How Did Life Arrive on Land? A Billion-Year-Old Fungus May Hold Clues
Scientists reported on Wednesday that they have discovered the oldest known fossils of fungi, a finding that may reshape our understanding of how life first arrived on land from the oceans.
Fungi are the invisible giants of the natural world, even if most people are only dimly aware of them as toadstools along a hiking trail, or mushrooms sprinkled across a pizza.
Scientists have identified about 120,000 species of fungi so far, but estimate there are as many as 3.3 million species in all. By comparison, all living mammals comprise fewer than 6,400 species.
The success of fungi results largely from their unique way of feeding. Rather than absorbing sunlight like plants or devouring other organisms like animals, fungi spew out powerful enzymes. These break down surrounding cells or even rock, which the fungi slurp up.
Some of them seemed to have partnered with plants that also left fossils behind in the rocks. Others appeared to have specialized in breaking down dead plant matter.
Until now, those fossils have been the oldest clear evidence of fungi. Many scientists considered them a snapshot of the early conquest of land. Fungi and plants came ashore together as ecological partners, it seemed. Together, they transformed barren lands into a soil-carpeted habitat.
Recently, though, some researchers have grown dissatisfied with this scenario.
By comparing the DNA of different species, scientists have drawn an evolutionary tree of fungi. If the Scottish fossils were among the earliest members of the fungal kingdom, you’d expect that living fungi would share a common ancestor not much before 407 million years.
But that’s not what the DNA trees tell us. The genes of living fungi indicate that their common ancestor lived over a billion years ago.
Could there be a 600-million-year gap in the fossil record? In recent years, scientists have searched for fungi in rocks older than those in Scotland, and they’ve found a few microscopic fossils that looked like they might be fungi. But they were too ambiguous to convince many experts.
The new fossils came to light during a geological expedition to the barren fringes of the Canadian Arctic. In 2014, Robert Rainbird, a research scientist at the Geological Survey of Canada, noticed black flecks on a piece of shale.
He knew that sometimes flecks like these turn out to be microscopic fossils. “I thought, ‘I should grab some of this stuff, because it looks juicy,’” he said.
Dr. Rainbird sent the material to Emmanuelle Javaux, a paleontologist at the University of Liège in Belgium. She asked Corentin Loron, a graduate student, to analyze them more closely.
Mr. Loron put the rocks into an acid bath to strip out the minerals. He ended up with a black paste of organic matter, which he smeared onto slides. When he looked at them under a microscope, he saw hundreds of tiny fossils.
The fossils were single-celled organisms. They were much bigger than bacteria, but Mr. Loron couldn’t determine exactly what they were. Dr. Rainbird’s analysis of the rocks showed that these organisms, whatever they were, had fossilized a billion years ago in an estuary, where a river flowed into a sea.
On an expedition in 2017, Dr. Rainbird, Mr. Loron and their colleagues discovered some peculiar fossils in the rocks. They were composed of spore-like spheres, often joined to long filaments that sprouted T-shaped branches — the kind of shapes found today in fungi.
Mr. Loron used electron microscopes to survey the fine structures, and found that the spheres and filaments had double walls — another hallmark of fungi. To see what molecules were contained in the fossils, Mr. Loron and his colleagues fired infrared beams at them and measured the light they released.
Three fossils produced a pattern that matches that of a protein called chitin. All fungi make chitin to build their tough walls. Only insects and a few other species do the same.
The researchers concluded they had found an ancient fungus, which they named Ourasphaira giraldae.
“This is the first evidence that fungi are a billion years old, even though we’ve thought they were for a long time,” said Mary Berbee, a mycologist at the University of British Columbia, who was not involved in the new research.
But Dr. Berbee and other experts said they would have more confidence in the findings with more data, especially about the fossils’ chemistry.
“I don’t have any doubt that they’re fossils, and that alone is fascinating,” said George Cody, an organic geochemist at the Carnegie Institution for Science in Washington. But the infrared results could have been produced by molecules other than chitin, he added.
If the Arctic fossils are indeed fungi, it’s a mystery how the tendril-sprouting organisms made a living.
Today, fungi that sprout tendrils can grow to huge sizes by feeding on vast amounts of plant matter. A single subterranean mat in Oregon is three times larger than Central Park.
A billion-year-old fungus would have had no land plants to feed on. The oldest known plant fossils are no more than 470 million years old.
“The fungi were probably colonizing the land before the plants,” Mr. Loron speculated.
The fungi had to be eating something. One possibility: bacteria. Researchers have found signs that crusts of bacteria were growing on land as long as 3.2 billion years ago.
It’s also possible that these ancient fungi lived on the bottom of the estuary, perhaps feasting on underwater mats of algae. Land plants evolved from green algae, and so perhaps the estuary habitat was where fungi and the ancestors of plants first formed partnerships.
It’s possible, Mr. Loron said, such ancient estuaries provided “the toolbox for everything that’s going on land afterward.”