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Pale, wormlike tubes in 890-million-year old rock may be ancient sea sponges, a new study concludes. If confirmed, that controversial claim would push back the origin of the earliest sponges by about 350 million years and make the tiny squiggles the oldest known fossils of animals, by far.

Crucially, these fossils would imply that animals emerged in environmental conditions previously thought unworkable for animal life, geologist Elizabeth Turner reports July 28 in Nature.

Early in Earth’s history, the ocean mostly lacked oxygen. It wasn’t until a large pulse of the gas to the atmosphere about 800 million to 540 million years ago, known as the Neoproterozoic Oxidation Event, brought atmospheric oxygen levels to within 10 to 50 percent of modern levels, boosting the amount of oxygen in surface ocean waters (SN: 12/11/19). “But sponges are different from other animals,” says Turner, of Laurentian University in Sudbury, Canada. “Some sponges in the modern world and in the rock record are known to be tolerant of comparatively low oxygen relative to modern ocean levels.”

Until now, the earliest, unambiguous fossils of sponges date to about 540 million years ago to the beginning of the Cambrian Period, when an extreme burst in the evolution of animal diversity took place (SN: 7/29/13). Some other animals are known from just a bit earlier, but go too much further back in time and identities become less clear (SN: 3/9/15). Based on genetic data and their relative simplicity, sponges are generally thought to have been the earliest form of animal life.