Relatives of the humble sea sponge have filtered Earth's waters for hundreds of millions of years or more, long before the first plants took to land. Their simplicity has led scientists to suggest sponges were the earliest animals to arise on our planet. But exactly when that happened remains under debate.
Now, a study published in the journal Nature suggests that mesh-like structures in an ancient reef may be 890-million-year-old sponges. If confirmed, the fossil sponges, found in the "Little Dal" limestones in northwest Canada, would predate the earliest undisputed fossils of any animal by more than 300 million years.
However, most claims of extremely old fossilized life kick up controversy. The creatures that flourished in ancient seas may have looked quite different than those that swim through oceans today, and scientists disagree about how much and which types of evidence can distinguish animals from other forms of life—or geologic structures. And the Little Dal fossils are no different.
"What we have is essentially something a bit like a Rorschach inkblot test, where there are some squiggles in a rock," says Jonathan Antcliffe, a paleontologist specializing in early life at the University of Lausanne, Switzerland.
During a Zoom interview, Elizabeth Turner, the sole author of the study, holds up a mustard yellow natural bath sponge—a modern relative to the newly proposed fossil sponge. She points out the network of flexible tubes that give the sponge its squish, explaining that the mesh is "identical" to the newly analyzed fossils, as well as to several younger mesh-like fossils recently identified by other scientists.
"It seems almost like a no-brainer," says Turner, a field geologist at Ontario's Laurentian University. But she acknowledges that the proposed animal identity will be controversial. "It's time for it to be published and go out to the community for discussion and challenge."
The newly described fossils were tucked in the nooks and crannies of the towering Little Dal reef. The structure formed at a time when warm, shallow seas flooded a vast tract of land through what is now North America—time and tectonics dried out the inland seas and turned the reefs to stone. Unlike many modern reefs that are built by corals, the architects of this ancient structure were cyanobacteria. These microbes grow in slimy sheets, forming layered mounds over time as sands catch on their sticky surfaces and minerals dissolved in the waters turn into solid bits of fluff.
When fossilized, the layered microbial structures are known as stromatolites. Some date as far back as 3.5 billion years ago, providing some of the earliest convincing traces of any kind of life on Earth.
Turner began studying Little Dal decades ago as a graduate student at Queen's University in Ontario. At the time, she was interested in how the cyanobacteria built the reef. But a series of strange samples with complex structures caught her eye.
"There was something fishy about it," Turner says. The Little Dal reef is mostly "just a streaky set of laminated things," she says. Yet some odd samples from the structure displayed tube-like forms that branch apart and then rejoin in a polygonal three-dimensional network. She didn’t know what to make of the odd forms' identity.
"I just kind of thought, you know, I'll just let this fester away in my mind," she says.
In recent years, however, clues to their possible identity began to amass. Researchers found strikingly similar winding networks in rocks much younger than the Little Dal reef in several locations. They suggested the branching mesh formations could be the fossil remains of a group known as keratosan sponges.
Many sponges build their skeletons out of tiny rigid structures called spicules, which are made from calcium carbonate or silica and shaped like toy Jacks. In fossils, the structures provide telltale signs of early sponges, but keratosan sponges lack these rigid skeletons. Instead, they get their squishy structure from networks of the protein spongin, which has a soft, spring-like texture that is ideal for their modern use for bathing.
By studying paper-thin sections of the rocks under a microscope, Turner documented the similarities of the tubular shapes and structures in the Little Dal samples to fossils that were previously identified as keratosan sponges, as well as to modern sponges.
Decades after she first spotted the strange forms, Turner finally felt ready to publish. "It's an ode to slow science," she says.
Sponge or something else?
The new study joins the lengthy debate about when the earliest animals arose—and what evidence is necessary to confirm a fossil as an animal. Over the last several decades, the use of geochemical tracers for early life, known as biomarkers, became a common way to identify possible creatures, explains Keyron Hickman-Lewis, a geobiologist specializing in ancient microbes at London's Natural History Museum. The fossil remains of various types of lipids, for example, are commonly used as biomarkers.
But since then, Hickman-Lewis says, much of this supposed evidence for early life has turned out to be false. Some of the would-be biomarkers were likely due to contamination, while other chemical traces were not surefire signs of animals. For example, scientists recently found that a combination of algae and geologic alteration could produce the same compounds that were previously identified as evidence of ancient sponges extracted from 635-million-year-old sediments in Oman.
So after much early excitement, Hickman-Lewis says, "we became very suspicious of an early origin for animals."
The study of the Little Dal mesh networks promises to further stoke the debate. "I think the evidence is strong," says Robert Riding of the University of Tennessee, Knoxville, who was a reviewer of the study. He recently published a study documenting similar fossils associated with a roughly 485-million-year-old stromatolite in New York.
The association of these sponges with the microbial reefs would make sense, Turner notes. Earth’s atmosphere was not always rich in oxygen, and the early date for the sponges places them before this life-friendly gas was common throughout the sea. But so-called "oxygen oases" would likely have existed around cyanobacteria reefs, where the photosynthetic microbes would have spit out oxygen that the sponges could have used.
"The fact that these two things occur together, it strengthens the case of both of them," Hickman-Lewis says.
Other experts are less convinced of the case, noting that the sponge-like network isn't as unique to the group as Turner and others suggest. "Basically, every area of life—bacteria, algae, the fungi, the plants, the animals—they can all make things that look like this," Antcliffe says.
In a 2014 review of the evidence for early sponges, Antcliffe and his colleagues found that the oldest convincing animal fossils are sponge spicules found in Iran dating to roughly 535 million years ago—and he says no recent studies have yet changed his mind.
Many analyses have identified what he calls "hints and whispers" of earlier sponge-like structures. But none sport indisputable characteristics, such as spicules or pores. The latter of these features were key to confirming the identity of the much-debated Archaeocyathid sponges, another group that lacks spicules but has been identified in rocks as old as 523 million years.
Partly the challenge comes down to the difficulty of identifying ancient sponges compared to other animals, says Drew Muscente, a paleobiologist at Cornell College in Mount Vernon, Iowa. Dinosaurs, for example, have an array of distinctive boney features—sockets, skull sutures, and more—that can help scientists tell their fossils apart from nonliving objects. "When you have a sponge or a sponge-like organism, you're missing all of those little details," he says.
Abiotic, or nonliving, chemical processes can also form structures that look surprisingly similar to life, adds Rachel Wood, a carbonate geologist at the University of Edinburgh. "She may be right. But I think you really have to explore and disprove all the other possibilities to make such a really strong claim like this." So for now, Wood says, "I don't think that she's really nailed that these are sponges."
Only further analysis can resolve the debate. Wood notes that crafting three-dimensional models of the tube network would help give a more detailed look at the structures. And Riding hopes the new study will inspire more scientists to take a closer look at other stromatolites to search for more of these meshy structures.
"I don't think this is the end of the story," Riding says. "This is just the beginning of a really interesting phase."