Sea sponges are nothing like the creatures you see in a zoo or on a wildlife show. They don’t have brains, eyes, or even organs.
But according to a new study, their ancient relatives may have been among the first animals to ever exist on Earth.
The search for Earth’s first animals
The scientists behind the study found what they call “chemical fossils” in rocks more than 541 million years old. These aren’t bones or shells. They’re molecules that once came from living things and somehow stuck around in the Earth’s crust through heat, pressure, and time.
What the researchers found were special forms of steranes – chemical compounds that come from sterols, like cholesterol. Sterols are used by almost all complex life to build and maintain cell membranes. In this case, the steranes had 30 and 31 carbon atoms – something very unusual.
Today, only a few organisms make sterols that long. One of them? A type of soft, filter-feeding sea sponge known as a demosponge.
These sponges are still around and can be found in oceans all over the world. They come in all shapes, sizes, and colors. And their ancient cousins might have looked a lot like them, minus the fancy skeletons.
Digging into the past
The research team included scientists from MIT, Caltech, and several universities across the U.S. and Europe. According to study lead author Lubna Shawar, the key to the mystery was asking the right question.
“These special steranes were there all along,” said Shawar. “It took asking the right questions to seek them out and to really understand their meaning and from where they come.”
Back in 2009, the same team found early hints of sponge-related steranes – specifically the 30-carbon kind – in rocks from Oman. But not everyone agreed that sponges were the source. Some said the molecules might have come from other microbes or even non-living chemical processes.
This time, the researchers dug deeper – literally. They collected rock samples from Oman, western India, and Siberia.
Evidence of ancient sponges
The samples dated back to a time known as the Ediacaran Period. This point in Earth’s history, between 635 and 541 million years ago, occurred right before life on Earth exploded in complexity during the Cambrian Period.
The rocks were full of both C30 and C31 steranes. To confirm where these compounds came from, the team also studied living sponges and even synthesized eight different C31 sterols in the lab.
The experts mimicked the natural processes that happen when something gets buried in sediment for hundreds of millions of years – heat, pressure, chemical reactions.
The results showed that only two sterols broke down into the same form of C31 steranes found in the rocks. The others didn’t match. That suggests a very specific biological source – one that lines up with modern-day demosponges.
Why sterols matter
Sterols are found in all eukaryotes – organisms with complex cells. Plants, animals, fungi, and some single-celled organisms all need sterols to function.
However, the kind of sterol an organism produces depends on its genes. In humans, cholesterol has 27 carbon atoms. In plants, sterols usually have 29.
“It’s very unusual to find a sterol with 30 carbons,” explained Shawar. That rarity is what makes the sponge connection so important.
“You’re not a eukaryote if you don’t have sterols or comparable membrane lipids,” said Roger Summons, one of the study’s senior authors.
The team says these findings are backed by three solid lines of evidence. “It’s a combination of what’s in the rock, what’s in the sponge, and what you can make in a chemistry laboratory,” noted Summons.
“You’ve got three supportive, mutually agreeing lines of evidence, pointing to these sponges being among the earliest animals on Earth.”
From sponges to humans
Now that scientists have solid proof that certain steranes come from ancient sea sponges, they plan to look for these chemical fossils in rocks from other parts of the world. That could help them narrow down the exact time when the first animals started showing up in Earth’s oceans.
“In this study we show how to authenticate a biomarker, verifying that a signal truly comes from life rather than contamination or non-biological chemistry,” said Shawar.
This research doesn’t just help us understand sponges. It helps us understand ourselves – where life came from, how it evolved, and what clues it left behind in the most unexpected places. Like a patch of stone in the desert that once held the floor of an ancient ocean.
Buried in that stone are the chemical fingerprints of one of Earth’s earliest animals, living long before the first fish, insects, or dinosaurs ever showed up.
The full study was published in the journal Proceedings of the National Academy of Sciences.
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