The fossilized fungus is called Tortotubus. In a study published Wednesday in the Botanical Journal of the Linnean Society, Durham University's Martin Smith describes some specimens that could be over 440 million years old.

"There is a little bit of uncertainty with dating that particular specimen, so it’s possible the date will creep up a bit," Smith told The Post. "It’s possible that more precise studies will revise the age, but I think it’s unlikely that this will make it younger than the second-oldest terrestrial fossil," which came at least 5 million years later, he said.

No one is arguing that Tortotubus was actually the first thing to live outside the water. The fossil record from that period is woefully incomplete, and adding a few million years to the timeline doesn't mean we've found the starting point. But the organism lived at a critical time of transition from sea to land — and based on the dating of other specimens, remained remarkably unchanged for some 100 million years — so Smith believes it played a major part in paving the way for other terrestrial life forms.

Smith, who started the research while studying at the University of Cambridge, studied a range of microfossils from Sweden and Scotland, each shorter than a human hair is wide. Some of the fossils were thought to come from two different organisms, but Smith showed that they were the opposite ends of one Tortotubus growth cycle.

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"Those two ends of the spectrum looked very different. It was like having stills from the beginning and the end of a movie," Smith said. "When I found stills from the middle and put the film together, I saw the movie in action." And when he did, he showed that the fossils represented an organism that grew mycelium — long, root-like filaments that fungi use to extract and transport nutrients.

That means that, like modern fungi, Tortotubus would have played a dual role in promoting plant growth: The mycelium that organism used to branched out in the ground — forming cotton-candy like networks in the dirt — would help keep soil stable enough to anchor vegetation. Meanwhile, the fungus would decompose dead organic things on the surface of the soil (its source of food) and help recycle important nutrients back into the dirt.

"It's transforming those chemicals into a form available to any plant life trying to get established," Smith explained.

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In other words, it takes a lot of rot and decay to make Earth livable – and Tortotubus may have been a decomposing pioneer.

So what sort of stuff was Tortotubus decomposing? It's likely that the fungus subsisted on algae and bacteria that failed to fossilize. But the existence of Tortotubus at least 440 million years ago has Smith wondering just how early plants actually came on the scene.

"I’ll certainly be keeping my eyes peeled, looking at older rocks, now that we’ve pushed the fossil record back further," he said. "It’ll be very exciting to see what kind of things crop up in the future."

Correction: An earlier version of this post referred to the fungus "Tortubus" when in fact its name is "Tortotubus".