DO these fossilized structures belong to Earth’s first animals?

2021-08-28

Tiny, wormlike structures embedded within a fossilized Canadian reef may have been formed by the skeletons of ancient sea sponges some 890 million years ago, a controversial study argues. If the claim bears out, the structures would represent the oldest animal fossils yet found. The results would also suggest animals thrived even before a massive surge in Earth’s oxygen levels that began about 800 million year ago—an event thought by many to have catalyzed the evolution of animal life. But other scientists say the paper doesn’t do enough to single out sponges as the source of the fossils.

The findings are intriguing and plausible, says Gert Wörheide, a geobiologist at Ludwig Maximilian University of Munich whose own genetics work predicts that sponges could have arisen this early. Still, he’s not convinced the fossils are truly sponges. “I’d prefer if there was a separate line of evidence.”

Elizabeth Turner, the study’s sole author and a geologist at Laurentian University, first discovered the fossils as a graduate student in the ’90s, when working in a remote part of the rugged Mackenzie Mountains that separate the Yukon and the Northwest Territory. The ancient reef, which was formed by photosynthetic bacteria known as cyanobacteria, has been dated using a number of geological methods to be about 890 million years old. There are no roads near the site; to collect samples, Turner had to helicopter in and engage in a bit of “sketchy” mountaineering, she says.

Initially, her research focused on the reef itself. She ground down fist-size chunks of rock into sections 30 microns thin, then analyzed their contents under a microscope. Some samples contained tiny, branching structures that appeared to her to be too complex to have been made by microbes; they also resembled structures she’d seen in much younger reefs. But neither she nor her colleagues could figure out what they were. Turner finished her doctorate and moved onto other research projects, but she never forgot the mysterious branching pattern.

In the ensuing decades, other researchers pinned down the process by which “horny sponges”—still used today as bath scrubbers—leave behind a branching pattern of the chalky mineral calcite that gradually replaces the sponge’s fibrous skeleton. The pattern looks identical to the structures Turner found in her ancient reef almost 20 years ago, she says. “This is a paean to slow science.”

The cyanobacteria that built the reef don’t make such complex patterns, Turner says. And the angles of the microscopic structures don’t match anything known to be produced by algae or fungi. What’s more, scientists like Wörheide have suggested sponges could have arisen perhaps 1 billion years ago. This is based on estimating how long it would take for modern sponge lineages to evolve—though these estimates are not universally accepted.

For these reasons, the samples may contain the fossilized remains of 890-million-old sponges, Turner argues today in Nature. That would make them about 350 million years older than the previous oldest known animal fossil—a flat, saucer-shaped creature known as Dickinsonia that lived on the sea floor almost 600 million years ago.

Turner is careful to call her fossils “putative sponges,” as she is aware that such a bold claim will invite skepticism. The planet 890 million years ago only had a fraction of the oxygen it has today. Many scientists believe it wasn’t until the so-called called Neoproterozoic oxygenation event between 800 million and 540 million years ago that animal life became possible.

Yet even before this event, reef-building cyanobacteria would have created an “oxygen oasis” for marine life forms, Turner argues. “Sponges could have evolved and trucked along for a few hundred million years doing nothing in particular evolutionarily,” she says, until a boom in oxygen levels sparked an evolutionary explosion.

Jonathan Antcliffe, a geologist and paleobiologist at the University of Lausanne, doesn’t buy it. “She’s found some wiggles in a rock, performed a Rorschach inkblot test on them, and said, ‘They sort of vaguely remind me of a sponge,’” he says. “Pretty much every major group of life can produce wiggly little structures.”

Still, Allison Daley, a paleontologist at the University of Lausanne, says Turner’s work merits further investigation. “It’s important to understand these ancient ecosystems, including the structures described in this paper, whether or not they are sponges.”

sciencemag.org, 28 July 2021
; https://www.sciencemag.org