Fossils dating back a billion years delay the evolution of the first mushrooms

Tiny fossils extracted from the distant Canadian Arctic could repel the first known appearance of mushrooms, about a billion years ago – more than 500 million years earlier than scientists predicted.

These mushrooms, described on May 22 in Nature¹, are microscopic and surprisingly complex, with structures resembling filaments. Chemical analyzes suggest that fossils contain chitin, a compound found in the cell walls of fungi.

If this analysis holds, it could help to better understand how fungi have evolved and whether they could have facilitated the movement of plants to the soil. But some researchers are not yet convinced that the discovery is really a mushroom. "It seems to me that there is reason to believe it's real right now," says Mary Berbee, a mycologist at the University of British Columbia in Vancouver, Canada. "But more data would really be useful."

The paleobiologist Corentin Loron of the University of Liege, Belgium, and his colleagues discovered fossils by exploring a region of the Canadian Arctic called the Grassy Bay Formation.

The team had to go to the site of the study, nestled among the spectacular cliffs of the region, by helicopter. Because the rocks were formed without exposure to high temperatures or pressures, the fossils they contain are remarkably preserved, explains the paleobiologist Emmanuelle Javaux, a Loron consultant at the University of Liège.

From there, the team carefully cut the fossils into thin sheets that could be analyzed under an electron microscope. These images revealed branched filaments ending in spheres. The filaments were divided into segments by septae, walls found in some modern mushrooms.

A mushroom among us?

Fossils were discovered in a rock a billion years old and the presence of chitin in the specimens further convinced the researchers that they were dead preserved mushrooms a billion years ago. 39; years. The team named the mushroom Ourasphaira giraldae.

Sylvain Bernard, geochemist at the Institute of Mineralogy, Materials Physics and Cosmochemistry of Paris, did not interpret their chemical analyzes correctly. The presence of many organic molecules could produce similar results, he explains, and the results of the chemical analysis also suggest the presence of molecules that are not usually found in chitin. "These data do not show that these microfossils originally contained chitin."

Loron says the samples could have contained chitin and other organic compounds. It also signals the presence of chitin-like chemical signals and chitin-like fibers on the surface of fossils. "Our results are more compatible with chitin," he says.

The team's findings also correspond to molecular studies that use the rate at which DNA changes accumulate in fungi to calculate the date of their onset, says Javaux. These molecular clock analyzes had already reduced the origin of fungi to about a billion years. For the paleobiologist Christine Strullu-Derrien of the Natural History Museum in London, previous molecular analyzes showed that the only living fungi a billion years ago were simple unicellular creatures – and did not possess the more complex filamentous structures than fossils.

Nevertheless, she hopes that later studies will confirm that fossils contain fungi. "I would like to believe it," she says. "This makes an important discovery in the world – if it's really a mushroom."