An international team of scientists from southern China accidentally discovered the oldest terrestrial fossil found, about three times older than the oldest known dinosaur.
Investigations continue and the observations will have to be verified independently, but the international team argues that the long-fingered fingers of this ancient organism closely resemble fungi.
Whatever it is, the eukaryote appears to have fossilized on Earth about 635 million years ago, just as the Earth was recovering from a global ice age.
During this massive glacial event, our planet resembled a large snowball, its oceans sealed from the Sun by 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 earth for the first time.
Fungi could have been one of the first life forms to colonize that cool space. The date of this new microfossil certainly supports the emerging idea that some fungal-like organisms left the oceans to live on land even before plants.
In fact, this transition could have been the one that helped our planet recover from such a catastrophic ice age.
“If our interpretation is correct, it will be useful for understanding paleoclimate change and the evolution of early life,” says geobiologist Tian Gan of Virginia Tech College of Science.
Today, the early evolution of fungi remains a great mystery, in large part because without bones or shells, these organisms do not fossilize easily. Not long ago, many scientists didn’t even think it was possible that fungi lasted that long.
The genome of modern fungi suggests that their common ancestor lived more than a billion years ago, branching out from animals at the time, but unfortunately there could be a break of 600 million years before the first obvious fungal fossil in our records.
In recent years, a series of intriguing and controversial discoveries have helped bridge this gap.
In 2019, scientists reported the discovery of a fungus-like fossil in Canada, which had been fossilized a billion years ago in an estuary. The implications were enormous, meaning that the common ancestor of fungi may have been much earlier than the common ancestor of plants.
In 2020, a fungus-like fossil was found in the Democratic Republic of the Congo that was fossilized in a lagoon or lake between 810 and 715 million years ago.
There is still controversy over whether or not these ancient organisms were fungi, and the new microfossil found in China will certainly stimulate a similar debate. After carefully comparing the characteristics of the organism 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 research,” says geoscientist Shuhai Xiao of Virginia Tech.
“The best way to say it is that we may not have disapproved of them being fungi, but they are the best interpretation we have at the moment.”
That said, the new discovery provides more evidence that fungus-like organisms may have been prior to terrestrial plants.
“The question used to be,‘ There were fungi in the terrestrial kingdom before the appearance of terrestrial plants, ’” Xiao explains.
“And I think our study suggests yes.”
The next question is: How did those fungi survive?
Today, many species of terrestrial fungi are unable to do photosynthesis. As such, they rely 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 thought that plants and fungi emerged together to help populate the land. But the oldest terrestrial plant fossil only dates back 470 million years.
The recently unearthed fungal-like microfossil is much older than that and was found hidden within the small cavities of calcareous dolostonic 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 snowball the Earth had 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 that live inside these bubbles.
These fungus-like life forms may even have lodged with other terrestrial microorganisms, which were also widespread at the time, such as cyanobacteria or green algae.
If fungus-like animals were equally ubiquitous, it is possible that these life forms helped accelerate chemical weathering, delivering phosphorus to the seas and causing a wave of bioproductivity in the marine environment.
On Earth, they could even have helped dig up clay minerals for carbon sequestration in the Earth’s soil, making it a fertile environment for plants and animals, and possibly changing the atmosphere of our planet.
“Thus,” the authors conclude, “Doushantuo fungal-like microorganisms, however cryptic, may have played a role in catalyzing atmospheric oxygenation and biosphere evolution after cryogenic global glaciation.”
The study was published in Communications on Nature.