Analysis of 700 microbial genomes points to collagenases as a factor in Ediacaran fossil preservation
A genomic analysis reports that collagen-degrading enzymes spread to oxygen-tolerant microbes as animals diversified, a possible reason for uneven soft-tissue fossilization.
A team led by Philip Vixseboxse, who did the work as a doctoral student at the University of Cambridge and the University of Edinburgh, traced the evolution of collagenases across 700 microbial genomes. Collagenases are the enzymes microbes use to break down collagen. The study, published in Current Biology, addresses why soft-bodied animals of the late Ediacaran Period (about 579–539 million years ago) fossilized in many kinds of seafloor sediment sciencedirect.com
Current Biologysciencedirect.comOpen source ↗.
Most of the animal fossil record consists of shells, bones and teeth. Soft tissue survives only when decay is prevented or delayed until the remains are stabilized by mineralization. Earlier explanations for Ediacaran preservation pointed to unusual seawater chemistry, different tissue types or microbial mats on the seafloor. Each works at some sites but not at all of them.
Where the genes came from
In 1999, paleontologist Jerzy Dzik proposed that microbes of that time had not yet evolved collagenases, so carcasses kept their shape long enough to fossilize. The new study tests this with genome data. The team searched the genomes for two collagenase genes and found the main one in 242 species. They then reconciled the gene trees with a time-calibrated tree of microbial life.
The main collagenase arose billions of years before animals, in obligate anaerobes such as methane producers and fermenters sciencedirect.com
Current Biologysciencedirect.comOpen source ↗. So the enzymes existed earlier than Dzik assumed, but they worked only in the oxygen-free zones deeper in the sediment.
Starting in the Neoproterozoic, the era when animals arose, the gene moved between microbial lineages far more often. Across the larger set of trees the team counted an average of 63 lateral transfers. Oxygen-tolerant and oxygen-using microbes acquired it, including groups that dominate decay today. According to the authors, these groups obtained collagenases in the latest Neoproterozoic or later.
Effect on the fossil record
In an oxygen-rich Ediacaran sea, a dead animal would have retained its collagen until the oxygen in the surrounding sediment was used up. The authors put this delay at years, long enough for minerals to build up on the tissue and preserve its shape. The collagen scaffold of the body would have acted as a kind of organic skeleton.
The authors report that the shift in where collagen is degraded within the sediment redox profile coincides with the decline of exceptional soft-tissue preservation in the marine realm sciencedirect.com
Current Biologysciencedirect.comOpen source ↗. Co-author Alex Liu said the results suggest an overarching biological mechanism that has shaped the fossil record of soft tissues across Earth's history.
