What were the first animals to appear on Earth? New research points to a surprising candidate: sea sponges.
A team of geochemists from MIT and their collaborators has uncovered chemical traces in ancient rocks that strongly suggest that sponge-like creatures were living in the oceans more than 541 million years ago, making them some of the earliest animals on the planet.
The study, published in the Proceedings of the National Academy of Sciences, focused on “chemical fossils”—molecules left behind by organisms that lived long ago.
Unlike bones or shells, which rarely survive from soft-bodied creatures, chemical fossils are the molecular remains of compounds such as sterols, which are found in the cell membranes of complex organisms.
Over hundreds of millions of years, these sterols become preserved in rocks as stable compounds called steranes.
The researchers found special types of steranes in rocks that date back to the Ediacaran Period, which lasted from about 635 to 541 million years ago.
This period immediately preceded the Cambrian explosion, when life on Earth suddenly diversified into a wide range of complex organisms.
The steranes they detected closely match those produced by demosponges, a diverse group of sponges that still live in today’s oceans as soft-bodied filter feeders.
Roger Summons, MIT professor emeritus of geobiology and co-author of the study, explained that while scientists can’t know exactly what these ancient creatures looked like, the evidence strongly suggests that they were sponge ancestors.
“They would have lived in the ocean, been soft-bodied, and likely lacked the hard skeletons we see in some later sponges,” he said.
This new work builds on discoveries first made in 2009, when the same group reported the presence of unusual 30-carbon steranes in rocks from Oman.
These compounds, derived from rare sterols, were suggested to come from ancient sponges.
The finding was significant because it implied that sponges existed long before the Cambrian explosion. But skeptics argued that these chemicals could have been produced by other organisms or by geological processes unrelated to life.
To address these doubts, the team went further in their latest research. They analyzed rock samples not only from Oman, but also from India and Siberia, all dating back to the Ediacaran. In addition to the 30-carbon steranes, they found another, even rarer compound: 31-carbon steranes.
These molecules are strongly linked to sponges because they are produced by a specific enzyme found in modern demosponges.
The scientists confirmed this connection in several ways. They examined living sponges and found that some species still produce the same unusual sterols that could lead to the C31 steranes preserved in ancient rocks.
In the lab, they also chemically synthesized eight possible versions of C31 sterols and subjected them to processes that mimic what happens in sediments over millions of years. Remarkably, only two of these laboratory-made sterols matched the steranes preserved in ancient rocks. This finding ruled out the possibility that the steranes formed randomly or from non-living sources.
Lead author Lubna Shawar, now a researcher at Caltech, explained that this multi-step process—finding the molecules in rocks, confirming them in living sponges, and reproducing them in the lab—provides compelling evidence that these biomarkers truly came from sponges.
“It took asking the right questions to recognize these compounds and trace them to their biological origins,” she said.
The results strongly support the idea that sponges were among Earth’s first animals. While other life forms may have existed alongside them, sponges appear to have left behind the earliest clear chemical signature of animal life.
The implications are significant. If sponges were thriving in Earth’s oceans during the Ediacaran Period, it means that animal life had already begun evolving before the dramatic explosion of biodiversity in the Cambrian.
These simple filter feeders may represent the earliest step in a journey that eventually led to the wide variety of animal life we see today.
For Summons and his colleagues, the next step is to search for these chemical fossils in rocks from other parts of the world. By expanding the geographic range of their samples, they hope to narrow down the precise timing of when sponges, and perhaps the first animals, emerged.
“This study shows how you can authenticate a biomarker, proving it really comes from life and not from contamination or random chemistry,” Shawar said. “And in doing so, it strengthens the case that sponges were among the very first animals on Earth.”
