A new geochemical analysis of ancient rocks reopens an old question: who were the first animal inhabitants of Earth? The latest interpretations of molecular traces suggest that the ancestors of modern sponges—specifically demosponges—belonged to exceptionally early branches of the animal tree. At the heart of the story are "chemical fossils" in the form of steranes, durable remnants of sterols (such as cholesterol), which have been preserved in sediments hundreds of millions of years old.

Why sponges are an important candidate for the "first animal"

Sponges (Porifera) are among the simplest multicellular animals. They lack a nervous or digestive system in the form we know from most animals, and their biology is dedicated to filtering seawater through a network of canals and chambers. Precisely because of this simplicity, they have long been considered a potentially earliest branch of the animal kingdom. Demosponges, as the most numerous group today, appear in an incredible morphological and ecological diversity—from tiny, soft forms that encrust rocks to larger, vividly colored colonies. Their ancient relatives, although they did not leave us an abundance of classic fossils, very likely shared the same key traits: a soft body, a marine lifestyle, and a reliance on specific sterol-rich membranes.

Chemical fossils as a window into the Ediacaran Period

The Ediacaran (approximately 635–541 million years ago) was the period just before the Cambrian "explosion" of life. During this era, we find few hard skeletal remains in the rocks, but we do occasionally encounter molecular traces that have survived geological pressure, temperature, and time. This is where steranes come in—stable, saturated variants of sterols. Since sterols are fundamental building blocks of eukaryote membranes, and bacteria mostly use different lipids, the presence of steranes in old sediments is a signal that a eukaryotic world, including early animals, was already flourishing in the seas at that time.

What geochemists are looking for exactly: C30 and C31 signals

Sterols differ in the number of carbon atoms and in the structural "additions" on their side chain. In humans, for example, cholesterol is a C27 sterol, while plant sterols are most commonly C29. What intrigued researchers about fifteen years ago, and again today, is the frequent occurrence of very rare C30 steranes in Ediacaran rocks. These C30 steranes were interpreted as products of 24-isopropylcholesterol, an unusual sterol that, in today's oceans, is synthesized in significant quantities only by some demosponges. In more recent works, attention has also been focused on even rarer C31 derivatives, whose biological precursors have also been recorded in certain demosponge species. The combined occurrence of C30 and C31 steranes in the same stratigraphic sequences increases the likelihood that the source of the signals was animal, rather than abiotic or exclusively algal.

From Oman to Siberia: where molecular traces are sought

To confirm the continuity and source of the sterane signals, geochemical teams acquire samples from various geological provinces and lithological facies. Drill cores and outcrops from Oman's Huqf Supergroup, basins in western India, or Siberian platform sequences are often analyzed. Such a spatial "mosaic" allows for checking whether the occurrence of C30/C31 steranes is a local anomaly or a broader, repeatable pattern characteristic of a specific period and environment. When the signal is detected in multiple distant areas and across different rock types, the reliability of the paleobiological interpretation increases.

How to "read" a chemical fossil

The analytical procedure usually begins with the isolation of organic fractions from the sedimentary rock. Solvents and fractionation are used to separate the saturated molecules, followed by the application of gas chromatography combined with mass spectrometry. This identifies specific sterane isomers and homologs in the complex mixture. Special attention is paid to stereochemistry (e.g., configurations at C-20 or C-14), as the ratio of stereochemical pairs sometimes carries information about the source and degree of thermal maturity. Additional verification includes comparison with reference standards—either commercially available or synthesized in the laboratory—to confirm the structure and avoid confusion with closely related but biologically different molecules.

What living sponges tell us

To rule out the possibility that ancient C30 and C31 steranes come from a wide range of modern or extinct organisms, researchers also analyze the tissues of present-day demosponges. It has been shown that some species do indeed produce unusual sterols with an increased number of carbon atoms (C30 and even C31), including structural motifs that, after diagenetic changes, lead to the very same steranes found in old sediments. If we combine this with geological findings, we get a strong indication of evolutionary continuity: modern demosponges have retained the biochemical legacy of their ancient ancestors.

Three lines of evidence: rocks, sponges, and the laboratory

The thesis of demosponges as possible earliest animals is not based on a single observation, but on a triple verification. The first line is the "chemical fossils" themselves in Precambrian rocks (with an emphasis on the Ediacaran), where C30 steranes are consistently found, and not infrequently C31 equivalents as well. The second line comes from the biology of living sponges, where sterols with the same number of carbon atoms and specific side chains have been documented. The third line is organic synthesis and experimental diagenetic transformation: when chemically synthesized precursors subjected to heat and pressure yield the exact sterane patterns detected in the rocks, the statistical probability of a chance coincidence drops sharply.

But science does not advance without debate

Of course, the hypothesis of demosponges as the main source of C30 steranes has also prompted alternatives. Some studies, for example, have pointed out that traces of similar compounds can be found in some algae or that microbial symbionts could play a larger role in the methylation of the sterol side chain. It has also been suggested that certain biosynthetic and diagenetic pathways might be more complex than initially assumed. This is precisely why recent works combine geochemistry, genomics, and biochemistry: the genes for methyltransferase enzymes in sponges and their symbionts are being examined, it is being tested whether bacterial enzymes can replicate the "sponge" signatures, and the results are then compared with the geological record across time and space.

Why the distinction between C30 and C31 is important

C31 sterols and their diagenetic products (C31 steranes) have so far been less frequently documented in nature than their C30 analogs, but their recognition in modern demosponges and in the fossil record is valuable for at least two reasons. First, the additional "signature" reduces the risk of misattribution that can occur if we rely on a single biomarker. Second, the C30:C31 ratio, in combination with other markers (e.g., the proportion of C29 steranes characteristic of green algae), can tell us something about the paleoenvironment: whether it was dominated by coastal, shallow-sea conditions, stronger algal production, or niches where sponges could thrive without competition.

What the Ediacaran tells us about the origin of complex life

The Ediacaran biota is known for its soft-bodied, often flat organisms whose relationships with modern groups are not always clear. In this world of soft bodies, the absence of hard skeletons makes it difficult for us to interpret evolution solely based on classic fossils. That is why molecular traces like steranes are particularly important: they fill in the gaps and provide an unprecedented insight into when the first animals truly appeared. If demosponges indeed left a C30/C31 signature before the "big bang" of Cambrian diversity, then their appearance would precede many later flourishing groups—which completely changes the arrangement on the "timeline" of early animal evolution.

Methodological rigor: how false leads are avoided

In the meantime, the geochemical community has tightened its protocols to avoid contamination and misinterpretation. Laboratories work with "blank" control samples, use isotopic and stereochemical analyses, and sample series come from multiple independent sources. Of particular importance is the consistency of the signal in stratigraphic sequences of the same area and its correlation with other paleobiological indicators. Only when a sufficient number of puzzle pieces are assembled—from geology and sedimentology to biochemistry and genetic data—does the hypothesis move from suggestive to convincing.

What modern enzymes tell us

A key breakthrough was the work on enzymes that "methylate" the sterol side chain. The identified genes and enzymes in demosponges, as well as in their symbionts, have been experimentally verified in vitro. This confirmed that natural biochemical pathways can indeed produce the unusual sponge sterols, which then, over time and under geological conditions, transform into the recorded C30 and C31 steranes. If the same functionality can be consistently linked to demosponges, and alternative sources are limited to traces without geological robustness, the hypothesis of sponges as the first animals gains additional weight.

The global picture: from individual sites to a planetary story

When the same or similar biomarkers emerge in the rocks of Oman, India, Siberia, and other regions, and their profiles match the expected diagenetic transformation and paleoenvironmental variations, we get a global mosaic of the early biosphere. In some sequences, the signal appears in higher concentrations, elsewhere it is more discreet, but what is repeated is the presence of sponges as silent but persistent protagonists of the pre-Cambrian seas.

What's next: the search for a finer time needle

To date, based on the analyzed samples, it can be said with certainty that the sedimentary records with C30 and (in certain cases) C31 steranes are located within the vast window of the Ediacaran. But the exact moment of the appearance of the earliest animals is still being calibrated. The next step is a systematic, global search for additional sequences—including less explored ones—to "sharpen" the time scale: did demosponges establish themselves in the mid-Ediacaran or only in its final phase, just before the Cambrian?

For readers: a short glossary

• Sterols/Steranes: sterols are membrane lipids of eukaryotes; in the geological record, we find their stable, saturated derivatives—steranes—as molecular fossils.


• C30/C31: a designation for the number of carbon atoms in sterols/steranes; they are more unusual than the common C27–C29 and are therefore used as a "signature" of certain biological sources.


• Demosponges: the largest group of sponges, very diverse, with proven abilities to synthesize unusual sterols.


• Ediacaran: the period between about 635 and 541 million years ago, immediately before the sudden increase in the complexity of life in the Cambrian.

Is this the end of the debate?

No; and it shouldn't be. Science progresses through verification, repetition, and re-examination. The fact that C30 steranes have been discovered in a wide range of Proterozoic and early Cambrian sediments is a strong argument in favor of demosponges, but every claim must be re-tested on new samples, with even stricter controls and increasingly sophisticated instruments. Parallels with modern demosponges, including the discoveries of rare C31 sterols in certain species, provide a consistent picture, but researchers are still actively mapping where this signal is unequivocal and where it could be confused with algal or microbial traces.

To whom the answer matters

The seemingly esoteric question of whether the first animal was a sponge is actually fundamental to understanding our own origins. If sponges were indeed the pioneers of the animal world, then key innovations—multicellularity, cell coordination, early forms of communication—are pushed deeper into the past than macrofossil collections alone have suggested. This, in turn, affects the ways we interpret the evolution of other animal groups, as well as environmental conditions: oxygen levels in the oceans, food webs, and the geochemical cycles of carbon and sulfur.

Looking ahead: what this means for future research

As new samples from different basins are accumulated and detection techniques advance, we can expect a finer "resolution" of the molecular record. Interdisciplinary syntheses are also in play: combining biomarkers with stable isotope signals, with traces of early bioturbation, with microfossils, and with analyses of ancient genomes. Although the story of C30 and C31 steranes is currently the most coherent narrative we have for the early trace of animals, it is still being supplemented. Every new sample, every new analysis can push the boundaries of understanding who and when first walked the path that leads to today's animal diversity.

For readers who want to delve deeper into the context, introductory concepts about the Ediacaran and sponges can be further explored through general encyclopedic overviews or educational websites. For example, an overview of the Ediacaran period is available here: Ediacaran Period – Britannica, and a concise description of demosponges here: Demosponges. Note: the links serve as an introduction to the topic and basic definitions of terms.