Ancient Chemical Fingerprints Point To Sea Sponges As Earth’s First Animals

Molecules preserved in 635-million-year-old rocks suggest sponges appeared long before the Cambrian explosion.

CAMBRIDGE, Mass. — Scientists have uncovered new molecular evidence that sea sponges, some of the simplest creatures on Earth, were likely among the planet’s first animals. The discovery centers on two previously unidentified chemical fossils found in rocks more than 635 million years old, offering fresh support for the idea that sponges emerged during a tumultuous period when Earth was thawing from extreme ice ages.

The research team, led by scientists at MIT and collaborating institutions, identified rare molecules preserved in ancient sedimentary rocks from Oman, Siberia, and India. These molecules work like fingerprints, matching the chemical signatures found in living sea sponges today but appearing only rarely elsewhere in nature.

The compounds they identified, known as 24-n-butylcholestane and 24-sec-butylcholestane, had never been recognized in the ancient rock record before. Their paper is published in the Proceedings of the National Academy of Sciences.

Matching Ancient Rocks to Modern Sponges

Finding these ancient molecules wasn’t straightforward. The research team had to create eight different steroid compounds in the laboratory to compare against the fossils, like making a lineup of suspects for identification. Only two of these lab-made compounds matched what they found in the rocks, confirming that the ancient molecules came from living organisms rather than random chemical reactions during burial.

The team also extracted and analyzed similar compounds from 11 modern sponge species. When they chemically altered these modern sponge molecules to simulate what happens over hundreds of millions of years, the results closely matched the ancient rock samples.

Interestingly, six types of sea sponges that produce abundant amounts of one previously identified sponge marker also yielded detectable amounts of both newly identified compounds. This pairing creates what researchers describe as “tandem biomarkers” that work together like a double signature, making it easier to identify ancient sponge presence with confidence.

One glass sponge species tested showed completely different patterns, producing mainly other types of compounds. This contrast helped confirm that the target molecules represent specific pathways in certain sponge groups.

When Sponges First Appeared on Earth

Finding these molecular fossils in rocks dating from roughly 635 to 541 million years ago places sponges on Earth well before the Cambrian explosion, a period when most major animal groups rapidly diversified. This timing matches recent fossil discoveries, including a late-era organism from South China identified as a possible sponge with an organic skeleton but no mineralized parts.

The research is more convincing because it addresses a period when Earth was recovering from severe “Snowball Earth” events, times when ice may have covered much of the planet. These extreme conditions may have created opportunities for multicellular life to emerge and diversify afterward.

Traditional sponge fossils from this era are scarce because early sponges likely lacked the hard skeletons that preserve well in rock. Chemical fossils fill this gap, providing evidence for life forms that left few physical remains.

One rock sample from Oman’s Masirah Bay Formation proved particularly revealing. As one of the least altered samples analyzed, it contained the highest amounts of both previously known sponge markers and the newly identified compounds. The ratio of key molecules reached 21.2, far higher than the 0.5 benchmark scientists often use to suggest sponge origin.

Think of it this way: if you were trying to prove someone had been in a room, finding one fingerprint might be suggestive, but finding multiple distinct fingerprints in high concentration makes the case much stronger.

Rocks from later periods showed sharply reduced levels of these sponge markers. This decline makes sense given that other suspension-feeding animals diversified and likely replaced sponges as dominant filter feeders in marine environments.

Ruling Out Algae and Other Sources

Scientists also addressed recent suggestions that similar chemical signatures in ancient rocks came from algae rather than sponges. Some scientists had proposed that single-celled organisms called Rhizaria or chemical alterations during burial could account for the unusual patterns seen in these ancient rocks.

But the researchers found critical differences pointing away from these alternatives. While Rhizaria produce only trace amounts of the relevant molecules, as little as 0.01% of their total content, the ancient rocks show abundant concentrations. Modern algae mainly produce other types of molecules and only trace amounts of the target compounds, making them unlikely main contributors to the ancient rock record.

The research team also looked at diagenesis, the chemical changes rocks undergo during burial. The pattern of molecules changed in ways consistent with heating during burial. This ruled out random alterations.

Advanced Methods Yield Molecular Secrets

Modern techniques made the discoveries possible. The research team used specialized equipment that can detect and identify molecules present in extremely low concentrations within complex mixtures, similar to how forensic scientists can find trace evidence at crime scenes. They verified their identifications by running samples through three different types of analysis and comparing the results.

These molecular signatures help scientists reconstruct ancient marine ecosystems and understand how early animals influenced ocean chemistry during a pivotal period in Earth’s history.

The discovery reinforces how chemical fossils can illuminate evolutionary events that left minimal physical traces. While debates about interpreting ancient markers continue, identifying multiple complementary molecular signatures builds a stronger case for when sponges emerged and provides tools for tracking their presence across different rock formations worldwide.