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Sponge fossils suggest animals already existed 890 million years ago

Microscopic calcite structures reveal the protein skeleton of an ancient sponge

E.C. Turner

The origin of animals may have happened 350 million years earlier than thought. Fossils that seem to be sponges, one of the first animals to evolve, have been found in rocks from 890 million years ago.

“It seems at first glance that this is a very radical paper,” says Elizabeth Turner at Laurentian University in Sudbury, Canada, who made the discovery. However, she says the fossils she found fit with other evidence.

Animals are mostly multicellular organisms whose bodies are made up of distinct tissues, and unlike plants they have to eat food to survive. For years, the earliest-known animal fossils were from the Cambrian Period, which began 541 million years ago. However, in recent years, some fossils from the earlier Ediacaran Period (635 to 541 million years ago) have been identified as animals. There are also 660-million-year-old chemical traces that may be from sponges.

Ancient sponges

Turner studied rocks from north-west Canada that contained the preserved remains of reefs from 890 million years ago, during the Tonian period. These reefs weren’t made by corals, like modern reefs, as these didn’t exist yet. Instead, they were made by photosynthetic bacteria living in shallow seas. The reefs, known as stromatolites, were many kilometres across and rose to heights of hundreds of metres above the seafloor. “These are spectacular reefs,” says Turner.

Within the rocks, Turner found the preserved remains of a network of fibres, which branched and joined up in a complex mesh. These are the remains of sponges, she argues, but “not a normal fossil”.

The bodies of modern sponges contain a mesh made of a protein called spongin, which forms a soft skeleton. Turner’s work suggests that when ancient sponges died, their soft tissues became mineralised, but the tough spongin didn’t. Eventually, though, it decayed, leaving hollow tubes within the rock that later filled with calcite crystals. These networks of calcite (pictured above) are what Turner then found – and the way the network branched looked just like spongin (pictured below).

The spongin skeleton of a modern sponge

The spongin skeleton of a modern sponge

E.C. Turner

Similar fossils from later periods have been convincingly identified as sponges, says Joachim Reitner at the University of Göttingen in Germany, who has studied preserved sponges. “We have no other organisms forming this type of network in this way.”

“I personally found this pretty convincing,” says Amelia Penny at the University of St Andrews in the UK.

Early origins

If sponges existed 890 million years ago, then the origin of animals must have occurred much earlier than previous fossils have suggested. “Molecular clock” studies that use modern DNA to estimate when key points of evolution occurred have indicated that animals emerged long before the earliest fossils. However, this approach is often thought to be less reliable when there aren’t any fossils available to calibrate the molecular clock. Turner’s finding “brings the fossil record back into line with the molecular clock estimates”, says Penny.

However, an earlier origin of animals changes two key aspects of their story on Earth. Firstly, there was little oxygen in the air until levels rose between 800 and 540 million years ago. This rise in oxygen is thought to have enabled the evolution of animals, but if animals already existed 890 million years ago, it suggests that the first and simplest animals could survive with little oxygen, says Turner. In line with this, Reitner says many modern sponges can tolerate low-oxygen conditions.

Secondly, most of the planet froze over in the period between 720 and 635 million years ago, becoming a “snowball Earth”. “It’s previously been thought these are really catastrophic events for life on Earth, certainly multicellular life,” says Penny. But it seems sponges, at least, survived the glaciations.

“They did not wipe out all the products of biological evolution to date, and life did not have to start all over again, because the things I’ve identified are essentially identical to sponge fossils [from much later],” says Turner.

Sponges vs comb jellies

Finally, there is the question of which animal groups were the first to emerge. Palaeontologists have generally assumed that sponges were first, but in the past decade some genetic studies have suggested that comb jellies – which superficially look like jellyfish – actually preceded them. The debate is ongoing: Penny would only say that finding early sponges doesn’t mean there weren’t also comb jellies very early, because such soft-bodied animals are rarely preserved.

Journal reference: Nature, DOI: 10.1038/s41586-021-03773-z

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