A 558-million-year-old fossil has been confirmed as the world's earliest known animal, scientists have announced.
The creature, Dickinsonia, has been the subject of scientific debate since it was first discovered 75 years ago, with researchers arguing over whether it was an animal, a type of fungi or a giant single-celled protist—an organism that doesn't fit into any other category.
However, after finding ancient fat molecules in a Dickinsonia, researchers have finally been able to confirm its position in Earth's history.
Dickinsonia could grow up to 1.4 meters in length. It was oval shaped with rib-like segments running along its body. It lived toward the end of the Ediacaran Period, about 20 million years before the Cambrian Explosion—a time when complex animal life all of a sudden appeared in the fossil record.
In 2013, Ilya Bobrovskiy, a Ph.D. student at the Australian National University, set about finding Dickinsonia fossils that contained some sort of organic material. He traveled to a remote part of Russia where he scaled cliffs to find samples.
"These fossils were located in the middle of cliffs of the White Sea that are 60 to 100 meters high," Bobrovskiy said in a statement. "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."
Eventually, he came across fossils that had tissue containing molecules of cholesterol—a type of fat that serves as a hallmark to animal life.
Jochen Brocks, lead senior researcher, told Newsweek: "Ilya Bobrovskiy contacted me in 2013 and told me he had discovered organically preserved, or 'mummified,' fossils of the Edicara Biota, 558 million years old. He said he wanted to try to extract fossil fat from these fossils to find out if, or not, they were animals. I thought it was the most crazy idea that I had ever heard.
"He proved me wrong. It worked. And it worked much better than anyone could have predicted. The results were extremely clear. Dickinsonia produced copious amounts of cholesterol and was, thus, an animal. I have to say that I almost fell off my chair. It is really incredible that it is possible to detect original fat molecules in a fossil of that age and that these molecules still exist. It's breathtakingly beautiful."
The findings, published in Science, show that animal life was abundant 558 million years ago and that they had already grown to be large in size. The study authors say this now settles the debate over what Dickinsonia was—and that it confirms it as Earth's earliest known animal fossil.
"The data is very strong and very clear, and we have very good evidence that the molecules are real and not later additions," Brocks said. "I rather expect that the molecular fossil evidence will put to rest the discussion whether Dickinsonia was an animal or not. After 72 years.
"But the heated debate will continue about what type of animal it was, something that still exists today or an extinct lineage. The debate will also continue about the origin of other Ediacaran oddballs that may or may not have been animals."
As to what Dickinsonia might have been like in real life, he added, "If I had one go at a time machine, I would go snorkeling in the Baltic Sea of Russia or in the Ediacara Hills of Australia to look at these animals 558 million years ago.
"We only know that they were lying flat on the surface of the sea in relatively shallow waters. They were consuming the underlying mat of cyanobacteria and green algae—but we do not know how.
"We do not know if they had a mouth and gut. Instead, some scientists believe Dickinsonia took up nutrients and food through their skin—and I consider this likely. They may have been translucent like a jellyfish, or colourful, I would really love to know."
The study has been criticized. Commenting on it, Jonathan Antcliffe, a senior research associate at the University of Lausanne, in Switzerland, said the research uses "cherry picked examples" from the biota and that the evidence is "not compelling."
"This study, if it is to be believed to not be contamination and it could easily be, only really limits the fossil to a position within the eukaryotes, not the animals," he said in a statement emailed to Newsweek.
"There is no one arguing for the alternative position that Dickinsonia is bacterial. No one thinks that Dickinsonia is bacterial. No one. So we already know it is a eukaryote of some type. There are very many different eukaryotes and the authors cherry pick a few examples and quickly reject them before moving immediately to an animal conclusion.
"Modern life is less than 1 percent of everything that has ever lived. We have lost the biochemical data from over 99 percent of everything that has ever existed so we cannot pretend to know exclusively which organisms can or cannot make certain biochemicals. Particularly biochemicals that are as widely distributed across the tree of life as the ones used in this study."
