If you were to ever find yourself in close proximity to a tuatara – a strange, lizard-like creature from New Zealand – you may notice something startling on the top of its head: a functioning third eye.
Much like the more prominent eyes on the side of a tuatara’s head, its third, or parietal, eye has a lens, a retina and nerve connections to the brain.
The fact that a vertebrate like a reptile, which is very much on our side of the tree of life, has such a sophisticated third eye may come as somewhat of a surprise. But the truth is, humans have one too.
Ours is called the pineal gland, and while today it lies buried deep within the brain, long since cut off from direct sunlight, it still plays a crucial role in dictating how our body responds to light and dark.
Now, a sweeping new hypothesis published in the journal Current Biology is attempting to explain exactly where that little gland came from. The findings reveal that our third eye comes from some of our most ancient ancestors. And understanding it may finally explain one of the deepest puzzles in the evolution of sight.
The problem with vertebrate eyes
Look closely at most animals on Earth – a fly, an octopus, a crab – and their eyes follow a surprisingly consistent plan.
The light-detecting cells in their lateral eyes belong to one ancient family, called rhabdomeric photoreceptors. A second family, called ciliary photoreceptors, sits quietly in the brain, generally doing non-visual jobs like tracking day length and sensing overall light levels.
Vertebrates – the group that includes fish, reptiles, birds and us – flip that rule on its head.
Our eyes are built from ciliary photoreceptors at the input end, wired into neurons of rhabdomeric origin at the output end.
It's a configuration that barely exists anywhere else in the animal kingdom, and nobody had a satisfying explanation for how it came to be.
“What is the original solution to vision, and to what extent have different species just copied or modified it to make it their own?” asks Prof Thomas Baden, a neuroscientist at the University of Sussex and co-author of the new study.
“What really are the patterns? As you do this over time, you start to wonder, what is the original eye?”
The cyclops ancestor
To answer those questions, Baden and his team looked back around 575 million years ago, with a relative of ours that few would recognise if they turned up for Christmas dinner.
Back then, we were little more than a small, maggoty-worm-like creature, grazing along the seafloor in shallow seas. Like most bilateral animals – those with a left side and a right side – it almost certainly had two lateral eyes for navigation and a simpler median eye on top of its head for tracking light levels and staying oriented.
Then, Baden and his colleagues propose, something changed. The ancestors that would eventually lead to vertebrates started to bury their heads in the sand – literally.
Burrowed down in sediment, filter-feeding on particles floating past, they no longer needed to navigate. The lateral eyes, which were now largely useless but energetically expensive to maintain, were lost.
All that remained was the patch of sensors and photoreceptors in the middle, still useful for telling up from down and day from 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.”
This burrowing phase, the researchers suggest, is why vertebrates are the odd ones out. While every other animal lineage kept its lateral eyes, we lost ours and would one day have to reinvent them.
One eye becomes three
Thankfully for us, some of these ancestors eventually left their burrows and returned to the open water as free-swimming filter feeders. Navigation, once again, became essential, and the only available material for building new eyes was the photoreceptive organ up top, which was a mixed system containing both ciliary and rhabdomeric cell types.
What followed, the researchers propose, was a slow process of that central eye becoming more complex and sprouting cup-shaped extensions that were sensitive to the direction of incoming light.
As these cups tilted sideways, they began to detect the way brightness and colour shift depending on depth, time of day and environment. More information, more evolutionary pressure to keep going.
Eventually, the cups migrated fully to the sides of the head, where they could support proper directional vision and navigation.
The original eye didn’t disappear, though. It persisted as the pineal gland, which can still be found across virtually all vertebrates, from lions to lizards to you.
In tuataras and some other reptiles, the cells remained as what is essentially a fully formed eye. In fish it is a simpler organ that still directly detects light. In mammals, it lost that ability, receiving light signals instead via a relay from the eyes.
An eye before an eye
This origin story has some striking implications. For the retina – the sheet of light-sensitive tissue at the back of your eye – it means a prototype version was likely already assembled in the median eye, with much of its sophistication inherited rather than invented in what we know as our eyes.
Calling the median eye an eye, however, is something of a stretch, Baden explains.
“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 says. Therefore, "the retina predates the eye, if that makes sense. I always thought that was a cute tagline."
And all of this is but the beginning of the strangeness. Another recent study published in Nature suggests our ancestors in fact had four eyes at one point, all complete with lenses and retinas.
The jury is still out on whether these theories are correct. Piecing together an evolutionary history spanning half a billion years is no small task. Still, Baden is confident we could have answers soon.
“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 says.
And yet regardless of how it got there, a simple fact remains. At the top of your skull, buried and lightless, sits a collection of cells that once looked to the sky. Without it, reading this article would be somewhat tricky.
Read more:
