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An international team of researchers claims to have found traces of cholesterol on a fossil Dickinsonia-A mysterious creature who lived during the primordial ediacarian period. According to the researchers, these proofs Dickinsonia the oldest known animal in the fossil record. But the discovery is not without critics, who say that the new work is not convincing.
Is it or is it not an animal?
That's the question that scientists have been asking for decades about Dickinsonia. Measuring up to 4 feet long (1.4 meters) and featuring rib-shaped segments that line its oval-shaped body, this enigmatic organism dates back to the Ediacaran (there are between 571 million and 541 million years ago), a period immediately preceding Cambrian – a time when animal life has exploded in terms of diversity and number.
New research published today in Science suggests Dickinsonia was a real animal, not a fungus, plant or protozoan (unicellular organisms), as suggested previously. The evidence used to support this claim is quite extraordinary: cholesterol molecules found in an age of 558 million years Dickinsonia fossil near the white sea northwest of Russia. The researchers, led by Ilya Bobrovskiy of the Australian National University, believe that cholesterol, a type of fat, was produced by the individual when he was alive and that cholesterol can only be produced by animals. Dickinsonia therefore deserves the designation of the animal.
"The fossil fat molecules we found prove that animals were large and abundant 558 million years ago, millions of years earlier than expected," said co-author Jochen Brocks of the new study and associate professor at ANU. in a report. "Scientists have been fighting for more than 75 years on what Dickinsonia and other strange fossils of biota Edicaran [the totality of all life within a particular geological period] were: unicellular giant amoebae, lichens, failed evolutionary experiments, or the first animals on Earth. The fossil fat now confirms Dickinsonia as the oldest known animal fossil, solving a decades-old mystery that has been the holy grail of paleontology.
Brocks said the mystery was "resolved," but other experts are not so sure that the evidence is both inconclusive and unconvincing.
The animals, also known as metazoans, are one of the three main kingdoms of the larger group of eukaryotes (complex multicellular organisms), the other two being plants and fungi. Scientists use various characteristics to identify animals. The most important, in addition to being eukaryotic, is that they can gain energy by engulfing other (ie heterotrophic) organisms; they can move spontaneously at some point in their life cycle (that is, they are mobile); they reproduce sexually; and they do not have a rigid cell wall.
The key to this discovery was the discovery of lipid biomarkers in the Dickinsonia fossil. A biomarker is essentially a substance that indicates the presence of biological processes. Incredibly, after 558 million years, this fossil still contains traces of organic matter in the form of lipids, namely cholesterolides, "trademark of animals," according to the researchers.
Bobrovskiy and his colleagues identified biomarkers of hydrocarbons (molecular fossils of lipids and other biological compounds) extracted from Dickinsonia fossil using a technique known as gas chromatography – mass spectrometry. This allowed them to detect specific molecules in the fossil and to measure the abundance of these compounds. A "striking abundance" of cholesterol molecules – more than 93% of the extracted organic matter – was detected in the sample. This is much more than the 11% detected in the surrounding sediments.
Importantly, the Dickinsonia the fossil was devoid of ergosteriods – a telltale biomarker of fungal life. According to other researchers, other organisms, such as choanoflagellates and filasteries (single amoeba-like organisms), would not likely produce the biomarkers observed in fossils.
"Our results make these emblematic members of the Ediacara biota the oldest confirmed macroscopic animals in rock recording, indicating that the appearance of Ediacara biota was indeed a prelude to the Cambrian explosion of animal life. "Conclude the researchers.
Alex Liu, a paleontologist at Cambridge University who did not participate in the new study, says the new document is "remarkable" in that scientists have been able to recognize traces of original organic molecules in a fossil as well. former. The significance of this discovery, he says, is that researchers have been able to confirm that a member of the Ediacaran biota – a group of fossils that has long been hard to identify – is very likely to be an animal.
That said, Liu believes Dickinsonia It was probably not the first animal on Earth.
"It's already a complex organism, and it would have its own ancestors," Liu told Gizmodo. "There are also several other fossils of animal candidates known to record Ediacarans, which may be tens of millions of years old, while molecular clock studies suggest that early animals may have evolved to 'to 100 million years before the appearance of Dickinsonia. The important thing here is to be able to confirm that an Ediacaran fossil as an animal – this has been very difficult to do on the sole basis of morphology, but we now have traces fossils, development data and biomarkers. "
Roger Summons, professor of geobiology at MIT, says the new study is "robust" and "demonstrates that it is possible to collect molecular information about ancient and enigmatic fossil tissue." This was done on much younger materials, "but never, to my knowledge, on a fossil anterior to the Cambrian explosion," he told Gizmodo.
Jonathan B. Antcliffe, principal investigator at the University of Lausanne in Switzerland, was less charitable when asked to comment on the new document.
"I find the study totally unconvincing," Antcliffe told Gizmodo. "There is a long history of very strong claims because of evidence from biomarkers that ultimately do not really represent much."
In particular, Antcliffe did not like the way in which researchers have concealed alternative hypotheses, rejecting, for example, the possibility that the fossil is contaminated, which "could easily be," he said. In addition, he believes the new study limits the fossil to a position among eukaryotes.
"There is no one arguing for the alternative position that Dickinsonia is bacterial, "said Antcliffe. "Nobody thinks that Dickinsonia is bacterial. No one. So, we already know that it is a type of eukaryote. There are many different eukaryotes and the authors select a few examples and reject them quickly before immediately going to an animal conclusion.
He says Dickinsonia could very well be one of the first animals, but there is "very little evidence" to suggest it. He recommends that researchers analyze the anatomy of fossils to further corroborate their claims.
So, is it an animal?
The debate, it seems, continues.
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