Thread-like filaments, attached to hollow spheres in 635 million-year-old rocks, represent the earliest evidence we have of life on land. They may also help explain how life rebounded from “Snowball Earth”, the periods when it was so cold even much of the equator was covered in ice.
The upper parts of the Doushantuo Formation in South China have provided an abundance of fossils, but the older ones were thought to predate anything of interest. Virginia Tech PhD student Tian Gan only stumbled across a set of preserved branching filaments and hollow spheres close to the formation's base by chance. "It was an accidental discovery," Gan said in a statement.
The only thing the filaments truly resemble is modern-day fungi, and the authors think that's probably what they were. Wherever it fits on the tree of life, however, Gan thinks it may explain one of paleontology's big questions: how did life rebound so quickly after being almost wiped out by snowball Earth?
Snowball Earth was not like more recent ice ages. The ice was so thick and universal we are unsure how any life survived at all, making the sudden appearance of the Ediacaran diversity soon after the world warmed a mystery.
Gan's discovery may answer that. “We realized that this could be the fossil that scientists have been looking for a long time. If our interpretation is correct, it will be helpful for understanding the paleoclimate change and early life evolution," Gan said.
In Nature Communications, Gan advances the theory fungi broke down rocks until the nutrients they contained washed to the sea, fertilizing the post-snowball era.
"Fungi have a mutualistic relationship with the roots of plants, which helps them mobilize minerals, such as phosphorus,” Gan explained.
"The question used to be: 'Were there fungi in the terrestrial realm before the rise of terrestrial plants'," said co-author Professor Shuhai Xiao "And I think our study suggests yes. Our fungus-like fossil is 240 million years older than the previous record. This is, thus far, the oldest record of terrestrial fungi."
Fungus or not, the fossil appears to have lived in sheet cavities in the dolostone rocks in which it was found, but Xiao acknowledged; “Little is known about how exactly they lived and how they were preserved. Why can something like fungi, which have no bones or shells, be preserved in the fossil record?" The spheres may be part of the same organism, or something else with which the fungus had a symbiotic relationship.
The cavities were sealed shut with natural cement 632 million years ago, which the authors believe rules out more recent colonization of the space.
Molecular clocks indicate the last common ancestor of fungi and other forms of life lived 900-1,500 million years ago, but this is presumed to have been in the oceans. It's been a mystery when a descendent on the fungal line first had the bright idea of taking over the land. A few candidates for terrestrial fungi of immense age have been found, but these were in estuary sediments, and could as easily have lived on location as been washed off the land.
