Sea Sponges: Earth’s First Animals — New clues from ancient rocks
In a study highlighted by scientists today, MIT geochemists report new evidence from some of the planet’s oldest rocks that suggests the ancestors of modern sea sponges could be among the first animals to emerge on Earth. The findings, published in the Proceedings of the National Academy of Sciences (PNAS), add a twist to the long-standing debate about when animal life began and what those first animals might have looked like.
The idea that sea sponges were among the earliest animals is not new, but the new geochemical signals described by the MIT team provide a fresh line of evidence linking sponge-like life to the late Precambrian era — a time when Earth’s oceans teemed with simple, multicellular life and the fossil record is surprisingly sparse. The researchers focused on very old sedimentary rocks, some billions of years old in their lineage, to search for chemical fingerprints that only living, filtering organisms could create and preserve in minerals over vast spans of time.
The evidence from ancient rocks
Geochemists study tiny clues embedded in rocks — isotopes, trace elements, and mineral textures — to reconstruct life’s early steps. In this study, the team examined carbon and silicon isotopes, trace metals, and micro-scale mineral features that can indicate biological processing. They looked for a combination of signals that would be difficult to reproduce through non-biological, abiotic processes alone. The pattern that emerged aligns with what scientists might expect from sponge-like organisms that fed by filtering seawater, creating distinctive chemical footprints as they interacted with their environment over millions of years.
What makes these findings compelling is the convergence of several independent lines of evidence. Not only do isotope ratios shift in ways consistent with biological activity, but the mineral textures in the rocks hint at microstructures that resemble sponge skeletons. While not a direct fossil in the conventional sense, these clues collectively point toward a life form with a simple body plan that could give rise to more complex animals in later eons.
How geochemistry helps trace early life
The field of geochemistry offers a unique lens on the very beginnings of life because physical fossils from the deepest time are often scarce or poorly preserved. By measuring how elements were partitioned and altered during ancient diagenesis — the chemical changes that occur as sediments become rock — researchers can infer whether microorganisms played a role in shaping those rocks. In this case, the researchers argue that certain isotopic signatures and mineral associations are best explained by sponge-like organisms carrying out filtering and nutrient cycling within primordial oceans. This interpretation, while cautious, provides a plausible account that complements existing fossil and biomarker evidence.
Isotopic clues
Isotopes act as tracers of ancient metabolism. In biological systems, certain isotopic ratios deviate from what we would expect in purely non-biological processes. The MIT team reported fractions of carbon and silicon isotopes that are consistent with the kinds of fractionation associated with microbial filtration and sediment processing. If confirmed, these signals could indicate that early metazoans — animals with multiple cells — included lineages that eventually evolved into modern sponges.
Microfossil and biomarker clues
Beyond isotopes, the researchers examined micro-scale textures and mineral coatings that can accompany sponge habitation in ancient oceans. Where preserved, these features resemble the silica-based skeletons characteristic of modern sea sponges. The work also engages with the broader search for biomarkers — chemical remnants of ancient organisms — that would further anchor the sponge hypothesis. Taken together, the clues form a coherent narrative about early animal life without depending solely on rare body fossils.
Implications for the story of early animal life
If the interpretation holds, the timing of the emergence of animals could be pushed back, and the lineage leading to sea sponges may be closer to the root of the animal family tree than some researchers had assumed. Sponges are among the simplest multicellular organisms, lacking nerves and muscles, yet they occupy a foundational role in marine ecosystems. Demonstrating a sponge-like ancestor among Earth’s first animals would underscore how early life experimented with basic body plans that later evolved into the diverse animal kingdom we know today.
Next steps for research
Scientists emphasize that while the evidence is intriguing, it does not yet constitute a smoking gun. Additional work — including targeted sampling of other ancient rock formations, independent replication of isotopic patterns, and further microfossil search — is necessary to strengthen the case. The study nonetheless highlights the value of combining geochemistry with paleontology to illuminate periods of Earth history where the fossil record is sparse and time itself is measured in billions of years.
Why this matters
Understanding when and how animal life began informs our comprehension of evolution, biodiversity, and the ways life interacts with environment over deep time. By shining a spotlight on sea sponges as potential earliest animals, this line of research nudges the scientific conversation toward a more nuanced view of the very start of animal life on our planet.
