635-million-year-old chemical fossils reveal Earth's first animals were sea sponges
By isabelle // 2025-10-02
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  • Scientists discovered chemical evidence of sea sponges from 635 million years ago.
  • This makes them the earliest known animals on Earth.
  • The evidence was found in ancient rocks from several continents.
  • Researchers confirmed the biological origin with modern lab techniques.
  • This pushes the timeline for animal life back before the Cambrian explosion.
In a discovery that rewrites the earliest chapters of life on Earth, scientists have found chemical fingerprints in ancient rocks suggesting that simple sea sponges were our planet's first animals. This groundbreaking evidence, uncovered by a team of researchers from MIT and other institutions, pushes back the timeline for animal life and reveals a profound new truth about our own biological origins. The research, published in the Proceedings of the National Academy of Sciences, identifies unique molecular fossils preserved in 635-million-year-old rocks from Oman, Siberia, and India. These ancient chemical signatures perfectly match the sterol compounds produced by modern sea sponges, providing a powerful new line of evidence in the quest to find Earth's first complex life. The team identified two previously unknown compounds, 24-n-butylcholestane and 24-sec-butylcholestane, which appear abundantly in these ancient rocks. Roger Summons, the Schlumberger Professor of Geobiology Emeritus at MIT, explained the significance of finding these rare molecules. "You’re not a eukaryote if you don’t have sterols or comparable membrane lipids," he stated, highlighting that these compounds are the hallmark of complex, cellular life.

Connecting past and present

To confirm their findings, the researchers conducted a meticulous forensic investigation. They synthesized eight different steroid compounds in the laboratory to compare against the ancient molecules. Only two matched, confirming a biological origin rather than random geological processes. The team also analyzed modern sponges, finding that when their sterols were chemically altered to simulate millions of years of burial, the results closely matched the ancient rock samples. Lead author Lubna Shawar described the moment of discovery. "These special steranes were there all along," she said. "It took asking the right questions to seek them out and to really understand their meaning and from where they come." This multi-pronged approach created what Summons called "three supportive, mutually agreeing lines of evidence, pointing to these sponges being among the earliest animals on Earth."

A new timeline for life

This discovery places sponges on Earth well before the Cambrian explosion, the period about 541 million years ago when most major animal groups suddenly appeared in the fossil record. These ancient sponges likely emerged as the planet was recovering from the "Snowball Earth" ice ages, a tumultuous period of extreme global freezing. The researchers addressed competing theories that suggested algae or other sources could have produced similar chemical signatures. They found critical differences, noting that while algae produce only trace amounts of these compounds, the ancient rocks show abundant concentrations pointing clearly toward sponge origins. The implications of this research extend far beyond academic curiosity. As Shawar explained, "In this study we show how to authenticate a biomarker, verifying that a signal truly comes from life rather than contamination or non-biological chemistry." This methodology opens new doors for investigating life's earliest history. What did these pioneering animals look like? While we may never know for certain, Summons offered a glimpse. "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." This humble beginning for animal life stands in dramatic contrast to the complexity that followed. The fact that such simple organisms were the starting point for all animal diversity, including humans, offers a profound lesson in biological humility. The search for life's true origins continues, but this discovery firmly plants the sponge's flag at the dawn of animal evolution. Sources for this article include: StudyFinds.org News.MIT.edu PopSci.com
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