In a discovery that reshapes our understanding of life’s origins, researchers at the Massachusetts Institute of Technology (MIT) have found new evidence suggesting that the first animals on Earth were ancient sea sponges. The team uncovered chemical fossils, traces of long-buried biomolecules, preserved in rocks more than 541 million years old. These fossils, linked to the ancestors of modern demosponges, indicate that sponges existed long before the explosion of complex life during the Cambrian Period. The finding suggests that the earliest animals were soft-bodied, ocean-dwelling organisms, challenging previous assumptions about when and how animal life began on our planet.
Tracing Earth’s first animal through ancient rocks
The MIT team analysed rocks from Oman, western India, and Siberia, searching for biomarkers known as steranes, stable chemical remnants derived from sterols, which are essential components of cell membranes in complex organisms. They discovered rare C30 and C31 steranes that could only have originated from demosponges. These chemical fingerprints, preserved for over half a billion years, serve as molecular clues to Earth’s first animals.Unlike bones or shells, these molecules offer an indirect but powerful glimpse into ancient biology, revealing that sponge-like organisms thrived in oceans well before most other forms of animal life appeared.
Sponges on steroids: a chemical breakthrough
The research builds on MIT’s earlier 2009 findings, which first linked a specific 30-carbon sterane to ancient sponges. The new study strengthens that link by identifying an even rarer 31-carbon sterane, a chemical fossil that closely matches compounds found in modern demosponges. To confirm their results, the researchers recreated the same molecules in the lab, demonstrating that only sponge biology could have produced them naturally.According to lead scientist Lubna Shawar, these findings confirm that “the steranes found in ancient rocks were indeed produced by living organisms, not by random geological processes.” This offers one of the strongest molecular signatures of early animal life ever detected.
Why sponges matter in the story of evolution
Sponges may seem simple, but they represent a monumental leap in evolution, the transition from single-celled organisms to multicellular animals. By filtering seawater for nutrients, early sponges helped oxygenate Earth’s oceans, paving the way for more complex species to evolve. Their emergence likely transformed global ecosystems and set the stage for the Cambrian explosion, when life diversified dramatically.The study also highlights how molecular evidence can fill gaps left by the fossil record, especially for soft-bodied organisms that rarely leave physical traces.
A window into the distant past
Now that researchers have confirmed the biological origins of these unique steranes, they plan to expand their search for similar molecules in rocks from other regions and geological periods. Each new sample could help narrow down the timeline of when the first animals appeared.As Professor Roger Summons of MIT notes, “We’ve got three supportive, mutually agreeing lines of evidence pointing to sponges being among the earliest animals on Earth.”
