The earliest ancestors of all backboned animals, including humans, may have viewed the world with four eyes, not just two. The remnants of those extra eyes persist in the human brain today as the pineal organ, which is deep inside our brain, regulating our sleep cycle, but no longer forms images.
Early vertebrates “had eyes like we do, but not just eyes like we do. They had four eyes,” the study co-author, Jakob Vinther at the University of Bristol, explained to us in an interview. “That's quite amazing to think that our ancestors were swimming around in the ocean like half a billion years ago, and used four eyes to see the world. They probably had a much greater field of view.”
The Kunming region in China is famous for the exceptional preservation of the fossil deposits from the early Cambrian period. Here, Sihang Zhang and Peiyun Cong found specimens of two myllokunmingid species, representing the earliest vertebrates. Both species, dating to about 518 million years, had exceptionally preserved the anteriorly located four black spots: two larger spots on the sides of the head (interpreted as eyes) and the second pair on top between them.
Researchers had previously believed that the second median pair was nasal capsules. However, this was a frustrating inconsistency as we know that early vertebrates at the time only had a single nostril. Under the electron microscope, the team found the presence of melanosomes, tiny packages that contain melanin. Melanin determines the color of the eyes and also absorbs the light to create an image.
Before this fossil, we had fossilized melanin no older than Carboniferous, about 300 million years old. These fossils are “quite exciting to show that we have melanin preserved that goes that far back (518 million years),” Vinther told us.
Researchers also found an impression of a lens inside these organs “so they felt like they were eyes,” the scientist says. This means that “the animal had two big eyes on the side and two small eyes on the top, and both of them were camera eyes.”
The paper proposes that our ancestors were at the bottom of the food chain, and the four eyes likely evolved amid Cambrian environmental pressures. Having the ability to detect more of the surroundings and a bigger angle of view is advantageous to evade predators.
Over time, the ecological niche changed from filter feeders to carnivores, and the second pair of eyes may have evolved into a non-sensory neuroendocrine organ called the pineal gland, responsible for producing melatonin and regulating the sleep cycle.
The results may help “paint a clearer picture of the early stages of vertebrate evolution,” says Elias Warshaw, a paleobiologist at University College London, who wasn’t involved in the study but agrees with the findings. “The hypotheses presented within the paper are thoroughly tested, and the results are interpreted reasonably”.
The study has been published in Nature.
