What was the first adventurous organism to try its luck on land hundreds of millions of years ago when all other complex life lived under the sea?

Scientists don't know for sure, but they have now identified the oldest known fossils of a land-dwelling organism — and it was neither a plant nor an animal.

Martin Smith, a researcher from the University of Cambridge and Durham University in the U.K., identified fossils of a fungus dating back to 445 to 443 million years ago, making it the oldest land organism fossil known.

Filaments of Tortotubus

By comparing a range of tiny fossils, Smith showed that individuals that looked like they could be different species were actually root-like parts of the fungus at different stages of growth. (Martin R. Smith)

The fungus, named Tortotubus protuberans, was likely key to making it possible for plants and animals to survive on land, suggests Smith.

That's because the fungus has cord-like structures similar to those in modern land-dwelling fungi. Modern fungi use similar cords to rot organic matter, producing soil plants can extract nutrients from — likely what Torotubus used them for too.

"Before there could be flowering plants or trees, or the animals that depend on them, the processes of rot and soil formation needed to be established," Smith said in a statement.

Microfossils

Smith identified the fungus from extremely tiny "microfossils," each shorter than a human hair is wide, discovered in Sweden and Scotland. By comparing a range of such fossils, he showed that individuals that looked like they could be different species were actually root-like parts of the fungus at different stages of growth.

The research was published in the Botanical Journal of the Linnean Society.

While the fragments were tiny, the entire organism may have been a fungal network many metres across, Smith told Reuters.

Fossil site

Some of the fossils were collected from the Burgsvik Sandstone, exposed by abandoned quarry workings in the Husryggen area of Gotland, a Swedish island in the Baltic sea. (Martin R. Smith)

Later fossils show the fungus survived for another 70 million years, as the first trees and forests sprung up around them.

The fungus was probably not the very first organism to live on land — microbial communities like bacterial films and lichen-like crusts probably survived on land as much as 1.2 billion years ago, Smith acknowledged in an email to CBC News.

"If only they had left fossils!" he added.

He added there's also some evidence that animals may have crawled out of the ocean for brief periods of time to do things like lay eggs as much as 500 million years ago, but "with no vegetation to speak of, there would have been nothing to eat and no reason to stay out of the water any longer than necessary."

Smith said that before the discovery of Tortotubus, the oldest known fossil of a land organism belonged to a nameless, crusty, plant-like organism that looked "a bit like cornflakes" and lived 443 to 433 million years ago.

With files from Reuters