The oldest animal fossils ever found, or just squiggles?

In a paper published in the journal Nature, Elizabeth Turner, a geologist at Laurentian University in Ontario, described the branching, tubular structures she observed while examining ultra-thin slices — about as thick as a human hair — of what were once reefs in a prehistoric ocean. Dr. Turner suggests the mesh-like structures closely resemble the fiber networks of modern keratose sponges, also known as horny sponges, found around the world today. But she concedes that what she saw under a microscope may not clarify when animal life first emerged on Earth.

“It could just be some microbial wiggles,” said Jonathan Antcliffe, an evolutionary biologist specialising in sponges at the University of Lausanne in Switzerland. The evidence backing up the claim that they’re remnants of an ancient sponge, he said, “is very, very thin.” The putative fossils were extracted from the 890-million-year-old Little Dal reefs of northwestern Canada, which are now exposed parts of the Mackenzie Mountains. If verified, they would predate the oldest undisputed sponge fossil by about 350 million years — a span of time longer than today and when dinosaurs first evolved.

“We’re talking about inserting hundreds of millions of years without a trace” of fossils, said Graham Budd, a paleobiologist at Uppsala University in Sweden who was not involved in the paper. “It would be sensational. It would be like finding a computer chip in a 14th-century monastery.” The research highlights the challenges of identifying and making sense of the earliest available fossil records when scientists cannot be sure exactly what to look for, and when there’s not much to look at. “If we expect the first animals to be tiny and soft,” said Maja Adamska, an evolutionary biologist at the Australian National University who was not involved in the new paper, “this is the best we can expect.” There have been other controversial claims regarding the timeline of the emergence of life on Earth. Discoveries of ancient, “spongelike” fossils have been reported — and later disputed — in Namibia, Russia, Australia, China, Newfoundland and Ukraine. In the early 1990s, scientists at the University of California, Los Angeles, claimed that they had found the world’s oldest, bacteria-forged fossils, which were later dismissed as odd-shaped minerals. In 2016, scientists suggested 3.7-billion-year-old conical structures found in Greenland extended the fossil record back by 200 million years; two years later, three-dimensional analysis refuted the fossil claims.

In the study of sponge evolution, horny sponges are notoriously challenging. They lack many of the animal’s most distinct and fossil-friendly features, including mineralised skeletons and skeleton-like spicule structures. Accordingly, there is no scientific consensus for how to tell their earliest potential fossils apart from the geological traces of ancient organisms such as fungi or bacteria — or something else that has since gone extinct. If her hypothesis bears out, the 890-mn-year old sponge fossils prompt other difficult questions: How did sponges survive ice ages and dramatic rises in oxygen? How did they manage to escape fossilisation or evade the human gaze until now? And when did animal life truly begin to evolve on Earth?

Renault is a journalist with NYT©2021

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