Researchers may have uncovered the reason why some of Earth’s earliest animals were commonly preserved as fossils – even though they were essentially fragile ‘bags of goo’ – while later soft-bodied creatures have largely disappeared from the fossil record.
The researchers, led by the University of Cambridge, say that the reason may lie in the evolution of microscopic organisms capable of breaking down collagen, a structural protein found in animal tissue.
The fossil record of animal life on Earth is highly incomplete. Hard body parts like shells, bones and teeth are far more likely to be preserved than soft body parts like muscles, organs and skin, so the majority of the fossils we have today are mineralised skeletons.
However, fossils from the late Ediacaran Period, between 579 and 539 million years ago, are different. Fossils from this time, which include the oldest known complex forms of animal life, are frequently preserved in extraordinary detail, even though none of these earliest animals had any hard body parts. In some sites, entire Ediacaran ecosystems are preserved, providing a rich window into early animal evolution.
For decades, researchers have debated how such common preservation of soft tissues was possible. Possibilities included unusual chemical conditions in the Ediacaran oceans, different tissue compositions, or the influence of a layer of microbial slime that covered the entire seafloor. However, while these explanations work in some places, none of them works everywhere.
But now, the Cambridge-led researchers say the answer may be the evolution of microbes that produce collagen-eating enzymes known as collagenases. Their results are reported in the journal Current Biology .
Collagen provides strength and support to animal tissues throughout the body. Once an organism dies, collagenases can rapidly break these tissues down, reducing the chances that they will be preserved long enough to fossilise.
“If you think about a modern jellyfish, for example, if it washes up on a beach and is left to decay, it will decompose in a matter of days,” said lead author Philip Vixseboxse, who completed the work while a PhD candidate at Cambridge’s Department of Earth Sciences. “It will never become a fossil, but similarly squishy animals from half a billion years ago did commonly get preserved.”
Collagen is in all animal tissue, but without any enzymes to break it down, it will persist, like modern plastic. “You need an enzyme to break down a biopolymer, or else it will sit around for ages until something comes along that can actually eat it,” said Vixseboxse.
Vixseboxse and his co-authors traced the evolution of collagenase genes in microbial organisms through 700 bacterial genomes, and compared them to a time-dated tree of life, to understand how microbes acquired the ability to break down collagen through time.
The researchers found that collagenases are much older than animals. Initially, they existed only in anaerobic, or oxygen-intolerant, microbes. Dead organisms in the oxygenated waters of the Ediacaran would therefore have held on to their collagen until the oxygen in the sediments surrounding them was used up, giving the organisms much more time to fossilise.
Around the transition between the Ediacaran Period and the Cambrian Period, however, collagenases began to spread among a greater range of microbial groups. They transferred into aerobic, or oxygen-using, bacteria, including the groups that cause modern decay. This shift coincides with a significant fall in the number of fossils of soft-bodied animals.
“Around this critical time where these soft-bodied animals stopped fossilising, we start seeing the appearance of collagenases in aerobic bacteria, meaning the ability to break down collagen-bearing tissues could start to spread into new environments as animal diversity accelerated,” said Vixseboxse. “Now that collagen could be degraded, you needed ever-faster fossilisation processes to outpace decay, which is likely why we start to see a big drop in the abundance and quality of the fossil record, in terms of the preservation of soft tissue.”
In some exceptional circumstances, soft tissues can be preserved in younger rocks, depending on how and where the animal died, and what happened after that. A dead animal buried beneath an avalanche of sediment soon after death, for instance, could be well-preserved and avoid decay, but these instances are rare.
“These results suggest there may be an overarching biological mechanism that has shaped the fossil record of soft tissues across Earth’s history,” said co-author Professor Alex Liu, also in the Cambridge Department of Earth Sciences. “The acquisition of key enzymes by microbes to exploit a growing food resource may have decreased the preservation potential of soft tissues.”
Vixseboxse says in the far future, there may be similar echoes with modern plastic. Like ancient collagen, plastic fails to decay until there is something that can break it down. “Bacteria had no reason to efficiently degrade collagen when it didn’t exist,” said Vixseboxse. “But once collagen became more abundant, and provided a new food source for these microbes, collagenases proliferated. Perhaps someday, microbes will exploit similar enzymes to break down the increasing abundance of plastic polymers we are putting into the environment, but that will be long after we’re gone.”
The research was supported in part by the Leverhulme Trust, the Natural Environment Research Council (NERC), and the Biotechnology and Biological Sciences Research Council (BBSRC), part of UK Research and Innovation (UKRI).
Current Biology
Temporal heterogeneity in the animal fossil record driven by proliferation of microbial collagenases
8-Oct-2026