By the start of the Ordovician Period around 490 million years ago, life in the oceans had been flourishing for around 100 million years, but complex life had not yet managed to take that first jump onto dry land. It is thought that the first plants to have appeared were tiny non-vascular organisms probably looking like modern-day liverworts. Alongside these early plants, perhaps living in a mutually beneficial relationship, were fungi.

Now, researchers have described what they believe to be the earliest fossils of these first colonizers of land, as they have found parts of tiny thread-like fungi preserved in 440-million-year-old rock. This early pioneer – thought to have lived in the soil – is known as Tortotubus, and shows similar branching structures to modern fungi still around today. “These tiny little filaments are as long as a hair is wide, that kind of look like little strands with little branches coming off the sides,” says Dr. Martin Smith, who authored the study published in Botanical Journal of the Linnean Society, to IFLScience. “They’re very hard to interpret because they’re quite simple organisms and it’s hard to know exactly what they are.” 

Yet these early fungi were no doubt crucial for the further evolution of plants that led to the full colonization and diversity of both plants and animals we now see. This is because before anything moved onto land, there was simply no soil. “They probably played quite an important role in preparing the ground for colonization by other organisms,” says Smith. “They would have helped to stabilize soil, they would have helped to move nutrients around, and to transform those nutrients into a form that is accessible to what would eventually become the earliest land plants, and ultimately the earliest land animals.”

Like modern species of fungi, Tortotubus and other species like it would have played a vital role in the nitrogen cycle. They would have been breaking down organic matter and converting the nitrogen-containing compounds back into nitrates, which plants can then use to grow. But considering these fungi existed before most other complex life, what were they breaking down to form the nitrates? According to Smith, it’s likely they were decomposing early bacteria and algae. Not only that, but the pattern of growth of the fossil fungi is similar to mushroom-forming species. “We haven’t found the mushrooms – it would be really nice if we could – but the fact that it’s moving nutrients around suggests that there was a mushroom the filaments were feeding,” says Smith.

While the fossil likely does not represent the first organism ever to live on land, which is thought to have lived between 500 and 450 million years ago, it is the earliest known example. Smith analyzed the fossil by first taking rocks of the right age, dissolving them in acid, then sorting through the microfossils that remained. The fossils were originally thought to be from different organisms, but Smith found that they actually represented just one at different stages of growth. By then reconstructing the organism not unlike a teeny jigsaw, he concluded that it represented the below-ground structures of fungi. 

^{Image in text: Scanning electron micrograph of the fossil fungus, showing its branching structure. Martin Smith/University of Cambridge}