Sponge fossil may be oldest animal known to science

Most of the major groups of animals, including arthropods, mollusks, and worms, first appeared in the fossil record during the Cambrian Explosion 541 million years ago. But, according to an article published in Nature, the sponge fossils of northwestern Canada may be 350 million years older, significantly delaying the date of the earliest known animals on Earth.

The momentous discovery provokes a debate among paleontologists, who have long wondered when the first evolution of complex animal life occurred.

“If I’m right, animals appeared long before the first appearance of traditional animal fossils,” says study author Elizabeth Turner, a sedimentary geologist at Laurentian University in Sudbury, Canada-. This would mean that there is a long history of animals that just have not been very well preserved. “

Discovery

Some scientists, however, they are not convinced that the microscopic patterns of Turner’s 890 million year old fossils indicate an ancient sponge, given the evidence provided and available in the study.

“It’s such an important claim that you really need to rule out all other possibilities – warns Rachel Wood, a geoscientist at the University of Edinburgh, UK, who studies fossil reefs.. Microbes, for example, produce strange and wonderful shapes. Sometimes the crystals also grow in a way that resembles patterns created by living organisms. Which means the rock samples Turner found might not be fossils at all. “

Turner replies that none of the known reef-building organisms that existed 890 million years ago, such as cyanobacteria or algae, can explain the complex structures of your samples.

The specialist collected the supposed fossils of ancient microbial reefs preserved in the rocks of the distant Northwest Territories of Canada, during his graduate studies in the 1990s.

When Turner examined slices of rock under a microscope, he saw branching networks of crystal tubes. He later realized that these structures resembled the internal scaffolding of modern horny sponges and aligned with the decay and fossilization patterns expected of sponges, a collagen protein that forms its scaffold. “These rocks are beautiful, but you wouldn’t expect to find anything so complicated or weird about them,” he said.

Animal origin

It is only in recent years that Turner observed studies describing similar structures in much younger rocks, at a time when sponges were known to exist, that she felt confident to publish her findings. But these studies are also disputed on the grounds that they might not be true sponge fossils.

The authors of one of these articles They took photos of many thin slices of their rock sample and used them to generate a 3D model of the putative sponge. Wood says a similar test would have reinforced Turner’s claim.

“If Turner’s structures turned out to be sponge fossils,” says David Gold, a geobiologist at the University of California at Davis, “that would be very exciting and help us clarify the initial story of animal evolution. , A topic that has been debated for decades.

But good “It’s easy to find things that look like sponges in the fossil record,” he continues, “it’s harder to back it up with other evidence. He and other researchers, for example, have supported fossil detections indicating rock samples containing traces of biological molecules bound to sponges. “Unfortunately, given the great age and type of Turner rock samples, this type of preservation is not really possible,” he adds.

“It is not inconceivable that the sponges could have predated the Cambrian explosion”, explained Phoebe Cohen, a geobiologist at Williams College in Williamstown, Massachusetts. Scientists estimate how long the ancestors of groups of living animals diverged using molecular clocks, which measure the rate of DNA and protein mutations over time.

Most of these estimates suggest that the last common ancestor of all animals living today evolved before the Cambrian Explosion, “but not in 350 million years,” according to Cohen. However, you might be convinced that Turner’s samples are sponge fossils if you saw more evidence, including studies of how hot sponges fossilize.“.

Molecular clock estimates

The paleontological community is divided over whether the scarcity of animal fossils before the Cambrian period is due to creatures that lived and then rarely survived as fossils to this day, or because the molecular clock estimates of animal origins are incorrect. Gold is convinced that there were sponges before the Cambrian, but for him exactly how far is a big unresolved question.

“If the ancestors of modern sponges actually lived 890 million years ago, it means that the first animals survived very difficult living conditions, such as extremely low oxygen levels and periods of snow during which the planet’s surface almost completely froze, ”Gold analyzed.

Turner argues that the sponges could have survived the oxygen-poor environment by living in cavities and crevices of the microbial reef alongside photosynthetic cyanobacteria, which release oxygen. ANDThe sponges may also have fed on an organic exudate produced by the bacteria. “There was probably a delicious and very abundant supply of ‘snot’ for these filter-feeding organisms to feed on,” he said.

Scientists are passionate about their vision of the beginning of animal life on Earth. “The document could revive the debate that has been quietly smoldering for decades,” concludes Gold. But I suspect that the mentality of those who have made a decision on the age of the animals is not going to change much. “

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