Conceptual image of a feathered buitreraptor. (© Stock Source Studio – stock.adobe.com)
In a nutshell
- Scientists temporarily reversed feather evolution in chickens by blocking a key developmental pathway, causing the embryos to grow simplified, unbranched feather structures resembling those seen in early feathered dinosaurs.
- The changes weren’t permanent, although the chicks hatched with patches of bare skin and dormant follicles, they eventually regrew normal feathers, showing how resilient and self-correcting feather development can be.
- The study offers rare experimental insight into how complex traits like feathers evolved, helping bridge the gap between fossil evidence and the genetic mechanisms that shape modern bird anatomy.
GENEVA — From dinosaur fuzz to eagle wings, evolution spent over 100 million years perfecting the feather. Now, Swiss scientists have found a way to reverse this process in the lab, causing chicken embryos to grow simple, ancient-style feathers that look startlingly similar to those of their dinosaur ancestors.
In a study published in PLOS Biology, researchers blocked a developmental pathway called “sonic hedgehog” (named after the video game character) in chicken embryos. This caused the chickens to grow simple, unbranched feathers instead of the complex feathers we see on birds today.
From Simple Beginnings to Complex Designs
Today’s bird feathers are engineering marvels. Birds have fluffy down feathers for warmth and sleek flight feathers for soaring through the air. But birds didn’t always have such sophisticated feathers.
The earliest feather-like structures on dinosaurs were most likely simple hair-like filaments that were more like fuzz than proper feathers. Over millions of years, these simple structures evolved into the complex feathers we see today.
The research team wondered if they could reverse this process. They used a drug called sonidegib to block the sonic hedgehog pathway in developing chicken embryos. They injected this drug into chicken eggs exactly 9 days into their development when the early feather structures were just forming.
While normal chicken embryos develop complex feathers with branches and follicles (the structures that anchor feathers in the skin), the treated embryos grew simple, unbranched feather buds without proper follicles. These simplified structures looked surprisingly similar to what scientists believe dinosaur “proto-feathers” looked like.
The Surprising Recovery
This change wasn’t permanent. As the embryos continued developing, the drug’s effects gradually wore off. When the treated chickens hatched, they had patches of bare skin where the feathers had been most affected. Under these bare patches were dormant, simplified follicles.
By about seven weeks after hatching, these bare patches unexpectedly disappeared as normal feathers grew from the once-dormant follicles. The chickens’ bodies essentially overcame the temporary disruption and got back on track with growing proper feathers.
To make sure this wasn’t just a fluke, the researchers conducted additional experiments. In one test, they first blocked the pathway with sonidegib and then activated it again with another drug (called SAG), which successfully restored normal feather development.
When given high doses of sonidegib, the pattern of gene activity changed from spots (which normally mark where individual feathers will grow) to stripes. It’s as if the template for laying out feathers temporarily shifted to a different design altogether.
Bridging Past and Present
While fossils can show us what ancient dinosaur coverings looked like, they can’t tell us about the genetic mechanisms that drove their evolution into modern feathers. By manipulating these mechanisms in living embryos, scientists can test theories about how evolution might have happened.
The study also shows how resilient biological development can be. Despite the researchers throwing a big wrench into the developmental machinery, the chickens’ bodies eventually figured out how to get back on track and grow normal feathers.
The sonic hedgehog pathway is involved in many developmental processes, including brain development, limb formation, and even some types of cancer. In fact, the drug used in this study, sonidegib, is currently used to treat certain skin cancers called basal cell carcinoma.
The boundary between modern birds and ancient dinosaurs is thinner than we thought. With a simple molecular tweak, ancient features temporarily resurface, reminding us that evolution works not by inventing entirely new structures, but by modifying what already exists.
Paper Summary
Methodology
The researchers used broiler chicken eggs kept under controlled conditions. At day 9 of embryonic development, they carefully removed a small section of eggshell, made the underlying membrane transparent with mineral oil, and precisely injected either a control solution or different doses of sonidegib into the embryonic bloodstream. They examined embryos at different development stages from day 10 to hatching and followed the chickens for 49 days after hatching. They used techniques including immunofluorescence to visualize protein expression, in situ hybridization to track gene activity, 3D light sheet microscopy, and RNA sequencing to analyze genetic changes.
Results
The study found that sonidegib treatment caused four main effects: (1) It temporarily changed the normal spotted pattern of sonic hedgehog expression into stripes; (2) It dramatically reduced feather bud outgrowth and prevented follicle formation; (3) It blocked the branching process that creates feather barbs, resulting in simplified, unbranched structures; and (4) These effects were dose-dependent, with higher doses causing more profound changes. While newly hatched treated chickens showed patches of bare skin with dormant follicles underneath, by 49 days after hatching, normal feather coverage had largely recovered, except for specialized flight feathers which remained permanently altered.
Limitations
The study had several limitations. The researchers couldn’t precisely control the drug concentration over time as it naturally decreased in the embryo. They only tested effects beginning at day 9 of development, so different timing might produce different results. The simplified feathers, while resembling proto-feathers, are modern structures with disrupted development rather than true evolutionary relics. The study also had mortality rates of 38-68% among treated embryos, which could introduce survival bias. Finally, gene expression analysis was performed on whole wing tissue rather than isolated feather buds, potentially diluting feather-specific signals.
Discussion and Takeaways
This research provides evidence that the sonic hedgehog pathway plays a crucial role in feather development and evolution. By temporarily inhibiting this pathway, researchers created simplified feather structures resembling those of early dinosaurs, supporting the hypothesis that modifications to this pathway contributed to feather evolution over time. The study demonstrates the remarkable resilience of developmental systems – despite significant disruption, the chicken skin eventually recovered its ability to produce normal feathers. The findings bridge developmental biology and paleontology, offering a way to test evolutionary hypotheses through experimental manipulation of modern embryos.
Funding and Disclosures
The research was supported by grants from the Swiss National Science Foundation, the International Human Frontier Science Program Organisation, and the European Research Council under the European Union’s Horizon 2020 research and innovation program. The authors declared no competing interests.
Publication Information
“In vivo sonic hedgehog pathway antagonism temporarily results in ancestral proto-feather-like structures in the chicken” was published in PLOS Biology on March 20, 2025. The authors are Rory L. Cooper and Michel C. Milinkovitch from the Laboratory of Artificial and Natural Evolution at the University of Geneva, Switzerland.
