The Cambrian Explosion is generally regarded as the kickoff for animal life, the evolution of which is thought to have begun about 541 million years ago, according to fossil evidence. A web of worm-like structures discovered by researcher Elizabeth Turner in a fossilized reef in subarctic Canada could turn this assumption upside down. She suspects that the net-like branches in the rock originate from the remains of a sponge’s skeleton that lived in the sea as early as 890 million years ago. This would make the structures the oldest animal fossils found to date. However, not all paleontologists share her hypothesis.
For a long time, paleontologists have argued about when complex animal life first evolved. Many scientists believe that until 800 million years ago, oxygen levels on Earth were too low for animals to survive. It was not until oxygen levels rose during the Neoproterozoic Era, 800 to 541 million years ago, that animal life became possible on Earth.
If the discovery by Elizabeth Turner, a geologist at Laurentian University in Canada and sole author of the study published in the journal Nature, proves correct, it would mean these fossils are 350 million years older than the earliest known animals. “If I’m right, animals emerged long, long before the first appearance of traditional animal fossils,” Turner tells Nature. “That would mean there’s a deep back history of animals that just didn’t get preserved very well.”
Turner discovered the fossils back in the 1990s as part of her graduate research while working in the Mackenzie Mountains of northwestern Canada. The rock is part of an ancient reef formed by photosynthesizing cyanobacteria and dated to be 890 million years old. Under the microscope at the time, she noticed branching patterns that she thought were too complex for microbes and more closely resembled structures from younger reefs. But no one could say what they were.
In later published studies, horn sponges of the taxonomic subclass Keratosa – still used today as bath sponges – have been described as leaving behind a branching pattern of the calcium mineral calcite that over time replaces the sponge’s fibrous skeleton, science journalist Michael Price writes in the journal Science. Turner says the pattern looks exactly like the structures she found in the ancient reef more than 20 years ago. Although these studies are also controversial, Turner felt encouraged by them to publish her discovery.
Other scientists also do not rule out the possibility that sponges have been around much longer than assumed, possibly even for more than a billion years. Gert Wörheide, a geobiologist at Ludwig-Maximilians-Universität in Munich, Germany, believes sponges could have evolved that early, based on his own genetic work, he tells Science. Turner’s findings are intriguing and plausible, but not convincing, he said.
Other skeptics argue even more clearly, and find Turner’s classification of the structures to be completely off-base. For example, Jonathan Antcliffe, a geologist and paleobiologist at the University of Lausanne, tells Science, “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’. Pretty much every major group of life can produce wiggly little structures.”
Rachel Wood, a geoscientist at the University of Edinburgh, is also not convinced, telling Nature that with such a big claim, all other possibilities must be ruled out. “Microbes, for example, produce weird and wonderful shapes and forms.” As she says, sometimes crystals also grow to look like patterns formed by living organisms, which means the rock samples Turner found may not be fossils at all.
“Sponges could have evolved and trucked along for a few hundred million years doing nothing in particular evolutionarily until a boom in oxygen levels sparked an evolutionary explosion.”Elizabeth Turner, geologist at Laurentian University, Canada and author of the study
Turner counters, explaining that cyanobacteria, the builders of the reef, do not form such complex patterns. Nor do algae and fungi create the angles visible in the structures. Instead, they resemble the internal skeleton of modern horn sponges and are consistent with expected decay and fossilization patterns. Therefore, it is entirely possible that the fossilized remains are from sponges that are 890 million years old, Turner said. The sponges may have obtained the oxygen, which was scarce at the time, from their immediate neighbors, cyanobacteria.
Turner gets support from Allison Daley, a paleontologist at the University of Lausanne, who thinks Turner’s work merits of further investigation: “It’s important to understand these ancient ecosystems, including the structures described in this paper, whether or not they are sponges.”
If Turner’s structures do indeed turn out to be sponge fossils, David Gold, a geobiologist at the University of California, Davis, told Nature “it would be very exciting, and help us nail down the early story of animal evolution.”
The debate over when animal life began on Earth has been going on among scientists for decades, Gold said, and the study could reignite it.
Julia Hager, PolarJournal
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