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An international team of scientists from southern China accidentally discovered the oldest terrestrial fossil ever found, about three times older than the oldest known dinosaur.
Investigations are still ongoing and sightings will need to be verified independently, but the international team say the long, wiry fingers of this ancient organism look a lot like mushrooms.
Either way, the eukaryote appears to have fossilized on Earth around 635 million years ago, as Earth was recovering from a global ice age.
During this massive glaciation event, our planet looked like a large snowball, with its oceans sealed from the Sun by more than a kilometer (0.6 miles) of solid ice. And then, in a geological “lightning”, our world began to inexplicably thaw, allowing life to flourish on earth for the first time.
Fungi may have been among the first forms of life to colonize this cool space. The date of this new microfossil certainly supports the emerging idea that some mushroom-like organisms have left the oceans for a life on earth even before plants.
In fact, perhaps it was this transition that helped our planet recover from such a catastrophic ice age.
“If our interpretation is correct, it will be useful in understanding paleoclimatic change and early life course,” says geobiologist Tian Gan of the Virginia Tech College of Science.
Today, the early evolution of fungi remains a great mystery, in large part because without bones and shells, these organisms do not fossilize easily. Not that long ago, many scientists didn’t even think it was possible that mushrooms last that long.
The genome of modern fungi suggests that their common ancestor lived over a billion years ago, breaking away from animals around this time, but sadly there could be a 600 million year hiatus before the first obvious mushroom fossil to appear in our records.
In recent years, a flood of intriguing and controversial discoveries has helped fill this gap.
In 2019, scientists reported the discovery of a mushroom-like fossil in Canada, which had fossilized a billion years ago in an estuary. The implications were huge – namely that the common ancestor of fungi may have existed much earlier than the common ancestor of plants.
In 2020, a similar mushroom-like fossil was found in the Democratic Republic of the Congo, and it was fossilized in a lagoon or lake between 810 and 715 million years ago.
Controversy remains over whether or not these ancient organisms were fungi, and the new microfossil discovered in China will no doubt spark a similar debate. After carefully comparing the characteristics of the organism to other fossils and living life forms, the authors identify that it is a eukaryote and a “probable fungus”.
“We would like to leave things open to other possibilities, as part of our scientific investigation,” says geoscientist Shuhai Xiao of Virginia Tech.
“The best way to put it is that we may not have disapproved that it was mushrooms, but it’s the best interpretation we have yet.”
That said, the new finding provides more evidence that mushroom-like organisms may have preceded plants on earth.
“The question was,” Were there fungi in the earthly realm before the rise of land plants, “Xiao explains.
“And I think our study suggests so.”
The next question is: how did these fungi survive?
Today, many species of terrestrial fungi are incapable of photosynthesis. As such, they rely on a mutualistic relationship with plant roots, exchanging water and nutrients from rocks and other stubborn organic matter for carbohydrates.
Because of this relationship, it was believed that plants and fungi emerged together to help populate the earth. But the oldest terrestrial plant fossil is only 470 million years old.
The recently unearthed mushroom-like microfossil is much older than this and has been found hidden in small cavities of dolomite limestone rocks, located in the Doushantuo Formation in southern China.
The rock in which the fossil was found appears to have been deposited around 635 million years ago, after our snowball melted. Once opened to the elements, the authors suspect that the carbonate cement began to fill the cavities between the limestone sheets, possibly burying the microorganisms living inside these bubbles.
These mushroom-like life forms might even have lodged with other terrestrial microorganisms, which were also prevalent at the time, such as cyanobacteria or green algae.
If mushroom-like animals were also ubiquitous, it is possible that these life forms helped accelerate chemical weathering, deliver phosphorus to the seas, and spark a wave of bioproductivity in the marine environment.
On Earth, they could even have helped dig up clay minerals for carbon sequestration in Earth’s soil, creating a fertile environment for plants and animals and possibly altering the very atmosphere of our planet.
“Thus,” the authors conclude, “Doushantuo mushroom-like microorganisms, cryptic as they are, may have played a role in catalyzing atmospheric oxygenation and biospheric evolution following global terminal cryogenic glaciation. “
The study was published in Nature communications.
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