A 558-Million-Year-Old Mystery Has Been Solved
Scientists have finally confirmed that a weird ribbed oval called Dickinsonia is an animal.
Around 558 million years ago, a strange … something dies on the floor of an ancient ocean. Its body, if you could call it that, is a two-inch-long oval with symmetric ribs running from its midline to its fringes. It is quickly buried in sediment, and gradually turns into a fossil.
While it sits in place, petrifying, waiting, the world around it changes. The Earth’s landmasses merge into a single supercontinent before going their separate ways. In the ocean, animal life explodes; for the first time, the world is home to eyes, shells, and mouths. Living things invade the land, coating it first in thin films of moss and lichens, and then covering it in huge forests. Insects rise, into existence, and then into the skies. A dinosaur empire rises and falls. Mammals finally take over, and one of them—a human by the name of Ilya Bobrovskiy—finally unearths the fossilized ribbed oval from its resting place.
All of which is to say: Five hundred fifty-eight million years is an incredibly long time.
But despite that almost unimaginable time span, and everything that happened within it, many of the simple molecules that once existed in the oval creature’s cells still persist. Bobrovskiy, a geochemist at Australian National University, has isolated, identified, and measured them. And they provide conclusive evidence that the creature, despite all appearances, is an animal. More specifically, it is the oldest animal ever discovered. It’s called Dickinsonia.
First discovered in the 1940s, Dickinsonia is one of the most iconic members of the so-called Ediacaran biota—a group of mysterious, soft-bodied organisms that existed between 541 and 570 million years ago. In a world that had been dominated by microbes, these were the first big, complex living things. They would have been visible to the naked eye, had eyes even existed at that point.
But what were they? Some looked like tall fronds; others, such as Dickinsonia, were flat mats. They were so unlike the animals, plants, and other organisms we know today that one scientist described them as “strange as life on another planet, but easier to reach.” Some paleontologists, including Dickinsonia’s original discoverer, classified them as animals, precursors to the more familiar forms that arose later, during the Cambrian explosion. Others have taken them for giant amoebalike protists, lichens, colonies of bacteria, or even a completely extinct kingdom of life.
Bobrovskiy recently came up with a new way of resolving these debates. While looking at Ediacaran fossils under a microscope, he noticed distinctive dark films. These were the unmistakable signs of organic compounds that had been left behind when their owners’ bodies had decayed. Large, complicated molecules such as DNA or proteins don’t survive long after an organism’s death, but smaller and more stable molecules can. If Bobrovskiy could recover them, he could look for distinctive chemical signatures that distinguish animals from bacteria and other kingdoms of life. “[My supervisor] Jochen [Brocks] said we could try it, but he was always sure that it was a stupid idea,” Bobrovskiy says. “Even I thought it would fail. But it didn’t.”
Bobrovskiy dug up eight new Dickinsonia fossils from Lyamtsa, a location on the edge of Russia’s White Sea. He used solvents to extract any ancient organic molecules from those specimens, and then identified all the chemicals in the extracts. He was looking for traces of one particular group: the steroids.
The word steroid is most commonly associated with performance-enhancing drugs, but it refers to a much broader class of chemicals found in the cells of all complex creatures, including animals, protists, and fungi. These steroids come in different varieties, and each is diagnostic of a different group of organisms.
For example, Bobrovskiy found that the clay and sandstone sediments around the Dickinsonia fossils were rich in stigmasteroids, which are indicative of green algae. By contrast, the steroids in the actual fossils were almost entirely cholesteroids. That ruled out the possibility that Dickinsonia was a lichen or a protist (since these contain very different blends of steroids) or a colony of bacteria (since these don’t produce steroids at all). Indeed, only one group of organisms produces cholesteroids to the exclusion of anything else: animals. Dickinsonia was one of us.
Bobrovskiy almost sounded disappointed when I talked to him about it. “I was hoping that it would be something stranger, but animal is the only possible interpretation,” he said.
“The question of whether the Ediacara biota was a prelude to complex, animal-dominated ecosystems or a failed evolutionary experiment is fundamental to our efforts to understand the emergence of biological complexity on our planet,” says Lidya Tarhan from Yale. Bobrovskiy’s study doesn’t fully settle that question—it doesn’t say if Dickinsonia was a forerunner to any recognizable animals, or part of a group that went extinct. But it does suggest that the animal kingdom has deeper roots than many scientists appreciate.
“It’s an excellent study,” says Jennifer Hoyal Cuthill from the University of Cambridge, who studies the Ediacaran biota. “It’s really wonderful that biomolecules from these animals have survived for more than half a billion years to reveal their secrets.”
Alexander Liu, who also works at the University of Cambridge, adds that extracting such molecules “would have been unthinkable even just a few years ago.” They support other lines of evidence from studies that investigated how the organism grew and developed. “I think the case for Dickinsonia being an animal is now very strong,” Liu says.
“It will be exciting to see whether this approach can be applied to other equally problematic members of the Ediacara biota, so we can begin to reconstruct” the whole community, Tarhan adds. Not every member will turn out to be an animal. For example, before Bobrovskiy worked on Dickinsonia, he tested his techniques on a different fossil, called Beltanelliformis, and showed that those round, half-inch discs were colonies of bacteria.
“It is a very open question what other parts of the biota were,” Bobrovskiy says. “They could still be pretty much anything.” But the point is that researchers can find out what they were. The long-running mystery of the Ediacaran biota doesn’t have to continue forever. It’s a puzzle with an answer, and those answers can be found.