Long before dinosaurs, trilobites, or even jellyfish swam the seas, Earth’s oceans were filled with creatures that looked like squishy tubes or blobs. They had no bones, no shells, and no nervous systems. Yet these primitive organisms may have been the first animals on the planet.
Now, a team of geochemists says they’ve found chemical fingerprints of these ancient pioneers locked inside rocks more than 541 million years old. The evidence points straight to sea sponges — specifically, ancestors of modern demosponges, the largest group of sponges alive today.
“It’s a combination of what’s in the rock, what’s in the sponge, and what you can make in a chemistry laboratory,” Roger Summons, an emeritus professor of geobiology at MIT, explained in MIT News. “You’ve got three supportive, mutually agreeing lines of evidence, pointing to these sponges being among the earliest animals on Earth.”
Sponges on Steroids
The story begins in 2009, when researchers studying ancient rock outcrops in Oman reported something surprising: an unusual molecular fossil called a C30 sterane. This compound is the fossilized form of a rare steroid. At the time, the team suggested it was a chemical leftover from ancient sponges.
The rocks dated back to the Ediacaran Period — a mysterious slice of Earth’s history between 635 and 541 million years ago. This was the prelude to the Cambrian explosion, when animal life suddenly flourished in spectacular diversity. If sponges really were present in the Ediacaran, they would push the origin of animals to well before the explosion.
But the claim sparked debate. Some scientists suggested that the chemicals could have been produced by bacteria, algae, or even through nonliving geological reactions. The discovery of C30 steranes, while intriguing, wasn’t enough to close the case.
Fast forward to the new study, published this week in the Proceedings of the National Academy of Sciences. Lead author Lubna Shawar, now at Caltech, and her colleagues looked this time not only for C30 steranes but also for an even rarer cousin: a C31 sterane.
“It’s very unusual to find a sterol with 30 carbons,” Shawar said in a press statement. “It took asking the right questions to seek them out and to really understand their meaning and from where they come.”
Building the Case for Ancient Sponges
The team collected samples from ancient rocks in Oman, western India, and Siberia. Inside those rocks, they detected both C30 and C31 steranes.
To confirm that sponges were the source, the scientists turned to living demosponges. They found that some species still produce the exact sterols that would eventually fossilize into C31 steranes.
They then took the final step. In the lab, they chemically synthesized eight different C31 sterols, simulating the long process of burial, pressure, and transformation that happens over hundreds of millions of years. Out of the eight, only two produced the exact fossil molecules found in the rocks. The match was uncanny.
“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,” Shawar said.
Unusual Animals
While they might not look the part, sponges are indeed animals — and the most basic of them all. These multicellular organisms lack cell walls (meaning they aren’t plants) and can produce sperm. Most of them live rooted in one spot all their lives in rivers or ocean bottoms.
Sponges are filter feeders. Their bodies contain a 3-D network of hollow channels to circulate water in and out. Simple as they are, they can filter up to 120,000 gallons of water per pound of tissue every day to vacuum-clean out organic matter such as bacteria, which they feed on.
Together, the evidence creates a molecular time capsule: sponges were filtering seawater millions of years before the Cambrian explosion rewrote the planet’s evolutionary story. Previously, researchers in Canada identified spangled stones as sponge fossils dated at 890 million years old. However, many experts disagree with the fossils’ spongy identity.
What does this mean for the bigger picture of life on Earth? If sponges were already thriving in the Ediacaran seas, animals may have been shaping ecosystems long before fossils like shells or bones appeared in the record. It also suggests that the origins of multicellular life weren’t explosive at all, but a slow burn that began with simple, squishy creatures.
The team now plans to look for these chemical fossils in rocks from other parts of the world. Each new sample could narrow down the timeline of when, exactly, the first animals emerged.
