If life arose in the oceans, when did it migrate to land?
A new study, published in the Botanical Journal of the Linnaean Society on Wednesday, pieces together the story of the world’s oldest terrestrial fossils, Tortotubus protuberans fungi.
This fungus lived on land at "a very important time in our planet’s history," says Martin Smith of Durham University, author of the new paper, in a phone interview with The Christian Science Monitor.
His discovery gives researchers a unique glimpse into the role of non-marine eukaryotic life forms.
The Earth in which Tortotubus developed was very different from our world today. A rocky, bare environment, early Earth would have looked much like Mars or the moon, says Dr. Smith. The only visible life was “simple, gunky, mat-like” algae structures floating on shallow seas.
Until these fungi helped transform Earth’s then-barren landscape, he says.
The fungi's main contribution was forming nutrient-rich soils, which lent stability to the planet’s early surface. Tortotubus grew mycelium, root-like strands that prevented soil from eroding during wind or rain storms.
Tortotubus also helped take out early Earth’s trash, breaking down dead algae or any other organisms that may have existed then.
Without decomposition, organisms live and die without providing resources for the next generation. Tortotubus's role in nutrient cycling in ancient soils served a vital role in encouraging the growth and diversification of plant species, says Smith.
"By the time Tortotubus went extinct, the first trees and forests had come into existence," Smith told Reuters. "This humble subterranean fungus steadfastly performed its rotting and recycling service for some 70 million years, as life on land transformed from simple crusty green films to a rich ecosystem that wouldn't look out of place in a tropical greenhouse today."
Fungi rarely leave fossils, Smith says. In order for these Tortotubus to have survived so long, they must have been sealed away from the atmosphere quickly, probably by fine silt or clay.
Smith's fungus samples came from across the globe, from North Africa to Scotland, from Sweden to upstate New York.
The North African fossils are some of the oldest, dating back 445 million years. The most recent fossils are about 385 million years old, says Smith.
More precise dating could push the date up slightly, Smith told the Washington Post, but not enough to lose its claim to being the oldest terrestrial fossil, as the second-oldest is 5 million years younger than Tortotubus.
Although full-sized fungi could have, in life, been anywhere between an inch and several feet long, these fossils are tiny.
“The longest ones,” says Smith, “are shorter than a human hair is wide.”
Researchers don’t exactly set out to find microscopic fossils, which are far too small to identify in the field. Instead, Smith says, field geologists collect a wide assortment of rocks. Only later, when they get back to the laboratory, do they discover the value in some of the samples they have collected.
In Smith’s case, he may have found the oldest fungus fossil in existence, a revelation that fills an important gap in the timeline of fungal evolution.
"This fossil provides a hint that mushroom-forming fungi may have colonized the land before the first animals left the oceans," said Smith in a press release. "It fills an important gap in the evolution of life on land."