"Nature": This spongy fossil may come from the earliest known animal on earth

Most major animal groups-including arthropods, molluscs, and nematodes-first appeared in the fossil record during the Cambrian outbreak 541 million years ago

This ancient discovery has sparked debate among paleontologists, who have long debated when complex animal life began to evolve

"If I'm right, animals have appeared long before traditional animal fossils first appeared," said Elizabeth Turner, a sedimentary geologist at Laurentian University in Sudbury, Canada and author of the study

Weird and wonderful shape

However, based on the evidence provided by the research, some scientists do not believe that the microscopic patterns in Turner's 890 million-year-old fossils indicate that they are ancient sponges

Rachel Wood, a geoscientist at the University of Edinburgh who studies coral reef fossils, said: "This is such a major assertion.

What caused the Cambrian explosion? 

Turner countered that none of the currently known reef-building organisms, such as cyanobacteria or algae, that existed 890 million years ago could explain the complex structure in her sample

She began collecting so-called ancient microbial reef fossils from rocks in the remote northwestern region of Canada during her graduate studies in the 1990s

When Turner examined the rock slices under a microscope, she saw the branching network of crystallized tubes

She said: "These rocks are beautiful, but you don't expect to find complicated or strange things in them

Early history of animal life

It was not until the past few years when she saw studies describing similar structures in younger rocks that Turner was confident of publishing her findings

The author of one of the papers took photos of many thin slices of rock samples and used them to generate 3D models of so-called sponges

If Turner’s structure does prove to be a fossil sponge, said David Gold, a geologist at the University of California, Davis, “It will be very exciting and help us determine the early stories of animal evolution.

But he said that although it is “easy to find sponge-like things in the fossil record,” it is more difficult to use other evidence to support them
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For example, he and other researchers pointed out that the rock samples contained traces of sponge-related biomolecules, thus supporting the fossil claim
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Unfortunately, given the age and type of Turner's rock samples, this method of preservation is impossible, he added
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Phoebe Cohen, a geologist at Williams College in Williamstown, Massachusetts, said it’s not incredible that sponges existed before the Cambrian eruption
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Scientists use a "molecular clock" (an instrument that measures the mutation rate of DNA and protein over time) to estimate the time of ancestral differentiation of existing animal populations
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Cohen said these estimates mostly indicate that the last common ancestor of all living animals evolved before the Cambrian explosion, but less than 350 million years
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Nevertheless, she said that if she saw more evidence, including research on how horny sponges became fossils, she would believe that Turner’s samples were fossil sponges
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Molecular clock estimation

For the lack of animal fossils before the Cambrian, paleontologists are divided: Is it because pre-Cambrian creatures rarely survive in the form of fossils to this day, or because the molecular clock's estimate of the origin of animals is wrong
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Jin is sure that there were sponges before the Cambrian, but he said how early the sponges appeared is an unresolved big question
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Ancient worm fossils can trace the origin of animal life 

Gold said that if the ancestors of modern sponge animals really lived 890 million years ago, it means that early animals survived under very difficult living conditions, such as extremely low oxygen levels and the "snowball earth" period.
During this period, the surface of the earth was almost completely frozen
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Turner believes that sponges can survive in a low-oxygen environment, relying on barely surviving in the caves and crevices of the microbial reef, next to them are cyanobacteria that can carry out photosynthesis, which can release oxygen
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Sponges may also be nourished by organic slime produced by bacteria
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She said: "These filter-feeding creatures may eat delicious and very rich'snot'
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Scientists are enthusiastic about their views on when animal life on Earth will begin
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Gold said that this paper may reinvigorate the debate that has been quietly brewing for decades
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"But I suspect that for those who have already decided on the age of animals, this will not change a lot of thinking
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