Researchers at MIT have discovered chemical evidence in rocks over 541 million years old suggesting that ancient sea sponges were among Earth's first animals. The findings, published in the Proceedings of the National Academy of Sciences, identify molecular fingerprints matching compounds from modern demosponges. This builds on earlier work and confirms the signals originate from biological sources rather than geological processes.
A team led by scientists from the Massachusetts Institute of Technology has uncovered traces of ancient life in Precambrian rocks, pointing to demosponges as likely precursors to the planet's earliest animals. The study analyzes steranes, stable remnants of sterols found in eukaryotic cell membranes, preserved in samples dating back more than 541 million years to the Ediacaran Period, which spanned from about 635 million to 541 million years ago.
The research revives and strengthens a 2009 discovery where high concentrations of 30-carbon (C30) steranes were detected in rocks from Oman. Skeptics had questioned whether these came from sponges or non-biological origins. Now, the team has identified even rarer 31-carbon (C31) steranes in the same Ediacaran rocks from Oman, western India, and Siberia. These molecules are tied to genes common in demosponges, soft-bodied marine filter feeders that thrive in today's oceans.
"We don't know exactly what these organisms would have looked like back then, 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," said Roger Summons, the Schlumberger Professor of Geobiology Emeritus at MIT.
To verify the biological source, the researchers examined living demosponges, which produce C31 sterols, and synthesized eight variants in the lab. After simulating geological burial, only two matched the ancient rock samples, ruling out random chemical formation. "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 explained. Lead author Lubna Shawar, now at Caltech, noted, "These special steranes were there all along. It took asking the right questions to seek them out."
The evidence supports demosponges evolving before the Cambrian explosion, when most animal groups diversified. The team, including Gordon Love from the University of California at Riverside and others, plans to search more global rock samples to refine the timeline of early animal emergence. This work was partly funded by the MIT Crosby Fund and NASA Exobiology Program.