The third eye is not just a metaphysical yoga concept.

Scientists have discovered the origins of the peculiar median cornea buried deep inside our head, our earliest ancestor from almost 600 million years ago. This tri-cornea theory was detailed in an eye-opening study published in the journal Current Biology.

The team had set out to shed light on the pineal gland, a pea-sized organ inside the human skull that dictates how our body responds to light in the dark, even though it’s long been cut off from sunlight.

Found in nearly all vertebrates, from cats to frogs and humans, this tertiary light receptor produces melatonin, the hormone that controls sleep and wakefulness.

While documented by doctors since ancient times, the origins of the “mind’s eye” were unclear — until now.

Study author Prof Thomas Baden, a neuroscientist at the University of Sussex and co-author, said that they wanted to find out the “original solution to vision,” as well as the extent to which “different species just copied or modified it to make it their own.”

“What really are the patterns?” he wondered, per the BBC’s Science Focus Magazine. “As you do this over time, you start to wonder, what is the original eye?”

To glean insight into the evolution of our optic organ, the team looked back to one of our earliest ancestors — a small, sea-dwelling maggot from 575 million years ago.

This critter sported two lateral eyes for navigation along with a third lens atop its noggin for monitoring light levels and staying oriented.

This design lasted until around half a billion years ago, when they started burrowing into the sediment, which meant they no longer needed their side eyes to navigate.

In accordance, our subterranean forebears ditched their lateral peepers to conserve resources, effectively leaving them with the third receptor that allowed them to discern up from down and day from night.

This cyclops-like configuration distinguished vertebrates from other lineages, which retained their lateral retinas.

What’s to account for the forward-facing lenses we have today? Eventually, some of the ancestors left the underground and returned to the sea as filter feeders, necessitating tools for navigation — in other words, we needed our eyes back.

Left with no other recourse, parts of the third eye evolved and migrated to the sides of the head, eventually developing into retinas. However, the third eye remained like an evolutionary straggler.

In doing so, they proved that both our retinas and pineal gland evolved from the same ancient organ and not separately as some experts have theorized.

Coincidentally, the study didn’t need to conduct an eye test to shed light on our ocular evolution. They simply reviewed existing scholarship and genetic data from animals such as fish and lampreys to determine how the sight organ’s function in our relatives.

Coincidentally, not all critters’ third eyes are hidden from view.

The tuatara – a bizarre, lizard-like creature from New Zealand – has a sophisticated middle orb situated prominently atop its head like some hybrid monster from Greek mythology.

Much like with humans, this eye is not used for sight, but rather to detect changes between light and dark and use this info to regulate its circadian clock.