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Volcanoes may have helped early microscopic organisms to colonise the land. The oldest known land-dwelling microorganisms lived in estuaries close to erupting volcanoes, which may have provided them with essential nutrients.

鈥淭his sedimentary environment, interacting with the volcanic environment, created the perfect conditions for life to spread,鈥� says at the University of Jena in Germany.

Long before animals and plants, the only life on Earth was single celled. Fossils of microorganisms similar to bacteria have been discovered in rocks from Pilbara, Western Australia, that are 3.5 billion years old, and there are disputed reports of older fossils.

Janse van Rensburg and his colleagues studied fossil microbes from 3.2 billion years ago that were found in the Barberton Greenstone Belt in South Africa. The fossils include some of the oldest examples of microorganisms living on land. 鈥淭his is a huge area where you find almost continuous [fossil] microbial mats in [what was] a tidal environment and a shoreline environment,鈥� says Janse van Rensburg.

The team focused on a little-studied section of the belt called the Moodies Igneous Complex. This contains preserved lavas and other substances ejected from volcanoes.

The combination and placement of the rocks reveals that the volcanoes erupted onto the plain of an estuary. 鈥淵ou have beautiful examples of lava that is interacting with sediments,鈥� says Janse van Rensburg.

There are shards of volcanic rock and other traces suggesting that lava hit water, with explosive results, and that it flowed over sand. Once the lava had cooled and solidified, water flowed over it and eroded it. Microbes lived in sheets called mats on and near the newly formed rocks. 鈥淟ava flowed on the surface into streams and onto beaches, and this was then weathered, and the microbial mats were living nearby,鈥� says Janse van Rensburg.

The volcanoes provided heat and energy-rich chemicals that the microbes could feed off.

鈥淚t is certainly possible that, in a volcanic environment interacting with terrestrial shallow water deposits and [microbial structures called] stromatolites, there was hydrothermal interaction with life,鈥� says at the Australian Museum in Sydney. There are plenty of modern analogues, she says. 鈥淲e see microbial life thriving in hot springs [and] living off chemical energy in and around hot vents.鈥�

The next step is to find out more about exactly how the microorganisms lived. Over the next few months, the International Continental Scientific Drilling Programme will . This should yield more information about conditions at the time and the metabolic strategies used by the microbes, says Janse van Rensburg.

The wider issue is how to interpret the fossils in the context of the early history of life. Janse van Rensburg thinks the Barberton fossils show the type of place where microorganisms first moved onto land, after originating in the sea. Like many biologists, he suspects life originated in hydrothermal vents on the sea floor.

Before the time of the Moodies Igneous Complex 3.2 billion years ago, there was no permanently exposed land, says Janse van Rensburg. 鈥淭he only place for life to grow was underwater.鈥� He says older fossil microbes, like those from Pilbara, lived in the sea.

However, in recent years, Djokic and her colleagues have published evidence that . 鈥淪tudies from the Pilbara and Barberton continue to reveal that microbial life was not just surviving, but likely thriving, very early on in Earth鈥檚 history, across the ancient landscape and within the shallow seas,鈥� she says.

Many researchers argue this suggests life originated on land. For Janse van Rensburg, though, the Pilbara landscape wasn鈥檛 fully terrestrial in the way the Barberton landscape was.

For instance, in Barberton, there is evidence of wind-blown sand dunes and other formations that were never underwater. These are absent from Pilbara, he says, suggesting the older ecosystem was only intermittently above water.