According to a recent study published in the journal Nature, the earliest ancestors of all vertebrates (including humans) may not have seen the world with only two eyes like we do today, but had four eyes. This startling discovery suggests that the extra "forehead eyes" of these ancient creatures did not disappear completely, but evolved into the pineal gland, which is buried deep in the human brain and is responsible for regulating our sleep cycles, although it no longer has an imaging function.
The study is based on fossils found in the Kunming region of China, an area known for its well-preserved fossil assemblage from the early Cambrian period (about 518 million years ago). Researchers Sihang Zhang and Peiyun Cong discovered two specimens of Kunming fish (myllokunmingid) here, which are believed to be the earliest known vertebrates. Four black spots on the front of the head are clearly preserved in the fossil: one larger spot on each side (the regular eyes), and a second pair of smaller spots between them, just above the head.
The scientific community has long thought that this middle pair of structures is the nasal sacs (the organ of smell). However, this explanation has always been questionable because early vertebrates at the time usually had only one nostril. To unravel the mystery, the team analyzed the fossils under an electron microscope and found melanosomes - tiny inclusions containing melanin - within these structures. Melanin not only determines the color of the eyes, but more importantly, it absorbs light to form images. Jakob Vinther, a co-author of the study at the University of Bristol, said that the oldest previously discovered melanin fossils only date back to the Carboniferous Period about 300 million years ago, but this discovery pushes this record forward significantly to 518 million years ago, which is very exciting.
In addition to melanin, the researchers also found impressions of the lens inside these organs, further confirming that they are indeed eyes. Winser explained that this means that these animals not only have two large eyes on the sides of their bodies, but also two small eyes on the top of their heads, and these four eyes are all "camera eyes" with imaging capabilities. Winser lamented: "Imagine that our ancestors used four eyes to observe the world when they were swimming in the ocean 500 million years ago. It is simply incredible. This probably gave them a wider field of vision."
The study pointed out that these early vertebrates were at the bottom of the food chain. As filter feeders, they faced tremendous pressure to survive. In the predator-filled Cambrian oceans, having a wider field of view and the ability to detect danger from above would have been a huge evolutionary advantage. Over time, the ecological niche of vertebrates has changed, evolving from passive filter feeders to active carnivores. During this process, the pair of eyes on the top of the head gradually degenerated, lost their visual function, and eventually evolved into the pineal gland, a non-sensory neuroendocrine organ responsible for secreting melatonin and regulating the biological clock.
Elias Warshaw, a paleontologist at University College London who was not involved in the study, expressed his approval of the discovery. He believes that the hypothesis proposed in this study has been fully tested and the results are reasonably explained, which will help scientists to more clearly depict the early stages of vertebrate evolution. This discovery not only rewrites our understanding of the appearance of our ancient ancestors, but also reveals the ancient origins of the mysterious organ deep in the human brain.