Around 410 million years ago, terrestrial life was relatively simple. There were no forests or prairies—land was largely dominated by slimy microbial mats. The types of plants that would eventually give rise to trees and flowers had only just evolved and would take another several million years to fully flourish and diversify.

A new discovery is rewriting the story of how these vascular plants, as they are called, spread onto land. Researchers may have finally resolved a debate about an enigmatic but widespread fossil called Spongiophyton: it seems to have been a strange life-form called a lichen that may have helped pave the way for plants to thrive on land.

Scientists have long debated whether Spongiophyton, which flourished around 410 million years ago, was lichen or algae. To determine the fossils’ identity, Becker-Kerber and his colleagues analyzed the underlying chemical properties of lingering organic material within the fossils.

Unlike algae, which are lined with cell walls, lichen contain fungi lined with chitin, the same material that makes up the exoskeletons of insects. Chitin is loaded with nitrogen, and the team’s results turned up an unmistakable nitrogen signal. “The more we tested it, the more consistent the signal became,” Becker-Kerber says. “It was genuinely exciting.”

There were other fungal traits present, too, such as a distinct branching pattern exhibited by growing fungal cells called hyphae. The results suggest lichen evolved around 410 million years ago, shortly after the initial spread of vascular plants 420 million years ago and just before the earliest known forests around 390 million years ago.

“It’s a major shift in how we view the complexity of life’s first steps onto land,” Becker-Kerber says. Spongiophyton “likely weathered rocks, stabilized sediments, cycled nutrients and contributed to the formation of protosoils just before forests developed.”

“If Spongiophyton was a lichen, it may have enabled the expansion of land plants into areas previously uncolonized,” says Matthew Nelsen, an evolutionary biologist at the Field Museum of Natural History, who was not involved with the research.

The new picture suggests lichen emerged near the beginning of terrestrial plant history and the unique fungi-algae relationship may have been essential to the spread of plants. “People often tell the story of life’s move onto land as a ‘plant story,’” Becker-Kerber says. “What our study shows is that fungi and lichens were also part of it.”