What the Question Was
For decades, scientists have debated what the very first animals looked like. While fossils from soft-bodied organisms are rare, genetic analyses have pointed in two directions: either today’s demosponges (sponges) represent the closest living relatives of the first animals, or comb jellies (ctenophores) might fill that role.
New research led by scientists at the Massachusetts Institute of Technology (MIT) shifts the balance toward the sponge hypothesis. By decoding chemical clues preserved in ancient rocks, the team argues that sponges emerged in the Neoproterozoic era, more than 541 million years ago, making them strong candidates for among the earliest animal life on Earth.
The Challenge of the Fossil Record
The problem is clear: soft-bodied organisms rarely fossilize well. Sponges can leave behind biomineralized spicules, but early sponges may have lacked these rigid structures altogether. Comb jellies, for their part, have leaned on genetic data to support their early appearance, yet direct fossil evidence remains scanty.
To overcome these gaps, researchers looked for chemical signatures—biomarkers—that could survive long after a soft-bodied creature disappeared. This approach demanded careful authentication to avoid mistaking non-biological signals for life’s fingerprints.
Unlocking Ancient Sterols
In a key advance, the team built upon earlier findings of sponge-like sterols in Marinoan rocks from Oman. These C30 sterols are rare fatty compounds tied to the membranes of many animals, including modern sponges. Their discovery suggested ancient sponge activity long before the Cambrian explosion.
MIT organic geochemist Lubna Shawar notes, “It’s very unusual to find a sterol with 30 carbons,” pointing to a link with sponge biology. Critics argued the Oman steranes could arise from non-biological or algal sources. The new study tackles this by identifying a second biomarker, a C31 sterol, produced by a sponge gene that also creates C30 sterols and appearing in some living demosponges today.
Shawar adds that this dual signal helps authenticate a chemical fossil—evidence that life, not chemistry alone, produced these molecules. MIT geobiologist Roger Summons emphasizes the strength of having three converging lines of evidence: rock chemistry, sponge biology, and controlled laboratory simulations of fossilization.
Lab Experiments That Strengthen the Case
The researchers simulated billions of years of rock processing by subjecting eight forms of synthetic C31 sterols to fossilization-like conditions. Remarkably, two samples preserved exactly as the ancient C31 remnants, reinforcing the interpretation that these molecules originated from ancient sponges.
“You’ve got three mutually supporting lines of evidence,” Summons says. “These sponges were among the earliest animals on Earth.” Shawar echoes the sentiment, highlighting that the special steranes were present all along, waiting for the right questions to reveal their meaning.
What This Means for Our View of Early Life
With ancient chemical signatures now tied more securely to sponges, scientists can reinterpret the earliest chapters of animal evolution. The findings don’t prove the exact appearance of the first animals, but they do strengthen the case that soft-bodied sponges inhabited the ancient oceans well before other multicellular groups.
Looking ahead, the team plans to examine additional geological samples to uncover more clues about our most distant ancestors. The fossil record may be sparse, but chemistry is offering a clearer map of life’s earliest branches.
Bottom Line
These results push the origin of animals deeper into the Neoproterozoic, with chemical biomarkers from ancient rocks pointing to sponges as among the very first animals. While absolute visuals remain speculative, the chemical story aligns with a sponge-like early animal—the frontier of life’s grand experiment.
