University of Bristol Researchers Reconcile Molecular and Paleontological Sponge Evolution Evidence
Sponges are among Earth’s most ancient animals, but exactly when they evolved has long puzzled scientists pursuing understanding of early animal origins. Genetic information from living sponges, as well as chemical signals preserved in ancient rocks, suggest sponges evolved at least 650 million years ago, predating the Cambrian explosion when most animal phyla appeared in the fossil record.
This molecular evidence has proved highly controversial as it predates the fossil record of sponge skeletal remains by a minimum of 100 million years, creating a persistent gap between genetic estimates and paleontological observations. Now an international team of scientists led by Dr. M. Eleonora Rossi from the University of Bristol’s School of Biological Sciences have solved this evolutionary conflict by examining the independent evolution of sponge skeletons across different taxonomic groups.
The research published today (January 7, 2026) in Science Advances provides compelling evidence that early sponges were soft-bodied organisms lacking the mineralized skeletons characteristic of modern sponge species, explaining why their fossils do not appear in ancient rock formations despite genetic evidence indicating earlier origins.
Living Sponges Possess Microscopic Glass-Like Skeletal Needles
Living sponges have skeletons composed of millions of microscopic glass-like needles called spicules that provide structural support and protection against predators. These mineralized spicules also have an extremely good fossil record, dating back to around 543 million years ago in the late Ediacaran Period when conditions favored preservation of these hard skeletal elements.
The conspicuous absence of sponge spicules from older rock formations has led some scientists to question whether earlier molecular estimates for the origin of sponges are accurate, suggesting the genetic clock calculations might contain systematic errors or that rock chemistry signals attributed to sponges actually originated from other organisms or non-biological processes.
Two-Step Analytical Approach Resolves Evolutionary Timeline Discrepancy
Dr. Rossi and her research team solved this long-standing paleontological mystery using a comprehensive two-step analytical approach combining multiple lines of evidence. Firstly, they integrated high-quality data from 133 protein-coding genes with fossil evidence to construct a new, more accurate timescale for sponge evolution using sophisticated molecular clock models accounting for rate variation across lineages.
They dated the origin of sponges to between 600-615 million years ago, substantially closing the gap with the fossil record while still preceding the first appearance of mineralized spicules by several tens of millions of years. Secondly, they investigated the evolutionary history of sponge skeletons using comparative genomics and statistical modeling, revealing that spicules evolved independently multiple times in different sponge groups rather than arising once in a common ancestor.
Dr. Rossi, Honorary Research Associate at Bristol, explained: “Our results show that the first sponges were soft-bodied and lacked mineralized skeletons. That’s why we don’t see sponge spicules in rocks from around 600 million years ago — there simply weren’t any skeletal elements to preserve in the fossil record.”
Modern Sponge Skeletons Built Using Entirely Different Genetic Pathways
Dr. Ana Riesgo, a world-leading expert in sponge evolution from the Museum of Natural Sciences in Madrid, Spain, noted that researchers already had some clues suggesting sponge skeletons evolved independently rather than being inherited from a common skeletal ancestor. Modern sponge skeletons may look superficially alike when examined under microscopy, but they’re built in fundamentally different ways using distinct biochemical processes.
Some sponge groups construct skeletons from calcite, the mineral that makes up chalk and limestone, while others utilize silica, essentially biological glass chemically similar to quartz. When researchers examine their genomes using comparative genomic approaches, they discover that entirely different genes are involved in skeletal construction across sponge lineages, providing strong molecular evidence for independent evolutionary origins.
This genetic evidence indicates that the ability to produce mineralized skeletons arose multiple times during sponge evolution, likely representing adaptive responses to similar environmental pressures such as predation risk, physical disturbance in wave-swept environments, or structural support requirements for larger body sizes.
Markov Process Statistical Models Reject Early Skeleton Hypothesis
In order to reconstruct sponge skeleton evolution with quantitative rigor, the research team used sophisticated statistical computer models commonly employed in diverse scientific and commercial applications. Dr. Joseph Keating, also an author on the study, explained the methodological approach: “We used a Markov process, a type of predictive model that’s widely applied in fields like finance, artificial intelligence systems, search engines, and weather forecasting.”
By modeling transitions between different skeletal types, including soft-bodied forms lacking any mineralization, the team found that almost all models strongly reject the idea that the earliest sponges possessed mineralized skeletons. Only an unrealistic model treating all mineral types as evolutionarily equivalent suggests otherwise, and even then the statistical results remain ambiguous without strong support for skeletal ancestors.
The Markov modeling approach allows researchers to estimate the probability of evolutionary transitions between different character states based on the distribution of traits observed in living and fossil species, accounting for phylogenetic relationships and the time available for evolutionary change along each lineage.
Early Sponge Diversification Drivers Remain Tantalizing Mystery
The results of this study raise intriguing questions about what ecological or environmental factors drove early sponge evolution and diversification if skeletal structures were not important initially. Professor Phil Donoghue, Professor of Palaeobiology at the University of Bristol, observed: “Given that nearly all living sponges have skeletons composed of mineralized spicules, we might naturally assume that spicules were important in early sponge evolution.”
“Our results challenge this assumption, suggesting that early sponge diversification was driven by something else entirely—and what those drivers were is still a tantalizing mystery requiring further investigation,” Donoghue continued. Possible alternative explanations include innovations in feeding mechanisms, chemical defenses against predators, reproductive strategies, or physiological adaptations to changing ocean chemistry during the Ediacaran Period.
Sponge Evolution Research Illuminates Earth-Life Coevolution Processes
Professor Davide Pisani, Professor of Phylogenomics at the University of Bristol, emphasized the broader significance extending beyond sponge biology: “But this is not only about sponges. Sponges are the first lineage of reef-building animals to evolve and might as well have been the very first animal lineage, although this evolutionary priority is still actively debated among researchers.”
Understanding sponge evolution provides key insights into the origin of the very first reef systems on Earth, which fundamentally transformed marine ecosystems by creating complex three-dimensional habitats, altering sediment dynamics, and modifying ocean chemistry through biological filtration. “This is about how life and Earth co-evolved, and how the evolution of early animals changed our planet forever, ultimately enabling the emergence of the animal life forms we are familiar with, humans included,” Pisani concluded.
Sponges maintain a fundamental role in modern marine ecosystems as important filter feeders processing enormous volumes of seawater, removing bacteria and organic particles while recycling nutrients back to the water column, supporting food webs and influencing biogeochemical cycles across coral reefs, deep-sea habitats, and coastal environments worldwide.
