Nearly 600 million years ago Earth’s continents were lifeless lands—but the oceans were teeming. Below the white-capped waves a dizzying variety of life-forms grazed blindly on gooey mats of microbes that covered the seafloor.
Thought to represent the earliest flowering of complex multicellular life on our planet, these creatures arose in a world devoid of predators, and had no need for hard protective carapaces or skeletons. Their soft, squishy bodies resembled, tubes, fronds or even thin, quilted pillows; they bore scant similarity to the anatomy of animals today. This ancient biosphere seems quite alien—and yet these organisms must be our early ancestors.
At least that is the story according to a new paper published today in Science, where researchers argue an iconic fossil from this time period is the oldest known animal that would have been visible without the help of a microscope. If correct, the finding settles a 70-year-old debate and could help explain the emergence of more advanced life-forms on our once-barren planet.
The fossils from some 575 to 541 million years ago, from a time known as the Ediacaran period, represent the earliest known complex life on Earth—meaning these creatures were neither microscopic single-celled organisms nor simple multicellular colonies of unicellular microbes. Yet because of their remoteness from us in planetary and evolutionary time—and the fact flesh fossilizes far less readily than shell or bone does—their true nature has mostly remained unknown. In 1947, for example, when scientists in Australia first discovered the segmented, pancake-shaped fossils of Ediacaran creatures dubbed Dickinsonia that can run up to one meter in length, they thought the strange organisms were an early form of jellyfish. And yet fossilized Dickinsonia and other Ediacarans exhibit no obvious characteristics such as appendages, a mouth or a gut that would link them to anything in the animal kingdom. As such, their place on the family tree of life has been quite contentious: If Dickinsonia were not jellyfish, perhaps they were instead annelid worms or mushrooms, or enormously oversize lichens or single-celled organisms.
The problem is that although Dickinsonia fossils have now been spotted at dozens of sites across the globe, they are typically found solely as two-dimensional imprints in sandstone. “It would be like trying to judge the structure of our modern world if all you had was footprints,” says Guy Narbonne, a paleontologist at Queen’s University in Ontario who was not involved in the study. Then in 2016, Ilya Bobrovskiy, a graduate student at Australian National University, made a startling discovery, stumbling upon Dickinsonia fossils in Russia that were essentially mummified in a mixture of clay and sandstone. “Just imagine finding a T. rex that is so well preserved you still have the hard-tissue, the skin, maybe even a mummified eye,” says Bobrovskiy’s PhD Advisor Jochen Brocks, a biogeochemist at the Australian National University. “Think about how much we would learn about dinosaurs! That’s in principle what my student found.”
The potential was enormous. “I’m in awe of this study because it’s a spectacular opportunity to get molecular information about a fossil that has been so enigmatic,” says Roger Summons, a geobiologist at Massachusetts Institute of Technology who was not part of the work. Indeed, when Brocks and his colleagues analyzed the samples, they uncovered cholesteroids: the molecular fossils of cholesterol, a distinctive signature of animal life. Whether animal, vegetable or otherwise, every Earthly organism is composed of cells bounded by layers of lipid molecules; only animals, however, have cholesterol in their cell membranes. So spotting cholesteroid meant Dickinsonia were in fact animals. Still, the detection could have been due to contamination—a careless brush of a finger against a fossil, for instance, could immediately transfer cholesterol-containing cells and produce an artificial signal of ancient animal life. So, Brocks and his colleagues carefully scrutinized the rocks surrounding the mummified Dickinsonia as well. There, rather than cholesteroids they found stigmasteroids, a molecular fossil commonly associated with green algae. That difference, they say, all but confirms the cholesteroid came from the fossils themselves and not from contamination, cementing Dickinsonia’s foundational status in the animal kingdom.
“I think the paper puts to bed any suggestion that they were otherwise,” Summons says. “To me, chemistry doesn’t lie.” Not only does it prove that Dickinsonia was an animal—it also now holds the record as the oldest macroscopic creature in the fossil record. And that is a crucial finding. At the end of the Ediacaran (which marks the beginning of the next period, the Cambrian), an evolutionary uprising overturned the simple and peaceful ecosystems that had reigned for 30 million years, setting the stage for our modern world. The Cambrian explosion, as it is called, produced animals with far more familiar anatomies and behaviors, such as creatures with shells, spines, thrashing limbs and tooth-rimmed jaws that could trap and devour prey. But scientists still do not know what sparked this eruption of life-forms.
Based on his team’s latest findings, Brocks argues the answer must lie somewhere within the evolutionary vagaries of Ediacaran biota. That is different from previous notions that suggested the Ediacarans were not animals at all, causing some scientists to argue the creatures were evolutionary dead ends wholly distinct from their Cambrian successors. For Dickinsonia, at least, scientists can now argue these strange soft-bodied beings were the progenitors of the Cambrian animals that swept over the planet, and thus were our ancestors.
Ultimately, this finding could help scientists better understand the complicated interplay of geology and biology that triggered the evolution of complex life on Earth—and perhaps on other worlds as well. Douglas Erwin, a paleobiologist at the Smithsonian National Museum of Natural History who did not take part in the study, is hopeful it will bolster the search for life elsewhere in the solar system because it demonstrates how faint chemical traces—rather than the more obvious analysis of fossil morphology—can uncover new, previously overlooked biological details. That is crucial because beyond Earth, he says, “we’re more likely to find fossils of something than we are to find something sticking its head up and waving.”