- Scientists traced the origin of the human pineal gland to 600 million years ago
- The pineal gland produces melatonin to regulate sleep and wake cycles
- The tri-cornea theory links the pineal gland and retina to one ancient organ
Scientists have discovered what they believe to be the ancient origin of a hidden light-sensing organ inside the human head, tracing it back nearly 600 million years. The finding was explained in a new study published in the journal Current Biology, reported NYPost.
The research focused on the pineal gland, a small pea-sized organ located deep inside the skull. Although it is no longer exposed to sunlight, the gland still helps the body respond to light and darkness. It produces melatonin, the hormone responsible for controlling sleep and wake cycles.
The study proposed what researchers called the "tri-cornea theory," suggesting that the pineal gland and the human retina may have evolved from the same ancient light-sensing organ.
The research team examined the possible origins of vision by looking back at one of humanity's earliest ancestors, a small sea-dwelling creature that lived around 575 million years ago.
According to the study, this ancient animal had two side eyes that helped it navigate and a third light-sensitive organ on top of its head that monitored light levels and helped it stay oriented.
Researchers explained that this body structure continued until around 500 million years ago. Over time, some of these creatures began burrowing into sediment and no longer needed side eyes for navigation. As a result, they lost the lateral eyes and relied mainly on the third light receptor to distinguish between up and down as well as day and night.
The study stated that this unusual arrangement helped separate vertebrates from other evolutionary groups that kept their side eyes.
Scientists said that some of these underground ancestors later returned to the sea as filter feeders. Once they needed navigation again, parts of the third eye gradually evolved and shifted to the sides of the head, eventually developing into retinas.
However, the original third eye remained behind as what researchers described as an evolutionary remnant. According to the study, this process suggests that both the retina and pineal gland evolved from the same ancient organ instead of developing separately.
The team did not conduct traditional eye experiments for the study. Instead, they reviewed previous scientific research and genetic information from animals including fish and lampreys to better understand how vision-related organs evolved.
Study author Professor Thomas Baden said the researchers wanted to understand the "original solution to vision" and examine how different species later adapted or modified it over time.
He explained that while studying patterns across species, the team began questioning what the original eye may have looked like. The study also noted that not all animals hide their third eye internally. One example is the tuatara, a reptile-like creature found in New Zealand.
The tuatara has a visible light-sensitive organ positioned on top of its head. Like the human pineal gland, this organ is not used for sight but helps detect changes between light and darkness to regulate the body's internal clock.
