In 2017, researchers found tiny, fossilized filaments that, at the time, appeared to be trace fossils of worms or other small marine animals between 542 and 555 million years old. Their astounding age easily made the fossils one of the oldest discovered instances of animals on Earth—if, that is, the traces truly came from animals.

According to new research published in Gondwana Research , the answer is a disappointing “no.” When a different team revisited the fossils with advanced imaging techniques, it uncovered evidence strongly suggesting that the filaments were algae and bacteria.…