Over 500 million years ago, the earliest known vertebrates exhibited an intriguing feature: an extra eye. Interestingly, humans may retain traces of this ancient evolutionary trait.

Significant fossils from two species of jawless fish, known as myllokunmingids, were discovered by Kong Peiyun. From 2019 to 2024, Kong worked alongside colleagues at Yunnan University in China, specifically around Dianchi Lake.

The fossils unearthed in the Chengjiang biota area, renowned for its exquisite preservation, date back to approximately 518 million years ago—a timeframe marked by a dramatic increase in life’s diversity during the Cambrian period.

Remarkably, the vertebrate fossils discovered by Kong’s team included well-preserved soft tissue and vital eye structures.

Complex eye structures evolved independently in various animal groups. Many invertebrates, like insects, possess compound eyes, which consist of numerous individual units, each with its own lens, enabling a mosaic vision.

Meanwhile, vertebrates such as humans and reptiles possess what scientists label as “camera eyes.” These comprise a spherical lens, retina, iris, and muscles that regulate eye movement. Additionally, they contain pigment structures called melanosomes that influence eye color.

Light focuses on the retina, generating a signal relayed to the brain via the optic nerve.

Under electron microscopy scrutiny, Kong and his team identified two eyes situated on the sides of the head, with melanin-rich melanosomes preserved, alongside two smaller enigmatic black marks between them.

Employing a lens impression to analyze the fossils, team members led by Jacob Vinther from the University of Bristol suggested that these ancient creatures possessed two pairs of camera-like eyes, allowing them to visualize their environment much like modern vertebrates. The decisive difference? They utilized four eyes instead of two.

The research team posits that this ancient additional eye has evolved into various organs known as the pineal complex. Some vertebrates, such as reptiles, possess a light-sensitive organ called the parietal eye atop their heads, while all mammals retain a reduced version that is the pineal gland, a key player in regulating sleep cycles through melatonin secretion.

“Early vertebrates likely used the pineal organs as functional eyes, enabling them to perceive their surroundings before evolving into sleep-regulating organs,” states Vinther.

These large eyes may have been optimized for high-resolution vision, complemented by smaller eyes that enabled detection of nearby threats—critical for survival in the predator-rich Cambrian seas.

According to Vinther, these creatures could likely discern objects with detail, estimating their shape and gain a degree of depth perception—all thanks to their remarkable four-eyed adaptation.

Tetsuto Miyashita, from the Canadian Museum of Nature in Ottawa, finds the interpretation of these fossils both “half-believable and half-doubtful.”

The structure located between the two eyes had previously perplexed researchers, but realizing it may indicate another camera eye was considered a “lightbulb” moment, he explains.

If indeed this is the case, it raises the question: where is the animal’s nose? “Most early fish evolution centered around nose development, suggesting that it would be unusual for the nose to not be preserved,” he notes.

Miyashita anticipates significant discussions will persist until experts can engage in a thorough debate regarding this exciting finding. “What function do so many prominent eyes actually serve?” he questions.

John Patterson, a researcher from the University of New England in Armidale, Australia, asserts that it is logical for prey species to have developed such visual capabilities to escape formidable predators.

The Cambrian era was evolutionary peculiar, showcasing animals displaying unusual behavior and not strictly developing pairs of eyes on their heads, but placing eyes in other regions as well.

Karma Nangle, a professor at the University of California, Riverside, aims to create a comprehensive map of the entire fossil body to investigate the potential existence of similar traces. Such findings could demonstrate that the second set of eyes may simply be a result of chemical processes during fossilization.

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