Two decades ago, geologist Elizabeth Turner set out to explore the ancient reefs locked away in the Mackenzie Mountains in north-west Canada.
Her heart was set on understanding how photosynthetic microbes had built huge reefs millions of years ago.
Instead, the then-PhD student walked away with a pile of rocks, a handful of which had some pretty unusual features.
But it turns out that Dr Turner may have stumbled across the earliest known animal fossils, according to a paper published today in Nature.
The tiny structures embedded in the 890-million-year-old rocks look remarkably similar to the skeletons of sea sponges, suggesting that these simple creatures were thriving in the oceans earlier than previously thought.
Dr Turner said the structures were too complex to have been created by algae or bacteria.
“The process of elimination says that it can't be these other things," said Dr Turner, who is now based at Laurentian University in Ontario.
But some scientists are sceptical about whether the fossils are sponges or may have even been created by a biological process.
Simple animals are difficult to find
Sponges are simple animals that were around long before the dinosaurs, making them useful for studying how life on Earth evolved from single-celled organisms to the animals we know today, including humans.
So far, the oldest fossilised traces of sponges in ancient rocks date to around 540 million years old, placing them at the beginning of the Cambrian — a period when evolution kicked into high gear and produced an extraordinary diversity of animals.
But the genes of modern sponges suggest they could have emerged up to 400 million years earlier than their fossils suggest.
This suggests they could have been around before Dickinsonia, a flat, oval-shaped creature that has left behind 558-million-year-old fossils, which currently top the list as the earliest known animal remains in the geological record.
Finding fossilised sponges that date back this far is no easy task, as their soft bodies don't preserve as well in rocks as animals with hard skeletons, making them difficult to distinguish from other types of fossils.
It also doesn't help that ancient sponges are much simpler than their modern counterparts, said Jochen Brocks, a geobiologist who specialises in ancient ecosystems at the Australian National University.
"If you go further and further back in time, much of the complexity disappears and it becomes harder and harder to recognise what you're looking at," said Professor Brocks, who was not involved in the study.
"The more simple it is, the more likely it is that something non-biological made it by pure accident."
But if the fossils Dr Turner found are indeed ancient sponges, it means they were thriving in oceans 90 million years before oxygen was abundant on Earth, suggesting animal life may have begun evolving before this event.
Dr Turner said the ancient reefs where these sponges lived could have been "oxygen factories", thanks to the cyanobacteria that they lived alongside.
"There may well have been oases of higher oxygen," Dr Turner said.
The findings also suggest some animal life made it through a massive ice age that took place between 720 and 635 million years ago.
"In all probability, these glaciations did not wipe out life and it didn't have to start out all over again afterwards," Dr Turner said.
Identifying the ancient sponges
Dr Turner unknowingly discovered the tiny 890-million-year-old fossils when she was looking at the hundreds of rock samples she had taken from her field site in the mountains.
While she was more interested in looking for traces of reef-building microbes in the rocks, a handful of samples stood out.
Dr Turner knew she was onto something, but decided to put the unusual rock slices aside for a deeper look some other time.
Twenty years later, Dr Turner finally got a chance to slip the samples under the microscope.
The slices contained an intricate pattern of tube-shaped structures that branch out in three dimensions.
"That's a pretty complicated structure," Dr Turner said.
The branching pattern of the fossils also looked remarkably similar to the skeletons of horny sponges that live today, and those seen in younger sponge fossils.
Professor Brocks says while more work needs to be done to confirm the fossils are sponges, their age matches with estimates of when they may have first appeared.
"It makes sense. That's sort of the timing where we think that could have happened," Professor Brocks said.
Is it a sponge, microbe or none of the above?
But Professor Brocks isn't totally convinced that the sponge-like structures are a smoking gun.
In fact, "they may have nothing to do with biology at all," he said, adding that carbonate minerals can also form in branching structures.
Jim Gehling, a palaeontologist at the South Australian Museum, added that the fossils don't show classic physical features of sponges, such as an opening for filtering seawater or expelling waste.
This makes it difficult to tell whether the mesh-like fossils are truly sponges, or colonies of marine algae that take on a similar appearance, he said.
"For a fossil to be a sponge it needs to preserve evidence of structural complexity," said Dr Gehling, who was not involved in the study.
"Unfortunately, it is very difficult to separate early fossil sponges from early bacterial colonies."
