In search of the oldest animal

In the latest instalment in the quest to find our oldest animal ancestor, a Canadian geologist has unearthed a sponge-like fossil on an ancient reef – from a mind-bending 890 million years ago. If confirmed to actually originate from a sponge, this will become the oldest known physical evidence of animal life on Earth.

If we trace our origins back through the tree of life, we will eventually arrive at the last common ancestor of all animals. But exactly when that oldest animal lived and what it looked like are a matter of fierce debate in the scientific community.

Researchers have estimated that it arose in the Neoproterozoic era, between 1,000 million and 541 million years ago. But we haven’t yet found the physical evidence – fossils – to prove this, because the first animals were soft-bodied creatures, without hard parts that could easily fossilise.

The oldest animal specimens date back to the Ediacaran period, 555 million years ago, but many researchers think that our greatest-grandparents are much older than that and potentially resembled sponges.

This is because sponges are today’s most basic animal type: mostly stationary critters without nervous, digestive or circulatory systems, and without much organisation of their tissues. While most modern sponges have silica-based hard body parts called spicules, early sponges don’t seem to have these same structures and so are absent in the early fossil record.

Now, geologist Elizabeth Turner from Laurentian University in Ontario, Canada, has unearthed what may have been the most ancient sponge we know.

In an 890-million-year-old reef built by cyanobacteria, Turner found rock samples that contain branching networks of tube-shaped structures, known as vermiform microstructure. These resemble the decayed tissue of a keratose or ‘horny’ sponge – a type of modern sponge that has a fibrous skeleton instead of spicules.

A purple sponge on the sea floor.
Chelonaplysilla violacea, a type of modern keratose sponge. Credit: Wikimedia Commons/Philippe Bourjon

Turner argues that when the softer material of the sponge dissolved, it left behind a more slowly decaying fibrous structure. As this disappeared, it left spaces that were then backfilled by calcium, which crystallised and preserved these networks of fibres.

If these samples are verified as a sponge body fossil, they would become the oldest physical evidence of animal life on the planet, pre-dating the next-oldest undisputed sponge fossils by 350 million years.

The study is published in Nature.

Jochen Brocks, an organic geochemist from the Australian National University who was not involved in this study, says that on first glance, the fossils are quite convincing.

“They look sponge-y – they definitely do not look like artefacts, they look like something that was alive in the reef,” he says.

Kathryn Hall, another independent researcher with experience in sponge taxonomy, agrees.

She explains that the structures are very similar to a modern keratose sponge skeleton; when the soft material dissolves, what’s left is a network of more robust fibres that take longer to decay, similar to what was found by Turner.

This may mean that these organisms haven’t changed much even over hundreds of millions of years.

“That’s not necessarily strange – they found an evolutionary solution that worked and they stayed with it,” says Hall, who works at the National Imaging Facility at the University of Queensland.

She adds that the close association between the fossils and the cyanobacteria reef gives more weight to this finding, as this sponge-cyanobacteria relationship is still seen to this day: “It’s going to have these microbes that are going to be growing inside of it, they’re going to be producing the dissolved oxygen that the sponge body is going to be utilising.”

But Brocks notes that these fossils will be difficult to verify as animals. Similar structures have been found before but later overturned; for example, a paper published in 2002 presented a biomineralised skeleton found within a microbial reef that looked similar to a sponge, but further research published just last year looked at 3D reconstructions of the fossil to find that it was instead constructed by microbes.

“The oldest ‘something’ appears every two years in Nature or Science, and then invariably gets kicked [out] until something’s really so good that it stays,” Brocks says. “But those are very, very rare, the ones that have really accepted forever. And some of those even get kicked out 40 years later. So most people will sit back and watch it.”

But he still thinks that these new fossils are interesting, whether they were made by microbes, sponges or something else entirely.

“They’re beautiful,” he says. “They are some living thing that lived in this reef, and it’s actually pretty exciting.”

If the oldest animal emerged 890 million years ago, it would have interesting implications – it means animals would have shivered through multiple global ice ages.

We know that single-celled life has existed on the planet for around 3.5 billion years, but some researchers believe that it took a massive climatic shift – Snowball Earth, an intense series of global ice ages from 720 to 635 million years ago – to pump oxygen into the atmosphere and create the conditions for multicellular life to evolve.

However, this discovery could mean animals arose even earlier and survived these massive glaciation events, perhaps by existing in habitats warmed by geothermal heat.

Further fossil discoveries will help shed light on the origin of animals, though Hall says specimens will be rare.

“It relies on a unique set of circumstances to form,” she says. “I don’t think the world is going to be riddled with masses of them.”

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