

Stromatolites in Tasmania! Left and right: Those yellow clumps are the communities of microbes that mark the top of the stromatolites. Centre: A slice through one of these structure reveals the characteristic layers of sediment placed down as the microbe mats grow through time. Image: Proemse et al. 2017

"The discovery of living stromatolites in Tasmania is highly significant because stromatolites are rare globally and not previously known from Tasmania except as ancient fossils," said Rolan Eberhard of the Department of Primary Industries, Parks, Water, and Environment. In fact, all known stromatolite fossils from Tasmania are over 450 million years old!

The top of a stromatolite is a living mat made up of microbial colonies – often multiple species together. As these cells grow, they excrete or trap various minerals, and ultimately the entire mat becomes a thin layer of cement as the new generation of microbes grows on top of it. Over time, this growth creates a characteristic column of sediment layers.

Not only are stromatolites the oldest known evidence of life – fossils from Western Australia are around 3.5 billion years old, and others from Greenland might be even older – but they were also extremely common in the distant past. These formations declined around the beginning of the Cambrian Period, about 540 million years ago, a time when the rise of microbe-munching animals left few safe spaces for stromatolites to grow. 

This fossil stromatolite from the Pilbara Craton in Western Australia is approximately 3.5 billion years old, among the very oldest evidence of life on earth. Image: Didier Descouens, Wikimedia Commons

Today, stromatolites are most often found in areas with extreme conditions where their predators struggle to survive, most famously in the super-salty waters of Shark Bay, Australia. There, columnar stromatolites can be up to one metre (three feet) tall, having grown continuously for thousands of years.

The Tasmanian wetlands, on the other hand, are pretty tame – the freshwater coming out of the natural springs isn't super-heated or hyper-saline, for example – so researchers were surprised to find stromatolites thriving there. But the same thing can't be said for the natural predators of stromatolites: snails. Piles of shells around the calcareous mud of the spring mounds suggest that the snails don't deal well with the local chemistry, which harms their shells.

"This is good for stromatolites because it means there are very few living snails to eat them," explained Bernadette Proemse of the University of Tasmania. That means the microbe mats are free to grow unimpeded.

The pale ring of calcareous mud on the outer edge of this spring mound is where the stromatolites were found to be thriving while their natural predators – snails – could not. Image: Proemse et al. 2017 

Not only are these stromatolites living in an unexpected habitat, but DNA analysis also showed that these microbial communities are a unique combination. This has some pretty exciting implications: it seems there are many more stromatolite secrets left to uncover, and these life forms might be present in more environments than researchers have come to expect.

"Fortuitously, these Tasmanian 'living fossils' are protected by the World Heritage Area and the sheer remoteness of the spring mounds," Proemse said. Stromatolites can be extremely sensitive to environmental disturbances, as changes to local conditions can interfere with the chemical comforts that allow them to thrive.



Top header image: Stromatolites near Hamelin Pool in Western Australia.

Credit: Node worx/Flickr