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Scientists at the University of California, Riverside, claim to have discovered the oldest known animal fossil, an old sea sponge that appeared between 660 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 world's first animal life forms. . The new evidence, presented by a UCR research team led by Gordon Love, suggest that marine sponges emerged as early as 660 million years ago, during the Neoproterozoic era, or at least 100 million years ago. years before the Cambrian explosion, a time when animal life has developed significantly in diversity and in number.
But not everyone is convinced by this latest study. An expert we spoke to said that these results are far from conclusive.
Finding the oldest fossilized animal in the world is a kind of Holy Grail for paleontologists. Unicellular microbial life appeared about 3.5 billion years ago – a lifestyle that has persisted for thousands of millions of years. Then, suddenly, in the Neoproterozoic era, a complex and multicellular life emerged, giving birth to the three kingdoms we still see today: animals (also called metazoans), plants and fungi. By discovering and studying the world's oldest animal fossils, scientists hope to gain important new information about early evolutionary processes and even 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 strange creature known as Dickinsonia, first appeared between 571 and 541 million years ago. The fossil evidence presented this week seems to be a little older, dating back about 100 million years. Interestingly, neither study provides traditional fossil elements, that is, fossils containing bone or soft tissue contours. On the contrary, these studies highlight the chemical traces left by ancient creatures. In the case of Dickinsonia, they were large molecules; in the case of sea sponges, it is a steroid compound called sterane. In both studies, scientists claimed that these molecular signatures could only be produced by animals.
"Molecular fossils are important for tracking the first animals, because the first sponges were probably very small, did not contain a skeleton, and did not leave well-preserved or readily recognizable body records," said the co-author. the study, J. Alex Zumberge, in a statement. "We were looking for distinctive and stable biomarkers that indicate the existence of sponges and other primitive animals, rather than unicellular organisms that dominated the Earth for billions of years before the dawn of a complex and multicellular life.
New fossils have been found in rocks and oil extracted from ancient formations in Oman, Siberia and India. The chemical analysis revealed the presence of a steroid compound known as 26-methylstigmastane, or 26-my for short. It is known that this distinctive chemical structure is produced only by demosponges, a species of modern marine sponges. This is a strong indication of the presence of eukaryotic cells, that is multicellular organisms such as plants and animals, according to the researchers.
"This biomarker of steroids is the first evidence that demosponges, and thus multicellular animals, thrived in ancient seas at least 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 discovered in southern Oman and dated about 650 million years ago. At the time, Love and his colleagues had stated that the 24-ipc steroid could be linked to early eukaryotic life, but critics pointed out that the compound could also be produced by demosponges and some modern algae, putting into effect doubt the results of this discovery. The new fossils contain traces of 24-ipc and 26-mes, the latter being unique to the demosponges, reinforcing the case.
The new evidence is "the first uniquely identified animal-specific steran marker in a geological archive" for chemical signatures known to come from demosponges, the authors conclude in their study. "These new findings strongly suggest that demosponges, and thus multicellular animals, were predominant in some Late Neoproterozoic marine environments."
It is not shocking that sea sponges are among the first forms of animal life on Earth. In a related study published in 2014, scientists linked genetics to paleontology to formulate a similar assertion, claiming that the first sponges had emerged about 650 million years ago.
Jonathan B. Antcliffe, a researcher at the University of Lausanne in Switzerland, said the new study did not prove anything.
"The biggest problem," he says, "is that UCR scientists claim that the sterol is made only by sponges. This is "contradicted by their own statements in the details of the additional information," Antcliffe told Gizmodo, adding that it was "where all the skeletons are still hidden".
"In some places of the newspaper, they say quite carefully that sterols have only been found in modern sponges, [but] it's quite different to say that they are made only by modern sponges, "he said. "By digging into the additional information, they freely admit that sterols in modern sponges could have been made by sponges, that they could be made from symbionts inside sponges or from something that the sponge would have eaten. We are there. Since sponges eat everything from bacteria to metazoan zooplankton, it does not help much to make an exclusive statement about the affinity of sponges in the depths. As usual, there is nothing here of real substance.
When Antcliffe says "as usual", he refers to his continued criticism of this work; it's he who has pointed out, for example, that 24-ipc can be produced by algae. The new document, it seems, has not changed its position on this line of research.
Large demands require great evidence, and hunting continues definitive proof of the world's oldest animal fossil.
[Nature Ecology and Evolution]Source link
