Sponges are among earth's most ancient animals, but exactly when they evolved has long puzzled scientists. Genetic information from living sponges, as well as chemical signals from ancient rocks, suggest sponges evolved at least 650 million years ago. The research is published today [7 January] in Science Advances.
This evidence has proved highly controversial as it predates the fossil record of sponges by a minimum of 100 million years. 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 conflict by examining the evolution of sponge skeletons.
Living sponges have skeletons composed of millions of microscopic glass-like needles called spicules. These spicules also have an extremely good fossil record, dating back to around 543 million years ago in the late Ediacaran Period. Their absence from older rocks has led some scientists to question whether earlier estimates for the origin of sponges are accurate.
Dr Rossi and her team solved this mystery using a two-step approach. Firstly, they combined high-quality data from 133 protein-coding genes with fossil evidence to construct a new timescale for sponge evolution. They dated the origin of sponges to between 600-615 million years ago, closing the gap with the fossil record. Secondly, they investigated the evolution of sponge skeletons, revealing that spicules evolved independently in different sponge groups.
Dr Rossi, Honorary Research Associate, said: "Our results show that the first sponges were soft-bodied and lacked mineralised 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 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."
In order to reconstruct sponge skeleton evolution, the team used a statistical computer model. Dr Joseph Keating , also an author on the study, explained: "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 mineralised skeletons. Only an unrealistic model treating all mineral types as equivalent suggests otherwise, and even then the results are ambiguous."
The results of this study raise interesting questions about early sponge evolution.
Professor Phil Donoghue , Professor of Palaeobiology at the University of Bristol said: "Given that nearly all living sponges have skeletons composed of mineralised 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 tantalising mystery."
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".
