For 160 million years, the sea sponge was a ghost.

Scientists were sure the humble sponge — brainless, gutless, and perfectly content to sit on the seabed — had evolved around 700 million years ago. The molecular clock, which ticks through the slow accumulation of genetic mutations, said so. But the rocks disagreed. Convincing sponge fossils only began showing up around 540 million years ago, leaving a yawning gap in the family album of early animal life.

Now a single, beautifully preserved fossil from the banks of the Yangtze River in China has stepped into that gap — and, rather politely, explained where everyone had been hiding.

A creature from the "lost years"

Writing in the journal Nature, an international team led by Virginia Tech geobiologist Shuhai Xiao describes a 550-million-year-old sea sponge that falls squarely within the missing interval. It is the earliest convincing member of its family yet found.

Professor Xiao first saw the specimen five years ago, in a photograph sent by a collaborator.

"I had never seen anything like it before," he said. "Almost immediately, I realised that it was something new."

Working with colleagues at the University of Cambridge and the Nanjing Institute of Geology and Palaeontology, the team began ruling out suspects one by one. Not a sea squirt. Not a sea anemone. Not a coral. That left one quietly thrilling possibility — an ancient sponge.

Why the early ones vanished

The team's explanation for the long silence in the fossil record is elegant. Modern sponges are braced by tiny needle-like structures called spicules, often made of silica or calcium — the sort of hard bits that survive being buried for half a billion years. But trace sponge evolution backwards through time, the researchers found, and those spicules become steadily less mineral, steadily more organic.

"If you extrapolate back, then perhaps the first ones were soft-bodied creatures with entirely organic skeletons and no minerals at all," Professor Xiao said. "If this was true, they wouldn't survive fossilisation except under very special circumstances where rapid fossilisation outcompeted degradation."

The new find comes from exactly that kind of rare circumstance: a thin seam of marine carbonate rock known to capture soft-bodied animals that the fossil record would otherwise lose forever.

A surprisingly grand little animal

Two things surprised the scientists most. The first was a delicate pattern etched across the fossil's surface — a grid of regular boxes, each neatly subdivided into smaller boxes.

"This specific pattern suggests our fossilised sea sponge is most closely related to a certain species of glass sponges," said first author Dr Xiaopeng Wang, of Cambridge's Department of Earth Sciences and the Nanjing Institute.

The second surprise was its size. Co-author Dr Alex Liu, also at Cambridge, had been hunting something modest.

"When searching for fossils of early sponges I had expected them to be very small," he said. "The new fossil can reach over 40 centimetres long, and has a relatively complex conical body plan, challenging many of our expectations for the appearance of early sponges."

Rewriting the search

The implications stretch far beyond a single fossil. If the earliest sponges really were soft, jelly-bodied creatures, then palaeontologists have been looking in the wrong kind of rock for the wrong kind of thing. The hunt for the origin of animal life — a question stretching back to Darwin himself — will now shift toward the rare sites where soft tissues survive.

"The discovery indicates that perhaps the first sponges were spongey but not glassy," Professor Xiao said. "We now know that we need to broaden our view when looking for early sponges."

Half a billion years on, the quiet ones are finally being heard.