A modern demosponge species, which makes the same 26-mes steroids that the researchers found in ancient rocks.
Image: Paco Cárdenas

Scientists from the University of California, Riverside, are claiming to have discovered the oldest known animal fossil—an ancient sea sponge that emerged between 660 million and 635 million years ago.

New research published this week in Nature Ecology and Evolution describes a probable biomarker, or “chemical fossil,” linked to ancient sea sponges—a group of creatures considered to be among the earliest forms of animal life on the planet. The new evidence, presented by a UCR research team led by Gordon Love, suggests sea sponges emerged as early as 660 million years ago, during the Neoproterozoic Era, which is at least 100 million years before the Cambrian Explosion—a time when animal life expanded significantly in diversity and number.

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But not everyone is convinced by this latest study. An expert we spoke with said these findings are far from conclusive.

Finding the world’s oldest fossilized animal is a kind of Holy Grail for paleontologists. Single-celled microbial life emerged around 3.5 billion years ago—a mode of life that persisted for thousands of millions of years. Then suddenly, at some point during the Neoproterozoic Era, complex, multicellular life emerged, giving rise to the three kingdoms we still see today: animals (also known as metazoans), plants, and fungi. By finding and studying the most ancient animal fossils on Earth, scientists hope to gain important new insights into the early processes of evolution, and even into how life might appear elsewhere in the galaxy.

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Last month, an international team of researchers claimed to have discovered the world’s oldest fossil—a bizarre creature known as Dickinsonia, which first appeared around 571 million to 541 million years ago. The fossil evidence presented this week appears to be a bit older, dating further back in time by around 100 million years. Interestingly, neither study provides traditional fossil evidence, that is, fossils containing the outlines of bones or soft tissue. Rather, these studies highlight the chemical traces left behind by ancient creatures. In the case of Dickinsonia, it was fat molecules; in the case of sea sponges, it’s a steroid compound known as a sterane. In both studies, the scientists claimed that these molecular signatures could only have been produced by animals.

“Molecular fossils are important for tracking early animals since the first sponges were probably very small, did not contain a skeleton, and did not leave a well-preserved or easily recognizable body fossil record,” study co-author J. Alex Zumberge said in a statement. “We have been looking for distinctive and stable biomarkers that indicate the existence of sponges and other early animals, rather than single-celled organisms that dominated the earth for billions of years before the dawn of complex, multicellular life.”

The new fossils were found in rocks and oil pulled from ancient formations in Oman, Siberia, and India. Chemical analysis revealed the presence of a steroid compound known as 26-methylstigmastane, or 26-mes for short. This distinctive chemical structure is only known to be produced by demosponges—a species of modern sea sponges. This is a strong indication for the presence of eukaryotic cells, i.e. multicellular organisms such as plants and animals, according to the researchers.

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“This steroid biomarker is the first evidence that demosponges, and hence multicellular animals, were thriving in ancient seas at least as far back as 635 million years ago,” Zumberge said.

This is a critical discovery for the team, which produced similar results in 2009, but using a different steroid biomarker: 24-isopropylcholestane (24-ipc). This biomarker was discovered in rocks found in South Oman, and dated to about 650 million years ago. At the time, Love and his colleagues said the 24-ipc steroid could be linked to early eukaryotic life, but critics pointed out that the compound can also be produced by both demosponges and a few modern algae, throwing the findings into doubt. The new fossils contain traces of both 24-ipc and 26-mes, the latter of which is unique to demosponges, thus bolstering the case.

The new evidence is “the first animal-specific sterane marker detected in the geological record that can be unambiguously linked” to chemical signatures only known to come from demosponges, the authors conclude in their study. “These new findings strongly suggest that demosponges, and hence multicellular animals, were prominent in some late Neoproterozoic marine environments.”

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It’s not shocking that sea sponges should be among the first forms of animal life on Earth. In a related study published in 2014, scientists combined genetics with paleontology to make a similar claim, saying early sponges emerged around 650 million years ago.

Jonathan B. Antcliffe, a senior researcher at Lausanne University in Switzerland, said the new study doesn’t prove anything.

“The biggest problem,” he says, is that the UCR scientists are claiming that the sterol is being made by the sponges alone. This is “contradicted by their own statements in the details of the supplementary information,” Antcliffe told Gizmodo, adding that fthis is “where all the skeletons are always hidden away.”

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“In some places in the paper they rather carefully say that sterols have only been found in modern sponges, [but] this is rather different from saying that they are only made by modern sponges,” he said. “So digging through the supplementary information, they freely admit that the sterols in modern sponges could have been made by the sponges, that they could be made from symbionts inside the sponges, or they could be from something that the sponge had eaten. So there we are. As sponges eat everything from bacteria to metazoan zooplankton that doesn’t really help much to make this an exclusive statement of sponge affinities in deep time. As usual there is nothing here of real substance.”

When Antcliffe says “as usual,” he’s referring to his ongoing critiques of this work; it was he who pointed out, for example, that 24-ipc can be produced by algae. The new paper, it would appear, hasn’t changed his mind about this avenue of research.

Big claims require big evidence, and the hunt continues for definitive proof of the world’s oldest animal fossil.

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[Nature Ecology and Evolution]