A strange fossil that looks a bit like a giant leaf, or a fingerprint the size of a coffee table, has intrigued scientists for decades.
Thousands of the fossils have been found over the past seven decades, revealing that it lived at the bottom of the ocean, without a mouth, intestines or anus, half a billion years ago.
Was it a mossy plant? A giant single-celled amoeba? A failed experiment of evolution? Or the earliest animal on Earth?
After digging one of these fossils off a cliff in Russia and analyzing its contents, researchers discovered molecules of cholesterol, a type of fat.
This confirms that the creature, known as Dickinsonia, is the Earth's earliest known animal, said the report Thursday in the journal Science.
"Scientists have been fighting for more than 75 years" over the nature of these "bizarre fossils," said associate professor Jochen Brocks from the Australian National University Research School of Earth Sciences.
"The fossil fat now confirms Dickinsonia as the oldest known animal fossil, solving a decades-old mystery that has been the Holy Grail of paleontology."
Dickinsonia contained rib-like segments the length of its oval-shaped body, which came in a variety of sizes and could grow as large as 4.6 feet (1.4 meters).
The analysis showed the animals were abundant 558 million years ago, millions of years earlier than previously thought, according to Brocks.
The creature was part of the Ediacara Biota that lived on Earth during a time when bacteria reigned, 542-635 million years ago.
The Edicarian Period was about 20 million years prior to emergence of modern animal life -- a period known as the Cambrian explosion.
"The question has been, is that real? Is that an event that happened in Earth history? Or have we just not found the older fossils?" David Gold, geobiologist and assistant professor at the University of California, told AFP.
"This paper is another really good line of evidence to support the idea that it is in fact an animal, and that animals are much older than the Cambrian."
Dickinsonia could be an ancestor of "many forms of animal life today" including worms and insects, added Gold, who was not part of the current study.
Scientists had a difficult time finding Dickinsonia fossils with organic matter still attached.
Many of the known fossils were in Australia, and had been exposed to too many elements over many millions of years.
The fossil for the current study came from cliffs near the White Sea in the northwest of Russia.
"I took a helicopter to reach this very remote part of the world -- home to bears and mosquitoes -- where I could find Dickinsonia fossils with organic matter still intact," said Ilya Bobrovskiy, a doctoral researcher at ANU.
"These fossils were located in the middle of cliffs of the White Sea that are 60 to 100 meters high. I had to hang over the edge of a cliff on ropes and dig out huge blocks of sandstone, throw them down, wash the sandstone and repeat this process until I found the fossils I was after."
Other researchers have previously claimed to have solved the mystery of the oddball animal.
In September 2017, British researchers said they were certain it was an animal, based on a study of multiple fossils.
Another team of researchers, including Gold, also concluded in 2015 that it was a relatively advanced type of animal because of the way its body grew differently than plants or mushrooms.
While the matter wasn't entirely settled, paleobiologist Doug Erwin said: "I think the consensus among the majority of people who have worked on these fossils for the last decade or so has been they're somehow related to animals."
The Australian team "tries to make it more of a controversy than I think is in fact the case. The paper is still a good paper," he told AFP.
"This is a new line of evidence which is certainly welcome."
In that sense, the report represents the first time that biomarkers have been used to determine that a fossil from the Edicarian Period is an animal instead of something else.
"It's part of a renaissance in the field in paleontology," said Gold.
"We're discovering, as our tools and technologies get better and better, we're finding that actually there are all sorts of organic matter -- proteins, fats, different carbon compounds -- things that we didn't think could survive for such a long period of time," he added.
"Because of that we can find things like preserved pigments to see the colors of dinosaurs, or in this case, the cholesterol content of 500-million-year-old fossil."