By George Dvorsky, Gizmodo
Ancient rocks from northwestern Canada have been found to contain structures consistent with sea sponges. At 890 million years old, they could be the oldest known animal fossils on Earth.
Simple, single-celled life forms first appeared on Earth about 3.4 billion years ago, but it took a while for more complex animal life to emerge. The Cambrian Explosion of complex lifeforms happened around 540 million years ago, which coincides with the oldest undisputed sponge fossils on record.
In 2018, the discovery of steroids—a known biomarker—in rocks dated to between 660 million and 635 million years ago pushed the emergence of sponges to the Neoproterozoic, which is at least 100 million years before the Cambrian. Genetic analyses of modern sponges likewise suggests an early origin for these sea creatures, further reinforcing the notion that sponges were the first animals to appear on Earth.
Paco Cardenas, a biologist at Uppsala University in Sweden who wasn’t involved in the new study but is an expert on sponges, said this discrepancy between the fossil record and the chemical and DNA evidence “has been highly debated these past years.” Hence the importance of the newly reported discovery, which has implications for how we understand the origin of all animal life on Earth.
Elizabeth Turner from Laurentian University in Sudbury, Canada, is the sole author of the new Nature paper, and she reports on the discovery of fossil-like structures found in rocks pulled from the 890-million-year-old Little Dal reefs in the Stone Knife Formation of Canada’s Northwest Territories.
“If these fossils are confirmed to be sponge fossils, they would definitely be the first fossils of animals in the fossil record, thus pushing back the emergence of animals to 350 million years before the Cambrian,” Cardenas explained in an email. This recent discovery “may finally reconcile those [previous] lines of evidence,” but it “also raises new questions,” he added.
Peering at the rocks with a microscope, Turner noticed tube-shaped structures covered by mineral calcite crystals. These features bore a striking resemblance to the fibrous skeletons of horny sponges, and they formed from the decay of these ancient creatures, she argues. Horny sponges are still around today, and you might even be using one as a bath sponge (so yeah, humans might actually be descended from bath sponges, and as a dedicated fan of Spongebob Squarepants, I’m actually very cool with that).
Turner was able to rule out other interpretations of the microstructures owing to the configuration of the fossil material.
“It consists of little tubes that branch divergently and then rejoin to form a complex, three-dimensional meshwork,” she said in an email. “Of the branching organisms that could be considered as alternative interpretations, none of them have that kind of three-dimensional meshwork—not true algae, not bacteria, and not fungi.”
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