Photo: Xiangtong Lei & Sihang Zhang
Fossils from China reveal complex visual system in 518-million-year-old vertebrates
Some of the earliest known vertebrates had four fully functional eyes. Paleontologists discovered this thanks to exceptionally preserved soft tissues in fossils dating back approximately 518 million years, reports IFLScience. The new study sheds light on how early vertebrates saw the world and what their descendants became.
Myllokunmingids, from Cambrian deposits in the Chengjiang region of southern China, are considered among the earliest vertebrates and closer relatives to modern animals than most other Cambrian forms. According to researchers, these creatures resembled tadpoles with two large lateral eyes on the sides of their heads. In addition, they had two smaller eyes positioned at the front center.
“These fossils preserve the eyes in extraordinary detail,” said Professor Peiyun Kong from Yunnan University, lead author of the study. He added that the presence of two additional fully functional eyes was a complete surprise to researchers.
The discovery, published in Nature, is particularly significant due to its age. Previously, the oldest evidence of eyes with lenses was 13 million years younger and came from the Burgess Shale in Canada.
Researchers believe that the inner pair of eyes in myllokunmingids later evolved into the pineal gland, often referred to as the “third eye.” Although in humans it is hidden inside the skull, its function is still linked to light detection for regulating sleep cycles. The difference is that humans perceive light through our eyes, whereas some reptiles can still detect it directly through the pineal gland.
“These animals didn’t just have a third eye — they had a fourth,” noted Dr. Jakob Winter from the University of Bristol. According to Kong, the pineal gland once functioned as a fully formed eye capable of creating images. Over evolutionary time, however, its role diminished, the organ lost vision, and it was repurposed to regulate sleep cycles. Fossils preserving eyes are extremely rare because soft tissues almost never fossilize; usually, scientists can only infer eye structures from the shape of eye sockets.
“These fossils provide a rare window into how extinct animals saw and perceived their world,” emphasized honorary professor Sarah Gabbott from the University of Leicester. She noted that even retinal pigments and lenses were preserved.
The team confirmed the four-eye structure in two myllokunmingid species, including Haikouichthys ercaicunensis. The similarity of inner and outer eyes, along with the presence of melanin, confirms that these were visual organs, not other structures.
Previously, dark spots on the crown were thought to be nasal sacs, but the new data refutes this. Winter stated that the four eyes may have given the animals a wider field of vision. At the same time, the additional eyes required significant energy, which likely explains why evolution eventually abandoned such a system.
