According to a new study, simple sea sponges were the first animal to appear on our planet about 640 million years ago, more than 100 million years before the Cambrian Explosion.
The results suggest that sea sponges are the source of an unusual molecule found in 640-million-year-old rock samples. The findings were published in the Proceedings of the National Academy of Sciences.
To learn more about life on Earth prior to the Cambrian Explosion, scientists turn to analyzing molecular signatures left in ancient rocks – so called molecular fossils.
“There’s a feeling that animals should be much older than the Cambrian, because a lot of animals are showing up at the same time, but fossil evidence for animals before that has been contentious. So people are interested in the idea that some of these biomarkers and chemicals, molecules left behind, might help resolve these debates,” said lead author Dr. David Gold from the Massachusetts Institute of Technology.
The team focused on a unique molecule known as 24-isopropylcholestane (24-ipc) which was found in 640 million year old rocks from Oman by scientists in 2009.
“It’s known that some modern sea sponges and certain types of algae produce 24-ipc today,” the scientists said. “But which organism was around to make the molecule 640 million years ago?”
Researchers identified a single-gene associated with turning on the production of 24-ipc, known as sterol methyltransferase (SMT). They then used evidence from the fossil record to determine when the first SMT gene showed up in sea sponges and algae.
No matter how they manipulated their analysis, they found that sea sponges evolved SMT much earlier than algae, and it did so around 640 million years ago — the same time period in which 24-ipc was found in rocks. The results strongly suggest that the 24-ipc signature found on the 640-million-year-old rocks came from a sea sponge, making them the oldest known animal.