In the quiet corridors of MIT’s Department of Earth, Atmospheric, and Planetary Sciences, a team of geochemists has cracked open a mystery buried deep in time, one that spans over half a billion years. Their discovery, nestled in the molecular whispers of ancient rocks, suggests that some of Earth’s earliest animals may have been the soft-bodied ancestors of today’s sea sponges.
Published today in the Proceedings of the National Academy of Sciences, the study reveals “chemical fossils” preserved in rocks that are more than 541 million years old. These molecular remnants, steranes, are the geological echoes of sterols, like cholesterol, that once formed the membranes of living cells. And in this case, they point unmistakably to a class of marine creatures known as demosponges.
“We don’t know exactly what these organisms would have looked like back then,” says Roger Summons, Schlumberger Professor of Geobiology Emeritus at MIT, “but they absolutely would have lived in the ocean, they would have been soft-bodied, and we presume they didn’t have a silica skeleton.”
Demosponges today are the ocean’s gentle custodians, colorful, squishy filter feeders that come in a wide range of shapes and sizes. Their ancient relatives may have played a starring role in Earth’s biological origin story.
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The study’s authors include lead researcher Lubna Shawar, formerly a Crosby Postdoctoral Fellow at MIT and now at Caltech, alongside Gordon Love (UC Riverside), Benjamin Uveges (Cornell), Alex Zumberge (GeoMark Research), Paco Cárdenas (Uppsala University), and José-Luis Giner (SUNY College of Environmental Science and Forestry).
This isn’t the team’s first brush with primordial sponge chemistry. In 2009, they reported the presence of 30-carbon steranes, a rare type of molecular fossil, in rocks from Oman dating back to the Ediacaran Period (541–635 million years ago). These steranes, they argued, were likely derived from ancient sea sponges, suggesting that sponges may have predated the Cambrian explosion of complex life.
But the scientific community wasn’t entirely convinced. Alternative theories emerged, proposing that the steranes could have originated from other organisms, or even from non-biological processes.
Now, the MIT-led team has returned with a stronger case. Their new study identifies an even rarer sterane, one with 31 carbon atoms (C31), in the same ancient rocks. And this time, they’ve traced its origin with remarkable precision.
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“These special steranes were there all along,” Shawar says. “It took asking the right questions to seek them out and to really understand their meaning and from where they come.”
To build their case, the researchers examined rocks from Oman, western India, and Siberia, all of which date back to the Ediacaran Period. They searched for steranes, the molecular fingerprints of sterols found in all eukaryotic life.
“You’re not a eukaryote if you don’t have sterols or comparable membrane lipids,” Summons explains.
In humans, cholesterol contains 27 carbon atoms. Plant sterols typically have 29. But the steranes found in these ancient rocks had 30 and 31 carbon atoms, an unusual signature that pointed directly to demosponges.
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The team didn’t stop at rocks. They analyzed modern-day demosponges and found the same C31 sterols. Then, in a feat of chemical sleuthing, they synthesized eight versions of C31 sterols in the lab and subjected them to simulated geological processes, heat, pressure, and burial. Only two of the synthetic sterols matched the ancient rock samples, ruling out random geological origins.
“It’s a combination of what’s in the rock, what’s in the sponge, and what you can make in a chemistry laboratory,” Summons says. “You’ve got three supportive, mutually agreeing lines of evidence, pointing to these sponges being among the earliest animals on Earth.”
“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 adds.
The implications ripple far beyond sponge biology. These chemical fossils provide a new lens for examining Earth’s earliest ecosystems. They suggest that life’s complexity began not with a bang, but with a squish, a soft-bodied sponge quietly filtering seawater in a world still finding its biological footing.
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With C30 and C31 sterols now validated as biomarkers of ancient sponges, the team plans to expand their search to other regions and rock formations. Their goal: to pinpoint when, exactly, the first animals emerged.
So far, the evidence suggests that these sponge ancestors date back to the Ediacaran Period. However, with more samples, the timeline could become sharper and reveal new chapters in the story of life.
This research was supported by the MIT Crosby Fund, the Distinguished Postdoctoral Fellowship program, the Simons Foundation Collaboration on the Origins of Life, and the NASA Exobiology Program.
From squishy sea creatures to molecular fossils, the journey of life is written not just in bones and shells, but in the chemistry of stone. And thanks to the patient work of geochemists, we’re learning to read it, one sterane at a time.
Journal Reference:
- Lubna Shawar, Gordon Love, Benjamin Uveges et al. Chemical characterization of C31 sterols from sponges and Neoproterozoic fossil sterane counterparts. PNAS. DOI: 10.1073/pnas.2503009122
