[ad_1]
Hundreds of millions of years ago, one of the very first animals on Earth died at the bottom of an ancient ocean. In life, he was a humble sea sponge; in death, he had no bones, teeth, or shell to leave as proof of his brief existence. But there were fat molecules – or so it seemed.
In 2009, a much later group of animals (human scientists) were studying a plaque of ancient marine sediment when they discovered the fossilized remains of what appeared to be these same sponge fat molecules, trapped among the rocks. . The sediment was dated to 635 million years ago – about 100 million years earlier than the oldest confirmed animal fossil on the record – but the ancient molecules were unmistakably biological in origin and matched those found in modern sponges, the researchers wrote. As more and more of these biomarkers were discovered on ancient seabed samples, a question arose: how could these early sponges be so prevalent and not leave a single fossil body behind?
Now two new articles in the journals Nature ecology and Evolution provide an answer. These ancient fat molecules weren’t from an animal at all, the researchers say, but rather from an old and rotten one. algae whose molecular remains have been transformed by the ravages of geological time. After mimicking these geological processes in a laboratory experiment, the researchers succeeded in changing modern algal molecules into fats identical to those found in ancient sediments.
“All of this means that the evidence previously linked to Earth’s oldest animals comes from algae and not from animals,” said Lennart van Maldegem, co-author of one of the studies and a geoscientist at the Australian National. University. “This moves the oldest definitive evidence of animals from nearly 100 million years ago, to Dickinsonia fossil print, about 558 million years ago. “
Secrets in the rock
The crucial molecule in all of these studies is a separate type of sterol, or fatty compound, called C30 24-isopropylcholestane – essentially, a drop of fat enveloped in 30 carbon atoms. While some sterols, such as cholesterol, are prevalent in animals (including humans), these unique C30 sterols are almost exclusively associated with demosponges – a diverse class of marine animals that comprises about three-quarters of all sea sponges known on Earth.
When the authors of the 2009 study found an abundance of C30 sterols in sediment samples 635 million years old, they interpreted it as evidence that ancient marine sponges had lived and died there. , a million centuries before Earth became filled with animals during an event known as the Cambrian explosion (about 540 million years ago). However, this is not the only plausible interpretation of these sterols.
“We now know that the first algae (the ancestor of modern plants) became prominent in all of the world’s oceans 200 million years before the Cambrian explosion,” van Maldegem said. “So, in our study, we looked again at the molecules preserved in these old sediments.”
In addition to the sterols attributed to sponges, the researchers also found an abundance of molecules associated with algae, as well as non-biological molecules resulting from a geological process called diagenesis, or the process that turns granular sediments into hard rocks over millions of people. years. Van Maldegem compared the process to baking bread; after being exposed to heat and pressure for hundreds of millions of years, an “irreversible chemical reaction” changes the molecular structure of the sediment, much like a hot oven turns a moist soft dough into a firm loaf of bread. Sometimes, says van Maldegem, these processes can also alter organic molecules.
In ancient sediment samples, sterols associated with sea sponges and molecules produced by diagenesis were often located right next to each other, suggesting that the two molecules emerged through similar geological processes, van Maldegem said. . To test whether these processes could have turned old algal molecules into the appropriate C30 sterols, the team removed various molecules from modern red and green algae samples and then heated them to over 600 degrees F (315 C) in a furnace and exposed them to intense pressure – conditions intended to mimic geological processes that normally take millions of years.
The process was successful in transforming some algal molecules into the same sponge-bound sterols present in the 635 million year old sediment. This means that these ancient sterols are not animal evidence at all, but algae evidence, van Maldegem said. If so, the first evidence of animal life on Earth may have just been canceled.
It’s still possible that animals had started evolving on Earth hundreds of millions of years before that, van Maldegem said, but scientists should no longer look to these particular fatty sponge molecules for proof of this.
“We took a close look at the first evidence for the existence of animals on Earth… and we have a better understanding of how the first animals evolved,” van Maldegem said.
Originally posted on Live Science.
[ad_2]
Source link