An international team of scientists from South China accidentally discovered the oldest terrestrial fossil ever found, about three times older than the oldest known dinosaur.
Investigations are still ongoing and the observations will have to be verified independently, but the international team claims that the long threaded fingers of this ancient organism are very similar to mushrooms.
In any case, the eukaryote appears to have fossilized on land about 635 million years ago, just as the Earth was recovering from a global ice age.
During this massive glacial event, our planet resembles a large snowball, its oceans being sealed by the Sun with more than a kilometer (0.6 miles) of solid ice. And then, in a geological “flash”, our world began to thaw inexplicably, allowing life to thrive on land for the first time.
Mushrooms could have been among the first life forms to colonize that fresh space. The date of this new microfossil certainly supports the emerging idea that some fungal-like organisms abandoned the oceans for life on land just before plants.
In fact, this transition could have helped our planet recover from such a catastrophic ice age.
“If our interpretation is correct, it will be useful for understanding paleoclimatic change and the early evolution of life,” says geobiologist Tian Gan of Virginia Tech College of Science.
Today, the early evolution of fungi remains a great mystery, largely because without bones or shells, these organisms do not easily fossilize. Not so long ago, many scientists didn’t even think it was possible for mushrooms to last that long.
The genome of modern fungi suggests that their common ancestor lived over a billion years ago, branching out from animals at the time, but unfortunately it could be a break of 600 million years before the first obvious fossil of mushrooms to appear in our records.
In recent years, a flow of interesting and controversial discoveries have contributed to closing this gap.
In 2019, scientists reported the discovery of a mushroom-like fossil in Canada that fossilized a billion years ago in an estuary. The implications were huge – namely that the common ancestor of fungi could have been much earlier than the common ancestor of plants.
In 2020, a mushroom-like fossil was found in the Democratic Republic of Congo and was fossilized in a lagoon or lake between 810 and 715 million years ago.
There is still controversy as to whether these ancient organisms were actually fungi or not, and the new microfossil found in China will no doubt stimulate a similar debate. After carefully comparing the body’s features with other fossils and living life forms, the authors identify that it is a eukaryote and “probable fungi”.
“We would like to leave things open for other possibilities, as part of our scientific investigation,” says geo-scientist Shuhai Xiao of Virginia Tech.
“The best way to say this is that I probably didn’t disapprove of them being mushrooms, but they’re the best interpretation we have right now.”
That being said, the new discovery provides more evidence that fungus-like organisms may have predated plants on land.
“The question was, ‘Were there fungi in the land before the terrestrial plants appeared,'” Xiao said.
“And I think our study suggests so.”
The next question is: How did those mushrooms survive?
Today, many species of terrestrial fungi are incapable of photosynthesis. As such, it is based on a mutualistic relationship with plant roots, exchanging water and nutrients from rocks and other hard organic matter for carbohydrates.
Because of this relationship, it was believed that plants and fungi appeared together to help populate the earth. But the oldest fossil of terrestrial plants dates back only 470 million years.
The recently discovered mushroom-like microfossil is much older than this and was found hidden in the small cavities of doloston limestone rocks, located in the Doushantuo Formation in southern China.
The rock where the fossil was found appears to have been deposited about 635 million years ago, after our Snow Earth melted. Once open to the elements, the authors suspect that the carbonate cement began to fill the cavities between the limestone sheets, possibly burying the microorganisms that live inside these bubbles.
These fungus-like life forms may have even inhabited other terrestrial microorganisms, which were also widespread at the time, such as cyanobacteria or green algae.
If mushroom-like animals were equally ubiquitous, then it is possible that these life forms helped accelerate chemical degradation, delivering phosphorus into the seas and triggering a wave of bioproductivity in the marine environment.
On land, it could even help unearth clay minerals to sequester carbon in the Earth’s soil, creating a fertile environment for plants and animals and possibly even changing the atmosphere of our planet.
“Thus,” the authors conclude, “Doushantuo-like microorganisms, no matter how cryptic, may have played a role in catalyzing atmospheric oxygenation and biospheric evolution after global cryogenic terminal glaciation.”
The study was published in Communications about nature.