Research provides evidence they may have pulled the whole planet out of an ice age
Tamas Firkala
Tiny, fragile, thready and branched, they were found deep in the remnants of a phosphate mine in China. But these fossils, of what may be fungi, may have helped defrost the ancient Earth, setting free the ancestors of all recognizable lifeforms today.
The fossils, about 635 million years old, were found at the base of the Doushantuo Formation in Weng’an, China. That was where, in the 1990s, phosphate miners unintentionally struck a rich bed of fossils that appear to go down about 55 meters (180 feet). The fossils were odd – resembling embryos, corals and more – and sparked a great deal of interest.
In the long run, it also annoyed the miners who were more concerned about the phosphate and who damaged or destroyed some fossil sites. But then, that is another story.
The current crop of fossils, even more interesting, drew the attention of Shuhai Xiao, a professor of geobiology at Virginia Tech and his colleagues at the Chinese Academy of Sciences, the Guizhou Education University, and the University of Cincinnati.
These fungi had left some organic traces behind, but were mostly replaced by pyrite – an iron sulfide people know better as “fool’s gold.” The fungi fossils – of lifeforms mostly found on land – were found in silica that date back to before the Cambrian explosion. That was a time that the predecessors of modern animals burst forth in the fossil record. This is also the setting for the Snowball Earth theory.
Snowball Earth
Evidence shows that the most of the planet has repeatedly endured ice ages. The idea is that a little excessive ice-formation led to runaway cooling when the new ice reflected sunlight back into space, instead of the Earth absorbing it and warming things up.
Once these early fungi showed up, some things may have changed on the frozen planet.
These fossils hark back to the Ediacaran period (starting 635 million years ago and ending at the beginning of the Cambrian period, 539 million years ago). It was a time the Earth was possibly recovering from the Snowball Earth phase. That would be a time the oceans were covered with ice over a kilometer (0.6 miles) thick. In these conditions, only microscopic life could survive – for instance, fungi.
Fungi have strong digestive abilities in that their special enzymes can chemically break down rocks and tough organic material. This encourages continental weathering, life forms that rely on geochemical factors (like simple plants), and interactions between living things on land. It is believed all these factors played an important role in regenerating the Earth. But till this work by Xiao’s team, there was insufficient evidence for the existence of fungi on land beyond 400 million years ago.
While some studies have described fungi in older fossils, those samples came from marine and estuarine environments or were not clearly of terrestrial origin.
The story of life on land
“The big question is, when was the land colonized by life – and what kind of life,” Xiao says. He points out that the two ways of finding out are by studying the fossil record or analyzing DNA. To jog your memory, that is the biological code string, written in a language of amino acids. DNA changes gradually over generations as accumulating errors increase or decrease the chance of survival of a species.
“My approach is to look at the fossil record,” Xiao says. “Those who study … ancient life in an ocean, have been really fortunate because there is a very good geological record of the ocean. The ocean is basically a gigantic basin that receives … and accumulates sediments. Sediments help to preserve the fossil record.
“Those who study terrestrial records are not as fortunate as the people who study marine records, because the land or the continent has been subjected to erosion. So instead of deposition, material is removed from the continent rather than accumulated on it.”
The researchers, led by Tian Gan, the first author of the paper, were not expecting much from the dolostone. Dolostone is a mineral in sedimentary rock consisting of calcium magnesium carbonate.
“It was an accidental discovery,” Gan admitted in a Virginia Tech press release. “At that moment, 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.”
The dolostone deposition was estimated to be around 635 million years old, going by radioactive dating. Such dating of the fossils relies on how much natural uranium is converted to lead in zircon. Zircon has almost no lead in it when first formed, but it can take in radioactive uranium and thorium. These radioactive metals do break down, forming multiple forms (isotopes) of lead, each with different numbers of neutrons. Since the change occurs at a steady rate, scientists can estimate when the zircon formed by measuring how much uranium has been converted to lead.
Implications for alien life
“I will continue looking at the question of terrestrial biota and terrestrial biogeochemical cycles,” he says. “See how [these] terrestrial microorganisms modify the environment and contribute to the habitability of the terrestrial realm.”
But he also believes his ideas may not just work on Earth.
“I think it also some implications for astrobiology – the search for terrestrial life on Mars or other planets in general,” Xiao says. “The way they’re preserved, the way they made a living. … We’ll continue working on this project from sedimentological, paleontological and geochemical points of view. We’re hoping to write up another paper to talk about the cave deposit in the dolostone.”
So why does he do research? What does he tell students considering the option?
“Curiosity, I think, is what motivates most people. So, ask questions, be curious, because curiosity drives people to explore, to think out of the box … Another piece of advice I give my graduate students is to do interdisciplinary research. [Don’t] just focus on the narrow field that you’re working on, but put it in a big picture context – so that you know the larger context of your research.”
Tamas Firkala has a Ph.D. in material science.
The original article was published in Nature Communications.