Fossilised embryos are the work of bacteria
Animal fossils are usually the remains of hard structures – bones and shells that have been petrified through enormous pressures acting over millions of years. But not all of them had such hard beginnings. Some Chinese fossils were once the embryos of animals that lived in the early Cambrian period, some 550 million years ago. Despite having the consistency and strength of jelly, the embryos have been exceptionally well preserved and the structure of their individual cells, and even the compartments within them, have been conserved in all their beautiful, minute detail.
They are a boon to biologists. Ever since the work of Ernst Haeckel in the 19th century, comparing the development of animal embryos has been an important part of evolutionary biology. Usually, scientists have to piece together the development of ancient animals by comparing their living descendants. But the preserved embryos give the field of embryology its very own fossil record, allowing scientists to peer back in time at the earliest days of some of the earliest living things. But how did these delicate structures survive the pressures of the ages?
Elizabeth Raff from Indiana University has a plausible answer. Her experiments suggest that fossil embryos are the work of colonies of ancient bacteria, which grew over the dead clumps of cells and eventually replaced their organic matter with minerals. They are mere casts of the original embryos.
Under a range of different conditions, Raff watched decaying sea urchin embryos (which are roughly similar to the fossil ones in both size and shape. Under normal conditions, she saw that dead embryonic cells destroy themselves within a matter of hours, through the actions of their own enzymes. To produce fossil embryos, this self-destruction is the first hurdle to clear and Raff found that it can be done quite simply by placing the embryos in oxygen-less environments.
When the fossilised embryos first died several million years ago, they must have sank into oxygen-deprived mud, which staved off the embryos’ destruction long enough for bacteria living on their surface to take hold. Raff found that dead sea urchin embryos are rapidly colonised by bacteria that form three-dimensional communities called biofilms. They construct these communities using the embryo’s own structures as scaffolding and the bacteria replicate the structures of the cells they consume, right down to the smallest feature.
Indeed, the fossil embryos still bear traces of these ancient bacteria. They have long, thread-like imprints that strongly resemble the shapes made by bacterial groups growing over sea urchin embryos in oxygen-less water. These biofilms stimulated the growth of minerals and Raff saw that needle-shaped crystals start forming around the bacteria within a week after the embryo’s death.
The crystals are mainly made of aragonite, a type of calcium carbonate typically found in the shells of molluscs and hard enough to withstand the pressures of time. By decaying the embryos, the bacteria lower the pH of the surrounding water, which creates the right conditions for the growth of these crystals.
Raff’s experiments with living embryos don’t by any means give certain answers about the origins of the fossil embryos, but they do at least provide a plausible origin story. Indeed, the degree of degradation in her sea urchin embryos mirrors that seen in the fossils – in some, little but the outer layer was preserved but in others, even the inner workings were sealed in glorious detail (even though the multiple steps put a question mark over the accuracy of the final structures).
The study suggests that the beautiful fossil embryos are not in fact preserved versions of the original cells, but uncanny facsimiles created by bacteria. They may have been created through a two-step process, where each layer acted as a base for sculpting the next one – animal to bacterial, and bacterial to mineral.
Reference: E. C. Raff, K. L. Schollaert, D. E. Nelson, P. C. J. Donoghue, C.-W. Thomas, F. R. Turner, B. D. Stein, X. Dong, S. Bengtson, T. Huldtgren, M. Stampanoni, Y. Chongyu, R. A. Raff (2008). Embryo fossilization is a biological process mediated by microbial biofilms Proceedings of the National Academy of Sciences, 105 (49), 19360-19365 DOI: 10.1073/pnas.0810106105
Related Topics
Go Further
Animals
- Octopuses have a lot of secrets. Can you guess 8 of them?
- Animals
- Feature
Octopuses have a lot of secrets. Can you guess 8 of them? - This biologist and her rescue dog help protect bears in the AndesThis biologist and her rescue dog help protect bears in the Andes
- An octopus invited this writer into her tank—and her secret worldAn octopus invited this writer into her tank—and her secret world
- Peace-loving bonobos are more aggressive than we thoughtPeace-loving bonobos are more aggressive than we thought
Environment
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- U.S. plans to clean its drinking water. What does that mean?U.S. plans to clean its drinking water. What does that mean?
- Food systems: supporting the triangle of food security, Video Story
- Paid Content
Food systems: supporting the triangle of food security - Will we ever solve the mystery of the Mima mounds?Will we ever solve the mystery of the Mima mounds?
- Are synthetic diamonds really better for the planet?Are synthetic diamonds really better for the planet?
- This year's cherry blossom peak bloom was a warning signThis year's cherry blossom peak bloom was a warning sign
History & Culture
- Strange clues in a Maya temple reveal a fiery political dramaStrange clues in a Maya temple reveal a fiery political drama
- How technology is revealing secrets in these ancient scrollsHow technology is revealing secrets in these ancient scrolls
- Pilgrimages aren’t just spiritual anymore. They’re a workout.Pilgrimages aren’t just spiritual anymore. They’re a workout.
- This ancient society tried to stop El Niño—with child sacrificeThis ancient society tried to stop El Niño—with child sacrifice
- This ancient cure was just revived in a lab. Does it work?This ancient cure was just revived in a lab. Does it work?
- See how ancient Indigenous artists left their markSee how ancient Indigenous artists left their mark
Science
- Jupiter’s volcanic moon Io has been erupting for billions of yearsJupiter’s volcanic moon Io has been erupting for billions of years
- This 80-foot-long sea monster was the killer whale of its timeThis 80-foot-long sea monster was the killer whale of its time
- Every 80 years, this star appears in the sky—and it’s almost timeEvery 80 years, this star appears in the sky—and it’s almost time
- How do you create your own ‘Blue Zone’? Here are 6 tipsHow do you create your own ‘Blue Zone’? Here are 6 tips
- Why outdoor adventure is important for women as they ageWhy outdoor adventure is important for women as they age
Travel
- This royal city lies in the shadow of Kuala LumpurThis royal city lies in the shadow of Kuala Lumpur
- This author tells the story of crypto-trading Mongolian nomadsThis author tells the story of crypto-trading Mongolian nomads
- Slow-roasted meats and fluffy dumplings in the Czech capitalSlow-roasted meats and fluffy dumplings in the Czech capital