Over two decades ago, Elizabeth Turner embarked on a journey, by helicopter, to the Mackenzie Mountains, a sweeping range tucked away in a remote part of northwestern Canada. The mountains offer a window into the ancient past: They contain the geological remnants of an ancient reef system, built by algae.
Turner, now a professor of geology at Laurentian University in Ontario, says she wanted to understand the reefs at a microscopic and macroscopic level. "They're as big as modern reefs; kilometers in diameter and hundreds of meters thick," she notes.
When she first arrived in the mountains, she says she was "stymied" — there was a lot of limestone, a type of rock formed with calcium, but it was mostly featureless. "The weathering is just completely gray and you can't see what's inside," she notes. So Turner collected a bunch of samples, from all across the 890 million-year-old ancient reef (known as Little Dal) and brought them back to the lab to investigate, cutting the rock into incredibly thin slices and looking at it under a microscope.
Among her samples, she spotted something unexpected.
"Just a handful of samples, literally a handful, had this unexpectedly weird, complicated thing in it that said, 'I really don't belong here'," she recalls. The "weird, complicated thing" was a series of worm-like microstructures. But they didn't relate to her Ph.D. and so she put the find aside. She thought to herself, "I'll deal with this later."
Later came much later, as is often the case in academia. Those unusual samples didn't gnaw away at her, but she knew they were important. She says they didn't appear to be microbial structures — they were too complicated for that. And they had a familiar texture she'd seen in younger rocks. Possibly, she thought, they were associated with sponges: porous, "basic" animals with no organs or nervous system that rank among Earth's oldest animal residents.
On Wednesday, in a study published in the journal Nature, Turner lays out her case for this hypothesis, suggesting the microstructures discovered in the ancient Little Dal reefs are indeed sponge microfossils, making them almost 350 million years older than the current oldest animal ever described.
It's a big claim — and one that is sure to be debated — but Fritz Neuweiler, a geologist at Laval University in Quebec who wasn't affiliated with the study, called the work "a good step forward," noting the paper is "well-founded, courageous and provoking."
Over 890 million years ago, the Earth was a very different place. There was just one supercontinent, known as Rodinia. North America was in the center of this huge land mass, with all of the other continents snuggled around its edges. The Little Dal reef system was tucked away within, submerged under shallow ocean water.
The reef was full of life at the time, primarily occupied by microbes like cyanobacteria, or algae, which use the sun to produce energy just like plants. Stromatolites, mats of bacteria that can grow a meter tall, were also present. Turner says she would have loved to snorkel over it.
"It would be like this beautiful meadow of stromatolites, it would've been colored blue-green because of cyanobacteria and it would have been, like, very slimy," she says.
You wouldn't be able to see these sponges, though — they were minuscule. Today's sponges come in a huge variety of colors, shapes and sizes, but they are composed predominantly of the "demosponges," which are true survivors, living through every major extinction event.
In the time of the Little Dal reef, the sponges couldn't compete for the surfaces exposed to the sun, so they likely existed in the in-between spaces; in between rocks and fissures, in between cavities and perhaps out on the flanks of the reef, too.
Living near the cyanobacteria, Turner says, would have been highly beneficial for sponges. Cyanobacteria would be producing oxygen, which the sponges can utilize, and "they probably would have provided a source of food as well, because cyanobacteria produce polysaccharides," she says. The sponges don't have a mouth or organs, as such, but let food drift through the water, in and out of their bodies. Living next door to the algae was like living next to a buffet and an oxygen factory all at once. Turner calls it "nirvana" for the sponges.
As the world changed, the reefs disappeared, but the potential signs of the sponges did not. During the sponges' lives they incorporated crystals of the mineral calcite — calcium carbonate — into their skeletons. The reefs were buried over time, but the calcite remained, leaving fossils within the rock for Turner to examine them 890 million years ago.
If Turner's findings are confirmed they could, quite literally, rewrite history.
Turner points to a lot of earlier research done by other groups in the last 10 years, looking at sponge body fossils in the Phanerozoic. The Phanerozoic era began around 541 million years ago, right around the Cambrian explosion, when animals begin to appear everywhere in the fossil record. The previous most definitive fossil evidence for sponges comes from this time, and a lot of work has been done to explore how signs of the creatures may survive to the present day. "In the last few years there has been promising work on how sponges get preserved," she says.
Turner calls it slow science. She's been able to build her hypothesis off the back of clear evidence in the literature of younger sponges and thinks her interpretation makes sense. However, she doesn't put a definitive full stop on it. "It would be foolish to be overly confident," Turner says. "I want to acknowledge that what I'm doing is making an informed suggestion."
And the truth is, scientists have been here with sponges and early life before.
Some evidence suggests sponges may have arisen as far back as 800 million years ago, but physical evidence is lacking. A 2016 study claimed to have discovered molecules indicative of sponges in 650 million-year-old rock. Further analysis of these molecules by a different group of scientists, including Jochen Brocks, a geobiologist at the Australian National University, suggested these molecules were likely formed by algae and geological forces, not sponges.
That means Brocks is not unfamiliar with overturning claims of the "oldest animal ever."
But Brocks isn't trying to pour cold water on Turner's ideas — quite the opposite. "I would like these things to be sponges, because it would be very exciting," he says. However, he mentions the case of Namapoikia, an organism known from microstructures in limestone that was originally regarded as animal life known before later evidence showed it may actually be formed by bacteria. That case study shows science working perfectly, and it's a good reminder to keep an open mind when it comes to earliest animals ever.
"Particularly in carbonate, the most one wondrous and beautiful biological-looking things can grow just simply by rearranging and regrowing crystals," he notes.
Unfortunately, the Little Dal reef system is fairly unique, making it difficult to find similar microstructures from other formations across the world. "There are no other examples of that kind of reef in that particular age of rock," Turner says. Finding other examples of these microstructures and confirming they are body fossils of ancient sponges, then, will take time.
But after two decades thinking about what these structures could be, Turner is used to going slow. It may well be that these structures were not formed by sponges at all, but something even more exotic we've not even considered yet.
"We're dealing with this this depth of time here," she says. "Maybe there was some other type of organism that made a three-dimensional anastomosing meshwork, right? We don't really know. We're looking."