Scientists discover secret 'third eye' hidden in your skull and the usual reason why its there | indy100

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Did you know humans have a 'third eye'?

Unlike the tuatara, a reptile from New Zealand with a visible functioning third eye located on the top of its head, the human third eye - called the pineal gland - isn't visible as it is located deep within the brain.

The function of the pineal gland is directing how our body respond to light and dark.

But the question is, where exactly did this gland come from?

A new hypothesis published in the journal Current Biology seeks to answer this, and it appears our third eye is derived from some of our ancient ancestors and could hold the key to understanding how human sight has evolved.

For context it's important to know that the eyes of vertebrates are built differently compared to the rest of the animal kingdom.

Typically, for most animals on Earth the light-detecting cells in their lateral eyes are a part of an ancient family, called rhabdomeric photoreceptors. Then for the non-visual jobs like tracking day length and sensing overall light levels, this is carried out by a second family called ciliary photoreceptors, as per BBC's Science Focus.

It's a different story for vertebrates - humans, fish, birds, and reptiles - as our eyes are built from ciliary photoreceptors at the input end, wired into neurons of rhabdomeric origin at the output end.

And no one know how this happened.

Prof Thomas Baden, a neuroscientist at the University of Sussex and co-author of the new study, questioned “What is the original solution to vision, and to what extent have different species just copied or modified it to make it their own?”

He further pondered if there were any patterns, and what is the original eye.

To get to the bottom of this, Baden and his researchers had to look back to 575 million years ago for a certain ancestor - a far cry from what we look like today (we're talking a small worm-like creature located on the seafloor).

This creature would've have two lateral eyes for navigation, plus an eye on the top of its head to monitor the light levels, and to help with balance.

From that point, Baden and his team's hypothesis is that some kind of change occurred where the ancestors that would evolved into vertebrates began to burrow down in sediment filter-feeding on particles floating past, they no longer needed to navigate.

As a result, the lateral eyes were lost due to them no longer being useful and taking to much energy to sustain.

What was left were sensors and photoreceptors can could still indicate direction and light level (difference between day and night).

“The need to know what time of day it is, or where is up and down if you're in deep water. That doesn't go away,” says Baden. “So, we speculate that that's when we lost the original side eyes, but we kept the original median eye, because that's what it's good for.”

Due to this burrowing, scientists reckon this explains why vertebrates have a different structure - many animals still have their lateral eyes, while ours were lost.

Thanks to evolution, some of the ancestors didn't remain burrowed in sediment but instead reverted to the open water as free-swimming filter feeders.

This meant they needed to be able to navigate, and so the new eyes included both ciliary and rhabdomeric cell types as only the photoreceptive organ was only available matter to be built from.

So what does this mean?

It means for the retina - the layer inside the back of your eye that detects light - an earlier precursor version were already made in the media eye, with us intermitting the intricate features that make up the eyes we have today.

That being said, Baden is hesitant to call the median an an eyes - here's why.

“The thing on top of the head originally is not one eye; it's more like a series of sensors, multiple patches of photoreceptors,” he explained Therefore, "the retina predates the eye, if that makes sense. I always thought that was a cute tagline."

Elsewhere, a different study recently published in Nature suggests our ancestor had four eyes with lenses and retinas at one point in time.

Given the sheer scale of evolution history (around half a billion years), it remains unclear if these scientific theories are correct - although Baden believes we'll soon get to the bottom of this.

“The central testable bits that we've put forward – I think with some funding and a few years – you can get a yes-no answer,” he said.

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