Spoiler alert: they are not animals at all.
It was quite a groundbreaking discovery when scientists in the Middle East stumbled upon 635 million-year-old fat molecules – so-called sterols – apparently left behind by sponges a decade ago. Since sponges are classified as animals, the molecules went down in the books as the oldest evidence for the presence of animals on Earth. And apparently animals were involved relatively early in the history of our planet. But a new study is undermining the spectacular results. The traces in question were not left at all by sponges, researchers argue in the magazine Nature Ecology and Evolution. Instead, they can be traced back to algae; much simpler organisms that were already known to exist on Earth 1 billion years ago.
Younger
In a way, it makes the find a lot less spectacular. And it also has major consequences for our understanding of the evolution of animals on Earth. Because where it was previously cautiously assumed that they already existed on Earth 635 million years ago, we now have to conclude that the oldest convincing evidence for their presence is almost 100 million years younger.
Doubts
It must be admitted that not everyone was immediately convinced when researchers presented their oldest evidence for the presence of animals on Earth ten years ago. “In recent years, several scientists have questioned these results,” says researcher Lennart van Maldegem. Scientias.nl. The doubts arose, among other things, from the fact that the researchers found spongy sterols in rocks that were once part of a seabed, which suggested that sponges were already abundant on Earth 635 million years ago, but at the same time there was never a fossilized print of a sponge. from that time has been discovered. And that is strange. But the researchers were never able to capitalize on their doubts until recently, says Van Maldegem. “This has everything to do with the fact that the traces that researchers found ten years ago and attributed to sponges can really only be produced by sponges today.”
Geological processes
However, Van Maldegem and colleagues have looked again at sterols and discovered that they can also arise in a different way. “We were able to demonstrate that certain molecules derived from common algae can be altered by geological processes to form molecules that are indistinguishable from those produced by spongy animals.”
We know about algae that they have existed on Earth for a long time. And now also that they can fool us quite a bit – with a little help from geological processes. Image: Ilya Bobrovskiy.
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New analysis
The researchers discovered the true nature of the molecular remains when they took a closer look at the supposed oldest traces of animals. In doing so, they again encountered sterols that we know can in principle only be produced by sponges. But they also found molecules that are known to form only when sediments undergo a geological transformation and turn into stone, or fossilize. And that made you think. Because if those molecules have changed because the sediments they were part of petrified, could the same thing have happened to the spongy molecules? And so are those spongy molecules in reality perhaps not created by sponges at all? “The process (whereby sediments turn into rocks, ed.) Is comparable to baking bread,” explains Van Maldegem. “You start with a soft, malleable dough, and after it has been in a warm oven for a while, you have a firm loaf. An irreversible chemical reaction takes place in the furnace, and the same principle applies to the molecules of dead organism in sediments, if exposed to heat and pressure for a long time, a chemical reaction can occur, and although this reaction does not normally change the molecules drastically. it can in some cases cause molecules to change just a little bit. After careful analysis, we found a strong correlation between the molecules derived from geological transformation and the ‘sponge molecules’. ”
Experiment
To ensure that the molecules previously attributed to sponges have indeed become ‘spongy’ due to geological processes and thus do not attest to the presence of sponges, an experiment was then set up. “In the laboratory we simulated these conditions (which are at play during the geological transformation in which sediments turn into stone, ed.) By heating sterols from algae and it was confirmed that algae sterols can indeed be modified with exactly the same results. These results mean that the fat molecules discovered in these ancient sediments, previously interpreted as diagnostic of sponges, come from algae, and are not the oldest evidence for animals on Earth. ”
Still surprising
Although there were already doubts about the nature of the sterols, the results did surprise Van Maldegem. He didn’t expect it to lead to the demise of that spectacular find ten years older, he says. But that has happened. “And our results push the oldest evidence for animals on Earth back by nearly 100 million years.” Because the oldest traces for the presence of animals that we still have today are ‘only’ 558 million years old. It is then a fossil of a Dickinsonia.
Fossil of Dickinsonia. Image: Ilya Bobrovskiy.
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Older finds
The big question is, of course, whether we will still find animals that are older in the future. Van Maldegem certainly does not rule it out. From so-called molecular clock studies (in which researchers can calculate on the basis of differences in the genetic code of every animal on earth when the common ancestor of all those animals lived) shows that animals evolved somewhere between 900 and 635 million years ago. “So there is certainly a chance that older evidence for animals will be found in the future. The problem, however, is that animals older than 550 million years did not yet have skeletons, shells or teeth and it is precisely those hard tissues that have a good chance of being preserved as a fossil. So in order to find animal traces we have to look for other traces, such as molecular fossils, or body prints of animals without skeletons, as in the case of the 558 million year old Dickinsonia fossil. ”
Undoubtedly, there is still much to discover. But the further we go back in time, the more difficult it gets. And yet researchers do not give up. Because it is very important to get clear when and how the animals on earth originated. “On the one hand, we are naturally curious about where ‘we’ come from,” says Van Maldegem. “On the other hand, if we understand how life on earth has evolved, we can also better understand how life on other planets, if any, develops.”
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