The remote and unforgiving slopes of Canada’s northern McKenzie Mountains contain tiny traces of what may be the earliest animal bodies preserved in the fossil record, reports a new study. Dating back some 890 million years—hundreds of millions of years before the next-oldest known animal fossils—these ancient tubelike structures resemble sponges, the most basic form of animal life.
If the fossils are proven to be sponges, they would provide the first physical evidence that animals emerged before the Neoproterozoic Oxygenation Event that has been linked to the flourishing of complex life, and would also also confirm that early animals survived punishing glacial episodes that began some 700 million years ago, according to a study published on Wednesday in Nature.
This extended timeline of animals in the fossil record represents a major potential discovery about the origins of the biological kingdom to which humans, among countless other creatures, belong. These ancient structures provide long-sought material evidence that could validate molecular clock data, which is information inferred from the mutation rate of biomolecules that can be used to trace the evolutionary origins of species.
“Most of this actually isn't very surprising,” said Elizabeth Turner, a professor of carbonate sedimentology and invertebrate paleontology at Laurentian University who authored the study, in a call. “It only seems surprising when you see 890 million years, which is a couple hundred million years older than the next comparable material, but it has been predicted by these other independent ways of looking back into deep time.”
That said, the specimens that Turner presents in her study, which contain networks of sponge-like tubes in a distinct veriform structure, are extremely rare and hard-won. They were acquired over many years of fieldwork in the McKenzie Mountains, located in the Northwest Territories, an environment that presents numerous challenges to scientists.
“This is a very difficult area to work in from a logistical view,” Turner explained. “You have to have money and you have to plan in advance for helicopters, freeze-dried food, and fuel caches. The permitting process is very onerous so just to have permission to work there requires leaping through some hoops.”
“Working there is another tall order because you spend weeks or months camped—generally just me and one other person—in two tiny tents,” she continued. “You spend most of your time just trying to stay warm and fed, and also, everyday, groveling up some mountainside to the place you want to work and then stumbling back down again with a pack full of 70 pounds of rock samples...That kind of explains why this type of work is not really common, and it also explains why the type of material that I worked with for this study is not more widely presented in the literature.”
Turner has been working in these challenging conditions for decades, and has long suspected that these specimens contain valuable evidence about animal life. However, her work came into sharper focus recently thanks to the efforts of other researchers who refined our understanding of sponge fossilization, which can produce “sponge mummies” and other structures that are distinct from other animal remains due to the fragile bodies of these lifeforms.
Sophisticated 3D reconstructions of the tubular meshworks within sponges enabled Turner to make the connection between fossilized sponges from more recent periods of Earth’s history with the microstructures she has identified in rocks from the Northwest Territories. The 890-million-year-old specimens contain tubes that are mere micrometres wide, but they closely resemble the fibre networks in modern horny sponges, or keratose demosponges, such as the Mediterranean bass sponge.
“The most joyful part of this whole story is that I get to be really old-fashioned and give credit where it's due,” Turner said. “In the last few years, a few people around the world have been working on this topic and describing how you wind up with a microstructure that is pretty much identical to the one found in these rocks, and how it's derived through the preservation of sponge tissue.”
“In early 2021, I realized that there had come to be a critical mass of this other literature that helped me; that served for me as a foundation on which I could stand and then present my own comparable, but much older, material,” she continued. “That's how it happened: there was no ‘aha’ or ‘Eureka’ moment. It was an incremental, gradual, and on-and-off progress: a labor of love and a bit of a cottage industry for me.”
That said, Turner emphasized that she is only proposing that the microstructures described in her new open-access paper are keratose demosponges, and that it will take a lot more work to confirm that these ancient meshworks really are the oldest known body fossils from the animal kingdom.
A low-mag view of an area of vermiform microstructure inside a cavity. Image: E.C. Turner
Either way, the new study has big implications for reconstructing the origins of animal life on Earth. If sponges lived in bygone reefs some 900 millions years ago, it suggests that simple animals could survive before oxygen levels on Earth became super-charged, suggesting early members of our storied biological kingdom did not require an oxygenation event to flourish.
“There's lots that we can do to interrogate the geological record,” Turner said. “We need to be looking not just for evidence of sponge-like material; we could be looking for the other, more complicated animals in the older rocks as well. Maybe they were there, too.”
“We need an open mind and we need to be looking without preconceived notions about what an ‘animal’ is,” she concluded. “If we're looking for something familiar, it's already too complicated because the earliest animals won't look like familiar things. That's why this particular microstructure that I have is so gratifying because it is kind of weird and difficult to understand.”