For decades, scientists have debated which animals first appeared on Earth and set the stage for the evolution of complex life. A recent study by MIT geochemists offers compelling evidence that the earliest animals were not dinosaurs or the unusual creatures of the Cambrian explosion, but ancient ancestors of today’s sea sponges. These soft-bodied, ocean-dwelling organisms likely represent the planet’s first multicellular animals, emerging long before more complex life forms evolved. By analysing chemical fossils preserved in rocks over 541 million years old, researchers identified molecular traces unique to sponges, revealing that these humble filter-feeders were among Earth’s pioneering animal species. This discovery reshapes our understanding of life’s origins and the early evolution of multicellular organisms.
Chemical fossils reveal the secrets of early skeleton-free animals
Fossils are typically thought of as bones or shells, but for creatures that had no skeletons, scientists must rely on a different type of evidence—chemical fossils. These are traces of biomolecules that once formed part of living organisms but became locked into rock sediments for hundreds of millions of years.In this case, researchers identified molecular markers called steranes, stable chemical remnants derived from sterols, which are essential components of cell membranes. The steranes found in ancient rocks were specifically linked to demosponges, a large and diverse group of sponges that still thrive in oceans today. The chemical fossils date back more than 541 million years, to the Ediacaran Period. This was a time when Earth’s oceans were dominated by soft-bodied organisms, many of which left no clear fossil record.For decades, scientists believed that the Cambrian explosion marked the true beginning of animal life, as it produced an abundance of complex organisms with shells and skeletons. However, this new evidence shows that sponges existed well before that dramatic evolutionary event, quietly filtering seawater and forming the earliest animal communities.
Global rock analysis confirms ancient sponges as Earth’s first animals
The MIT team analysed rock samples from multiple regions, including Oman, western India, and Siberia, to ensure their findings were globally relevant. Within these rocks, they discovered steranes containing 30 and 31 carbon atoms—chemical signatures not produced by plants, fungi, or bacteria, but only by certain genes unique to sponges.This rare discovery provided a molecular fingerprint, confirming that sponges were present long before other multicellular animals diversified. One challenge with chemical fossils is proving that the molecules truly come from ancient organisms and not from chemical reactions in rocks. To rule out a geological explanation, the team conducted laboratory synthesis experiments, recreating the conditions under which these molecules might form.Their results showed a clear biological origin, confirming that the steranes were genuine remnants of ancient sponge life.Roger Summons, an emeritus professor at MIT and co-author of the study, explained that this discovery brings together three lines of evidence—molecular traces from rocks, comparisons with modern sponges, and laboratory chemistry—to firmly establish sponges as Earth’s first known animals.
From simple sponges to dinosaurs: Evolution’s first steps
Dinosaurs often capture the imagination as symbols of prehistoric life, but they appeared hundreds of millions of years after sponges. These early sponges lacked bones, eyes, or nervous systems. Instead, they survived by filtering water for nutrients, making them simple yet remarkably resilient organisms.By predating the Cambrian explosion, sponges provided a critical evolutionary stepping stone. Their existence highlights how multicellularity and animal life emerged gradually, long before the rise of more complex creatures like fish, amphibians, reptiles, and eventually dinosaurs.Building on Past ResearchThis study builds upon earlier work from 2009, which first detected sponge-related chemical fossils in ancient rocks. At that time, scientists debated whether the evidence truly pointed to animals. The new findings strengthen that claim, providing far more robust molecular data and experimental validation.The research therefore not only confirms the sponge origin but also reshapes the timeline of animal evolution on Earth.
How studying sponges reveals the roots of animal diversity
The MIT team plans to expand their research by studying more ancient sediments around the world. By comparing rock samples from different continents, they hope to narrow down the precise period when the first animals appeared.This global approach could uncover new evidence about how early life adapted to Earth’s changing environments and reveal why sponges thrived before the Cambrian explosion. The discovery of sponge ancestors as Earth’s first animals has profound implications. These simple creatures represent the origins of multicellular life, paving the way for the rich diversity of species we see today.By studying sponges, scientists can better understand how basic cellular structures evolved into more complex systems, eventually giving rise to animals with skeletons, nervous systems, and advanced body plans.Also Read | Witness stunning night skies this week as Northern Lights and the Autumn Star shine across these states; when, where and viewing tips
