Eyes are really weird when you think about them.
They’re like little gel filled sacks sticking out of our faces.
They take all these wavelengths of light and somehow make pictures out of them.
But how did they evolve to be like that?
Hey seers Julia here for DNews
Eyes are extraordinarily complex.
And the theory of evolution posits that such complexity arose from the process of natural
selection over millions of years.
But extraordinary claims demand extraordinary evidence.
So how did eyes evolve?
Well with each generation of offspring having slightly altered eyes with ever increasing
improvement, that gave them a decisive advantage over predators and peers.
Back when life was just a bunch of cells in a soup, eyes probably got their start in bacteria
over 600 millions years ago.
A random mutation gave some bacteria a collection of light sensitive cells.
These cells helped the bacteria know how bright sunlight was.
And some bacteria, who could sense this light, moved away from it, which helped them since
bright sunlight also carries harmful UV rays.
After a few more random mutations these little “eyespots” as they’re called turned
into a kind of early eye.
Early eyes, if you can even call them that, were just a collection of cells called photoreceptors
with photosensitive cells with proteins called opsins.
They lay on top of the photoreceptor and catch photons coming into the eye.
They trigger a series of chemical reactions that causes the photoreceptor to send an electrical
message towards the brain.
Their appearance in the tree of life can be traced back more than 580 million years and
can be found in everything from jellyfish to insects to dogs and humans.
One study published in the journal National Review of Neuroscience found that evidence
like this “strongly supports the notion that a common ancestor of sea-squirts and
vertebrates possessed...many of the building blocks that are fundamental to light signalling
in our own eyes.”
And we can see some of this process happening in the fossil record.
We can find evidence for early eyes in fossils of trilobites, a marine arthropod, that lived
around 543 million years ago.
As in most cases of evolution the delicate balancing act, or should I say arms race between
predators and prey, drove the evolution of the eye.
So you’re probably wondering, how could we tell what their eyes looked like, eyes
don’t fossilize well.
And you’re mostly right, except trilobites had a type of bacteria in their eyes that
laid down a thin layer of minerals.
And it’s these traces that we can see, but only with super high powered x-rays.
According to a study published in the journal Science Reports, trilobite eyes looked like
flowers.
Under a lens, their sensory cells are shaped like petals, surrounding a photoreceptor.
While basic, these eyes served well enough for these bottom of the ocean dwellers.
They worked so well in fact, a relative of the trilobite, the horseshoe crab, still has
them.
As time wore on, the arms race drove the evolution of more and more complex eyes.
Rather than looking for fossilized evidence we can examine the eyes of modern animals
that haven’t changed all that much.
Take a look at the turbellarian worm.
Their eyes are more like little cups lined with pigment cells.
These cells are opaque and don’t let light pass through, so they act as a way to direct
light towards the photoreceptor cells.
After a few more years of evolution, the eye cup becomes more of a chamber and the opening
narrows.
This shape allows greater ability to tell where the light is coming from and an early
way to form images of shapes and things.
This is called a pinhole eye and can still be seen on primitive animals like the beautiful
nautilus.
Along the evolutionary way, animals evolved spines and skulls, and with it, their eyes
got more advanced.
According to a study published in the journal Nature Reviews Neuroscience, researchers found
we could trace our own complex eyes all the way back to early lamprey-like vertebrates
who lived over 500 million years ago.
Their eyes start to look more like how we think of eyes.
A lens forms from a thin layer of adapted epithelial cells covering the opening to keep
out infection.
They even have a retina and ocular muscles that allow for eye movement.
These eyes would have been able to form a simple image over a broad range of wavelength.
But there was a problem.
These eyes evolved for living in water.
Once our ancestors moved onto land, our eyes needed to evolve again.
Around 430 million years ago, according to a study published in the Journal of Experimental
Biology, tetrapod eyes took on a more elliptical shape to compensate for seeing in the air
rather than water.
And the development of evolution even decides where eyes are located.
For instance prey like deer tend have their eyes on the side of their heads so they can
keep a watchful eye out over nearly 360 degrees for a predator.
And predators, like wolves, tend to have eyes on the front of their head so they have depth
perception.
All the better to hunt you with my dear.
So our eyes aren’t perfect and evolution isn’t a perfect process.
But as Charles Darwin said “from so simple a beginning endless forms most beautiful and
most wonderful have been, and are being, evolved.”
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So speaking eye ball weirdness, ever look up at the sky and see all those strange floaty
bits?
WHAT THE HECK ARE THEY!?
SHOULD I BE WORRIED ABOUT THEM?
Trace has the whole scoop in this episode right here.
So eye balls how cool are they?
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