Some 558 million years ago, the Dickinsonia—a mysterious blob-like organism measuring roughly four feet long and bearing rib-like segments across its squishy surface—left its imprint on the ocean floor, beginning the fossilization process that would preserve its likeness to this day.
Now, Maya Wei-Haas reports for National Geographic, a group of international researchers has analyzed a recently discovered Dickinsonia fossil and arrived at a surprising conclusion: The ovaloid undersea creature, which belongs to the enigmatic Ediacaran biota, is not a fungus, plant or single-celled protozoa, but a full-fledged animal. In fact, the team’s findings—newly published in Science—suggest the Dickinsonia may be the world’s earliest known animal.
According to Gizmodo’s George Dvorsky, researchers led by Australian National University’s Ilya Bobrovskiy identified cholesterol molecules on a more than 500-million-year-old Dickinsonia fossil unearthed near the White Sea in northwest Russia. The scientists believe the oddball organism produced this cholesterol, which represents a type of fat, during its lifetime. And, since animals are the only organisms capable of producing cholesterol, they argue that the molecules offer definitive evidence of Dickinsonia’s status.
“Scientists have been fighting for more than 75 years over what Dickinsonia and other bizarre fossils of the Ediacaran biota were: giant single-celled amoeba, lichen, failed experiments of evolution or the earliest animals on Earth,” study co-author Jochen Brocks, a professor at the ANU Research School of Earth Sciences, said in a statement. “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.”
Scientific American’s Shannon Hall notes that researchers have debated the Dickinsonia‘s origins since 1946, when Australian scientists first discovered fossilized specimens of the creature. Like other members of the Ediacaran biota, which includes forms of life found on Earth between roughly 571 to 541 million years ago, Dickinsonia exhibits few animal-like characteristics, such as a mouth or visible appendage. As paleontologist Adolf Seilacher summed up in a 2007 study, the Ediacarans were as “strange as life on another planet, but easier to reach.”
According to Science Translational Medicine‘s Derek Lowe, the Dickinsonia was essentially a “flat inflated bag” with a jellyfish-like consistency. Lacking a mouth, the creature was likely forced to secrete enzymes that allowed it to absorb bacterial food through its surface.
BBC News’ Paul Rincon writes that the Ediacarans thrived until about 541 million years ago, when a surge of diversification known as the Cambrian explosion gave rise to more complex creatures that drove the simpler animals into extinction. Even as the Dickinsonia and fellow Ediacarans yielded to these new species, their legacies remained, permanently preserved in Earth’s fossil record.
Lead author Bobrovskiy unearthed the Dickinsonia fossils at the heart of the new study back in 2016.
“Just imagine finding a T. rex that is so well preserved you still have the hard-tissue, the skin, maybe even a mummified eye,” Bobrovskiy’s PhD advisor Jochen Brocks, a biogeochemist at the Australian National University, tells Scientific American’s Hall. “… That’s in principle what my student found.”
Bobrovskiy and his colleagues extracted molecules known as sterane hydrocarbons (Rincon notes that traces of these molecular structures can linger for millions of years) from the fossils and found they contained cholesterol levels of up to 93 percent, as opposed to the 11 percent seen in surrounding sediment.
“The problem that we had to overcome was finding Dickinsonia fossils that retained some organic matter,” Bobrovskiy tells Rincon. “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.”
Still, some scientists remain unconvinced of the new findings. In an interview with Gizmodo’s Dvorsky, Jonathan B. Antcliffe, a researcher at the University of Lausanne in Switzerland, described the study as “completely unconvincing.” He argues that the authors failed to account for potential fossil contamination and the possibility that Dickinsonia is not an animal, but another type of eukaryote. As Dvorsky explains, eukaryotes are a group of complex, multicellular organisms that encompasses animals, plants and fungi.
According to BBC News’ Rincon, researchers have already explored these alternative options, previously classifying Dickinsonia as “lichens, fungi, protozoans, evolutionary dead-ends and even as an intermediate stage between plants and animals.”
Bobrovskiy acknowledges the potential controversy surrounding the new classification, but as he tells National Geographic’s Wei-Haas, confirming the presence of biomarkers—or stores of molecular information such as the cholesterol found on the Dickinsonia specimen—“removes a large part of this uncertainty.”
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