Remarkable fossils unearthed in southern China have shed light on a surprising characteristic of early human ancestors. Jawless fish that populated the planet 518 million years ago possessed four eyes. An international research team reconstructed the appearance of one such creature to comprehend how complex vision and brain structures developed in the forebears of modern humans.
The subject of this study is the Myllokunmingiids, agnathan fish that existed during the Cambrian period. At that time, larger apex predators began appearing in the ocean, rendering the habitat extremely perilous for small vertebrates. Contra the standard two eyes found in most vertebrates, these ancient beings sported a pair of two visual organs. According to study co-author Dr. Jacob Winter, an Associate Professor of Macroevolution at the University of Bristol, these four eyes likely afforded the animals a broader field of view. This capability was critical for survival and necessitates a revision of scientific understanding regarding early evolution.
By meticulously examining fossilized specimens of two distinct species from the Chengjiang fossil site, researchers identified two prominent eyes situated on the sides of the head, alongside two smaller eyes positioned centrally between them. The exceptional preservation of soft tissues in such ancient remains is a significant stroke of luck. The team utilized high-powered microscopy and chemical assays to confirm the presence of all four visual organs and analyze their structure.
Peiyun Cong, the lead author from Yunnan University, stated that initial investigations focused solely on the large eyes, but the discovery of two smaller, yet fully functional, organs located between them came as a major surprise. Both of the lesser eyes featured a circular shape, light-absorbing pigments, and lenses capable of image formation, mirroring the primary eyes.
This finding bridges a critical gap between ancient anatomy and contemporary biology. Certain extant reptiles, amphibians, and fish retain a “third,” or parietal, eye, which in humans evolved into the pineal gland (epiphysis). This brain appendage, connected to the pineal gland, manages sleep cycles in humans via melatonin regulation. Half a billion years ago, the precursor organ to the epiphysis functioned as a complete, image-forming eye for the Myllokunmingiids, aiding evasion of predators; only later did it lose its visual role to assume its modern function as a sleep regulator.
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