Scientists have identified the earliest known animal in the geological record.
It's a 558-million-year-old oval-shaped creature that may have borne a superficial resemblance to a segmented jellyfish.
Researchers found specimens of the creature, known as Dickinsonia, that were so well preserved they still contained molecules of cholesterol.
This fat is a hallmark of animal life, the team reports in the journal Science.
Dickinsonia belongs to a group of life forms known as the Ediacaran biota. They were the first complex multi-cellular organisms to appear on Earth.
But they have been extremely difficult to classify, and their position on the tree of life has been one of the greatest mysteries in palaeontology.
Different teams of scientists have variously classified them as lichens, fungi, protozoans, evolutionary dead-ends and even as an intermediate stage between plants and animals.
The new analysis of a specimen found in north-west Russia places Dickinsonia firmly within the animal kingdom.
"The fossil fat molecules that we've found prove that animals were large and abundant 558 million years ago, millions of years earlier than previously thought," said co-author Jochen Brocks, an associate professor at the Australian National University (ANU) in Canberra.
"Scientists have been fighting for more than 75 years over what Dickinsonia and other bizarre fossils of the Ediacaran Biota were," he explained, adding: "The fossil fat now confirms Dickinsonia as the oldest known animal fossil, solving a decades-old mystery that has been the Holy Grail of palaeontology."
The Ediacaran biota appeared around 600 million years ago, and flourished for tens of millions of years before the event called the Cambrian explosion.
This massive diversification of life occurred around 541 million years ago; it's when most of the major animal groups appear in the fossil record.
The Ediacaran species largely disappear when the Cambrian explosion happens. As such, they straddle an ancient age when the Earth was dominated by bacteria and a later age of dominance by animals.
Most multicellular life leaves behind stable molecules called sterane hydrocarbons which can be preserved in sediments for millions of years. The molecular structures and abundances of these compounds can be specific to particular types of organism.
Team member Ilya Bobrovskiy, from ANU, extracted and analysed molecules from inside the fossil.
He found that Dickinsonia fossils contained very high levels of cholesterol molecules - up to 93% - compared with the surrounding sediment, where levels were roughly 11%.
Furthermore, the fossils lacked the types of stable molecules that are sometimes left behind by fungi.
"The problem that we had to overcome was finding Dickinsonia fossils that retained some organic matter," said Ilya Bobrovskiy.
"Most rocks containing these fossils, such as those from the Ediacara Hills in Australia, have endured a lot of heat, a lot of pressure, and then they were weathered after that - these are the rocks that palaeontologists studied for many decades, which explained why they were stuck on the question of Dickinsonia's true identity."
Follow Paul on Twitter.