Scientists have long investigated the precise timing of sponge evolution. Genetic data from extant sponges and chemical markers in ancient rocks indicate sponge evolution occurred at least 650 million years ago. This timeframe has been subject to debate due to a discrepancy with the sponge fossil record, which dates back approximately 543 million years.
Resolving the Evolutionary Timeline
An international research team, led by Dr. M. Eleonora Rossi from the University of Bristol’s School of Biological Sciences, addressed this discrepancy by examining the evolution of sponge skeletons. Their findings were published on January 7 in Science Advances.
Living sponges possess skeletons composed of microscopic, glass-like needles known as spicules, which have a robust fossil record extending to the late Ediacaran Period, around 543 million years ago. The absence of these spicules in older rock formations led some researchers to question earlier estimates for sponge origins.
Dr. Rossi's team employed a two-step methodology. First, they integrated high-quality data from 133 protein-coding genes with existing fossil evidence to establish a revised timescale for sponge evolution. This analysis placed the origin of sponges between 600 and 615 million years ago, narrowing the gap with the fossil record.
Evolution of Sponge Skeletons
Second, the team investigated the evolution of sponge skeletons. Their research indicated that spicules developed independently across various sponge groups.
Dr. Rossi stated, "Our results indicate that the initial sponges were soft-bodied and did not possess mineralized skeletons. This explains the absence of sponge spicules in rocks from approximately 600 million years ago."
Dr. Ana Riesgo, from the Museum of Natural Sciences in Madrid, noted prior indications of independent skeletal evolution. She commented that while modern sponge skeletons may appear similar, their construction methods vary significantly, with some composed of calcite and others of silica, involving distinct genes.
Methodological Approach
To reconstruct sponge skeleton evolution, the team utilized a statistical computer model. Dr. Joseph Keating, a co-author, explained the application of a Markov process, a predictive model. By modeling transitions between different skeletal types, including soft-bodied forms, the study's models largely rejected the premise that the earliest sponges had mineralized skeletons. Only models treating all mineral types as equivalent provided ambiguous results.
Implications for Early Life
The study's findings generate further questions regarding early sponge evolution. Professor Phil Donoghue, Professor of Palaeobiology at the University of Bristol, suggested that while mineralized spicules are prevalent in nearly all living sponges, their presence might not have been central to early sponge diversification. The factors driving this early diversification remain an area for further investigation.
Professor Davide Pisani, Professor of Phylogenomics at the University of Bristol, highlighted the broader significance, stating that sponges represent the earliest lineage of reef-building animals and potentially the first animal lineage overall. Understanding their evolution offers insights into the origin of the first reef systems and how early animal evolution contributed to the co-evolution of life and Earth.