A new study suggests that the earliest sponges on Earth were soft-bodied and without mineralized skeletons, explaining the absence of their fossils from 600 million years ago. Led by researchers at the University of Bristol, the findings reconcile genetic evidence with the fossil record. The research indicates that sponge skeletons evolved independently in different lineages.
Sponges are among the oldest known animals, with genetic and chemical evidence pointing to their emergence at least 650 million years ago. However, the oldest confirmed sponge fossils, featuring microscopic glass-like structures called spicules, date back only to about 543 million years ago in the late Ediacaran Period. This gap has puzzled scientists for years.
To address it, an international team led by Dr. M. Eleonora Rossi from the University of Bristol's School of Biological Sciences analyzed data from 133 protein-coding genes and fossil records. Their work, published in Science Advances, places the origin of sponges between 600 and 615 million years ago. The study reveals that the first sponges were soft-bodied and lacked mineralized skeletons, which is why no spicules appear in older rocks.
"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," Dr. Rossi explained.
Further analysis using a statistical computer model, a Markov process, demonstrated that spicules evolved independently across sponge lineages. Modern sponges have skeletons made of different materials, such as calcite or silica, involving distinct genes. Dr. Ana Riesgo from the Museum of Natural Sciences in Madrid noted: "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."
Dr. Joseph Keating, a co-author, described the modeling approach: "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."
The findings challenge assumptions about early sponge success. Professor Phil Donoghue, a palaeobiology professor at the University of Bristol, stated: "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."
Professor Davide Pisani added broader implications: "Sponges are the first lineage of reef-building animals to evolve... Understanding their evolution provide key insights on the origin of the very first reef systems. This is about how life and Earth co-evolved."
The study reconciles paleontological and molecular evidence, offering new perspectives on the dawn of animal life.