When did life first appear on Earth?
A new study has found that ancient chemical traces known as biomarkers show that sponges existed at least 635 million years ago, perhaps back 660 million years, making it the oldest known animal life form.
A group known as Spongy vulgaris still exists today. According to the study, modern sponges produce the same chemicals that their ancient ancestors left behind: a steroid compound that only these sponges can produce.
These sponges may be the oldest animal life, but single-celled life on Earth should be there longer. For example, a microbial fossil in South Africa dates back 3.2 billion years and is currently the oldest fossil record of life on land, while other fossils dating back 3.5 billion years may be the oldest evidence of life on Earth.
Some early animal life, such as Digenthonia, lived 558 million years ago, leaving their mark on the rocks. But the bodies of ancient molluscs usually did not form fossils. In search of evidence of these elusive organisms, the researchers turned to biomarkers: molecules of the steroid 26-methyl stigma-motherrene.
In 2009, Love and his colleagues discovered another ancient compound, 24-isopropylchosterane (24-ipc), which is also thought to be a biomarker for sponges. The rocks of the Arabian Peninsula 635 million years ago. But it's hard to determine if sponges are the source, as certain species of algae can also produce 24-ipc.
On the other hand, according to this study, the steroid 26-mes has only been associated with modern demyelinating.
So far, it has not appeared in the bodies of any kind of animal, only on sponges.
The team analyzed sponge fossils found in rocks from around the world. The researchers reported finding traces of 26-mes molecules, as well as traces of 24-ipc, which are abundant in rock and oil excavated from India, Eastern Siberia and Oman.
The study found that sponges inhabited Earth's oceans at least 635 million years ago.
Chemical puzzles
However, there are still some unanswered questions about the use of biomarkers to identify ancient life.
Attributing biomarkers to specific animals is problematic. Why? In extinct organisms, it is almost impossible to check whether all possible sources match a particular chemical.
For example, 26-mes appeared in dinosaurs, not only the dinosaurs themselves had this substance, but also came to its food.
In fact, the 26-mes found in modern sponges may even have originated in another organism, such as a symbiotic bacterium, or something that the sponge had eaten.
Another puzzle is that perhaps steroids could allow early sponges to engulf and safely utilize nutrient-extracting symbiotic microbes, or that it could serve as a chemical defense against predators, but that's just speculation.
Nor do we know what life today has in common with the earlier sponges. Although sponges are the oldest animal life forms identified to date, they may not be alone, and there must have been many species that were thriving during the same period in the ancient oceans.
What do you think about the coolest old creatures that could be "sponges"? Welcome to leave a message in the comment area to participate in the topic discussion~
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