Sponges rank among the oldest known animals on Earth, yet scientists have struggled to pinpoint exactly when they first appeared. DNA from living sponges and chemical traces preserved in ancient rocks indicate they emerged at least 650 million years ago. The findings are reported in Science Advances.
That early date has sparked debate because it predates the oldest confirmed sponge fossils by at least 100 million years. To resolve this long standing conflict, an international research team led by Dr. M. Eleonora Rossi of the University of Bristol’s School of Biological Sciences took a closer look at how sponge skeletons evolved.
The Fossil Record and the Missing Spicules
Modern sponges contain skeletons made of countless microscopic, glass-like structures called spicules. These durable elements fossilize well and appear in rocks dating back about 543 million years to the late Ediacaran Period. However, scientists have not found spicules in older rocks. That absence has fueled skepticism about whether sponges truly originated as early as genetic evidence suggests.
To address the discrepancy, Dr. Rossi and her colleagues used a two part strategy. First, they analyzed data from 133 protein coding genes alongside fossil information to build a revised evolutionary timeline. Their results place the origin of sponges between 600-615 million years ago, narrowing the gap between genetic predictions and fossil evidence. Next, they examined how sponge skeletons developed over time and discovered that spicules arose independently in separate sponge lineages.
Dr. Rossi, Honorary Research Associate, 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 to preserve.”
Independent Evolution of Sponge Skeletons
The idea that sponge skeletons evolved more than once is supported by both structural and genetic differences among living species. Dr. Ana Riesgo, a world-leading expert in sponge evolution from the Museum of Natural Sciences in Madrid (Spain) said: “We already had some clues that suggested sponge skeletons evolved independently. Modern sponge skeletons may look alike, but they’re built in very different ways. Some are made of calcite, the mineral that makes up chalk, others of silica, essentially glass, and when we examine their genomes we see that entirely different genes are involved.”
To reconstruct this evolutionary history, the team relied on a statistical computer model. Dr. Joseph Keating, a co author of the study, described their approach: “We used a Markov process, a type of predictive model that’s widely applied in fields like finance, AI, search engines, and weather forecasting. By modelling transitions between different skeletal types, including soft-bodied forms, we found that almost all models strongly reject the idea that the earliest sponges had mineralized skeletons. Only an unrealistic model treating all mineral types as equivalent suggests otherwise, and even then the results are ambiguous.”
Rethinking the Rise of the First Animals
The findings raise new questions about what drove the earliest stages of sponge evolution. Today, nearly all sponges have mineralized spicules, which might suggest that skeletons played a key role in their early success. However, the new analysis challenges that assumption.
Professor Phil Donoghue, Professor of Palaeobiology at the University of Bristol said: “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 idea, suggesting that early sponge diversification was driven by something else entirely — and what that was is still a tantalizing mystery.”
The implications extend beyond sponges alone. Professor Davide Pisani, Professor of Phylogenomics at the University of Bristol, concluded: “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 is still debated. Understanding their evolution provide key insights on the origin of the very first reef systems. 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.”
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