Is this strange oval-shaped fossil a plant? A fungus? A giant amoeba? A failed experiment in evolution?
Well, it’s none of these things.
The true identity of one of evolution’s greatest mysteries has finally been revealed by ancient fat molecules extracted from 558 million-year-old fossils discovered in Russia.
It’s an animal.
The creatures preserved in the fossils, known as Dickinsonia, are among the earliest known animals in the geological record, according to an international team of scientists.
The team’s discovery, published today in the journal Science, puts to rest a 75-year-old debate about the nature of these organisms, which lived between 570 million and 540 million years ago.
The beautifully preserved fossils were discovered in 2013 by PhD student Ilya Bobrovskiy at two fossil sites in cliffs above the White Sea in northwest Russia.
“We got extremely lucky and found fossils almost right away,” Mr Bobrovskiy said.
“The fossil we found first had such a thick layer of organic matter that I knew immediately fossils from this layer will tell us what Dickinsonia was.
“To recover more Dickinsonia from this layer and to find larger specimens with higher content of biomarkers. However, we had to dig out tonnes of rock.”
Mr Bobrovskiy brought the fossils to Jochen Brock’s laboratory at the Australian National University to be analysed.
“I didn’t believe that he had these amazing fossils, I didn’t think it could exist,” Dr Brocks said.
But Mr Bobrovskiy’s hunch was right.
“It was loaded with cholesterol, which is the hallmark of animals,” Dr Brocks said.
Life got very big
Dickinsonia was a soft-bodied creature that belonged to a weird group of 80 organisms known as Ediacaran biota. They lived at a time when life suddenly super-sized up from microscopic organisms to 2-metre creatures.
“Ediacaran biota is when life got very big, very fast,” Dr Brocks said.
“Then 540 million years ago, they go extinct.”
After that, he said, we start to see fossils of animals with claws, armour and teeth in what is known as the Cambrian explosion.
“If we want to understand where we come from, how animals evolved, when they evolved, you really need to know what these strange creatures were.”
The first Ediacaran fossils were discovered in the Ediacara Hills in the Flinders Ranges in 1946.
Since then, scientists have been fighting about whether Ediacaran fossils were the remnants of lichen, fungus, a giant amoeba, an animal or just some failed experiment of evolution.
Finding fat in ancient rocks
To get to the bottom of the mystery, the team looked for chemical signatures of molecules preserved in organic material left after the creature rotted on the bottom of a shallow sea.
“What is most hardy and preserved for the longest is fat. These molecules can be extremely stable for millions of years,” Dr Brocks said.
To get the samples, the team dripped acid onto the surface of the rock and removed pieces of organic matter, which were then placed into solvents and analysed for their chemical signature.
The team compared chemical signatures covering the fossils to those in microbial mats — fossilised algae — surrounding the fossil.
“We get a totally clear signature of the molecules from green algae and looking at the fossil we get this beautiful cholesterol signature,” he said.
Cholesterol is essential for the functioning of every cell in an animal’s body.
“Only animals have this high concentration of it,” Dr Brocks said.
“It really shows it was related to us.”
Fossil cholesterol has a different structure to living cholesterol so it could not be mistaken, he added.
“Even if you touched the fossil you would not be able to contaminate it with that type of thing.”
John Volkman, an expert on lipid biomarkers, said the team’s work was very high quality and contamination was unlikely.
“Clearly from the steroidal pattern that they’ve found it’s most likely to be an animal rather than some other organism,” said Dr Volkman, an honorary fellow at the CSIRO who was not involved in the research.
He said sediments of this age typically have a chemical signature of 29 carbon atoms.
“That’s usually attributed to green algae, which flourished at that time,” Dr Volkman said.
Cholesterol has a signature of 27 carbon atoms.
“They can quite definitively say they’ve found this predominance of C27 steroidal compound in there.”
He said the fossil is so well preserved that there are even signs of bacterial decay of the fat molecules.
“That’s actually contributed to the preservation of the skeleton.”
Australia’s rich history of Ediacaran life
Palaeontologist and Ediacaran fossil expert Jim Gehling from the South Australian Museum welcomed the study’s findings as a “breakthrough”.
“I am one of the few living palaeontologists who has consistently argued that such Ediacaran organisms … were animals,” said Dr Gehling, whose work has identified Dickinsonia and its “footprints” in the red sandstone rocks of the Flinders Ranges.
“[Dickonsonia was] like a ‘living doormat’ that moved from one place to another on a lawn, killing the grass beneath and, perhaps, digesting it before moving to another site.”
“For years the palaeontology community has written off fossils of the Ediacaran biota as members of a lost kingdom — a ‘failed experiment’ in the history of life,” Dr Gehling said.
“The lack of hard parts, mineral skeletons, or familiar animal appendages, led most palaeontologists to claim that the true origin of animals was the so-called Cambrian Explosion of marine animal life.”
But, he added, the rise of more aggressive animals not only destroyed evidence of soft-bodied creatures but also led to their demise.
“Most unarmoured organisms simply disappeared from the fossil record in the last 500 million years of life on Earth.”
Dr Gehling said the work done by Mr Bobrovskiy and his team could never be replicated in fossils found in South Australia.
“Our Ediacaran fossil beds, as rich and diverse as they are, preserve only imprints of these soft-bodied marine animals in sandstone. They preserve no trace of organic chemicals.”
“But what we lack in organic geochemistry, we make up for in richness of the excavated samples of seafloor bearing fossils.”