New research led by a University of Wisconsin-Milwaukee geoscientist provides insight into how one of the most primitive living vertebrates evolved its uniquely simple visual system. The study, published in Biology Letters on May 27, examines fossilized hagfish eyes to reconstruct how their modern, reduced structure came to be.
The study’s lead author, Victoria McCoy, assistant professor in UWM’s Department of Geoscience, collaborated with researchers from the Field Museum of Natural History, the Lauer Foundation for Paleontology, Science and Education, the David and Sandra Douglass Collection and the University of Ottawa.
Hagfish, often described as the most primitive living vertebrates, possess extremely simple eyes and are nearly blind. Scientists have long hypothesized that their eyes evolved from more complex vertebrate eyes through a gradual loss of structures, likely as hagfish adapted to the dark, deep-sea environments they inhabit today.
To test this idea, McCoy and her colleagues analyzed exceptionally well-preserved fossils of three extinct hagfish species from the approximately 308 million-year-old Mazon Creek site in northeastern Illinois. Unlike modern deep-sea habitats, this ancient environment was near shore and well lit.
The fossil evidence revealed that these early hagfish had eyes more developed than those of modern hagfish, yet still less complex than typical vertebrate eyes, representing an intermediate stage in eye reduction.
“This study shows that the hagfish eye didn’t become simple all at once,” said Arjan Mann, curator of early tetrapods at the Field Museum. “Instead, it evolved through a gradual, stepwise loss of complexity over millions of years. Importantly, some of these changes appear to have happened before hagfish moved into the low-light environments where they live today.”
These findings challenge the long-held assumption that deep-sea conditions alone drove the degeneration of hagfish vision, suggesting instead that aspects of this evolutionary process began earlier.
The research emphasized the importance of fossil evidence in understanding evolutionary transitions.
“Fossils like those from Mazon Creek give us a rare window into evolutionary history,” said McCoy. “They allow us to see intermediate forms that bridge the gap between ancient organisms and their modern relatives, helping us understand how dramatic changes, like the reduction of a complex eye, actually unfold over time.”
The study highlights the importance of combining paleontological data with evolutionary biology to better understand how major anatomical transformations occur.
The study will be available upon publication via this link.
About UWM
The University of Wisconsin-Milwaukee has an ambitious mission as both a top-tier research university and an access institution, striving to ensure that students have equitable opportunities to earn a college degree. UWM educates a diverse student body of more than 23,000 students from 83 countries. About 43% of its undergraduates are first-generation college students. Its unique and top-rated programs include Wisconsin’s only accredited schools of architecture and public health, the only North American school dedicated solely to freshwater sciences and a film program ranked among the top 50 in the world. It has the largest and top-rated online education program in Wisconsin. UW-Milwaukee partners with leading companies to conduct joint research, promote entrepreneurship, provide student internships and serve as an economic engine for southeastern Wisconsin. The Princeton Review named UW-Milwaukee a 2026 “Best Midwestern” university based on overall academic excellence and student reviews.
About the Field Museum of Natural History
The Field Museum connects people to the natural world and the human story. Located on Chicago’s iconic Lake Michigan shore, the Field is home to nearly 40 million artifacts and specimens including the Chicago Archaeopteryx and the iconic SUE the T. rex. Much like the Earth itself, the Field Museum is always evolving. Every day, we work to uncover the history of life on Earth and find ways to safeguard its future, while sharing the beauty and diversity of our planet with visitors from every corner of the globe.