In the dark depths of ancient seas, hagfish ancestors could see. Victoria E. McCoy and Jason D. Pardo, researchers at the University of Wisconsin-Milwaukee and the Lauer Foundation for Paleontology, have discovered something remarkable in fossilized hagfish eyes: evidence of a gradual, three-stage journey toward blindness that unfolded over millions of years.
Hagfish are strange creatures—eel-like fish with a skull but no vertebral column, living today as nearly blind scavengers on the ocean floor. Their eyes are shrunken, pigmentless, lensless, and buried beneath soft tissue, rendering them almost useless for detecting light. Yet the fossil record now reveals they were not always this way. By studying high-resolution anatomical data from three extinct hagfish species, McCoy, Pardo, and their colleagues have mapped how vision gradually disappeared, offering a window into how sensory organs can become simplified rather than more sophisticated.
The team examined fossilized eyes using advanced imaging techniques, measuring the eyes' size and shape, the soft tissue covering them, the presence or absence of lenses, and the distribution of melanosomes—pigment-containing structures in the eye. What they found was a stepwise story. The earliest hagfish fossils studied contained eyes that were somewhat larger and equipped with lenses, suggesting they could form basic images. The next stage showed smaller eyes losing their lensing capability, meaning the animals could no longer focus properly. By the final stage—modern hagfish—the eyes had shrunk further, lost almost all pigmentation, and retained only spherical melanosomes, rendering them nearly non-functional.
"Reduction of the eyes occurred gradually across the Paleozoic, with an initial stage of size reduction and loss of cylindrical melanosomes from the retinal pigmented epithelium, an intermediate stage with loss of image-focus capability and finally a near-complete loss of vision," the researchers wrote in their paper, published in Biology Letters.
This process did not happen overnight. The initial and intermediate stages of eye simplification likely occurred in nearshore marine environments before the Permian period, when hagfish eventually colonized the continental slope and deeper waters where vision mattered less. As hagfish moved into darker habitats, the evolutionary pressure to maintain functional eyes simply dissolved. Over time, what had once been useful became metabolically expensive to maintain, and natural selection favored individuals whose bodies invested energy elsewhere—perhaps in other senses like touch and smell, which remain critical for finding food in the murky depths.
What makes this discovery particularly illuminating is its challenge to a common assumption: that evolution always drives sensory organs toward greater sophistication. Hagfish show the opposite trajectory, yet their simplification likely served them well. Modern hagfish thrive in their ecological niche without eyes, relying on other sensory modalities to navigate and hunt. Their journey from sighted to nearly blind ancestors is not a story of decline but of adaptation to new circumstances.
The fossils housed in the Lauer Foundation for Paleontology and the Field Museum of Natural History tell a quiet story about how life adjusts when conditions change. For hagfish, losing their eyes may have been one of evolution's wisest moves.