It's coming up on the five year anniversary of my PRK eye surgery. PRK (photorefractive keratectomy) is a lot like the more well-known LASIK (laser-assisted in situ keratomileusis, commonly known as Laser Eye Surgery), where a laser is used to reshape the cornea to correct vision problems. At the time of my surgery, I was just excited that I didn’t have to wear thick glasses anymore. But recently, I’ve been thinking of what an amazing contraption an eyeball actually is, and how we got eyes in the first place.
To understand how eyes evolved, you first need to know how they work. Your eyes act much like tiny cameras installed into your head, taking in the light your brain needs to build images. The parts you can see when you look in the mirror are the sclera, cornea, iris, and pupil. Inside there is a lens, retina, and a jelly-like tissue called the vitreous humor. All of these components are packaged together into an organ that is roughly the size of a ping pong ball.
The sclera is the white part of your eye. Covered in tiny blood vessels, it is made out of a tough material which protects the delicate inner components. The cornea is the dome of transparent tissue on the outer eyeball. It acts like a window that allows you to see outside, while protecting everything inside. It is also the way light gets into your eye. Beneath the cornea is the iris, which is the colourful part of the eye. It is a flexible, ring-shaped membrane that can contract or expand to control how much light enters the eye. The black circle in the middle of the iris is the pupil. While it may look solid, it is actually the hole that allows light to enter into your inner eye!
Beyod the pupil, inside the eye, there is the lens, retina, and the vitreous humor. The lens sits right behind the pupil and helps focus light as it comes through the pupil. Light that has entered the eye travels through the vitreous humor, before finally hitting the retina, which is a special kind of tissue that is embedded with millions of light sensitive cells. Each eyeball has an optic nerve attached to the end of it that sends all the information your eyeballs receive to your brain to process.
All together the eye is a serious piece of work!
From Simple to Complex
Scientists think the earliest version of the eye was formed in unicellular organisms, who had something called ‘eyespots’. These eyespots were made up of patches of photoreceptor proteins that were sensitive to light. They couldn’t see shapes or colour, but were able to determine whether it was light or dark out. These unicellular organisms would use photosynthesis to create food for themselves, so being able to determine where the most light was coming from created a huge advantage for them.
Over time, the unicellular creature would evolve, and its eyespot evolved along with it. Scientists believe a depression formed around the light sensitive spot, creating a pit that made its ‘vision’ a little sharper. Eventually, the pit’s opening could have gradually narrowed, creating a small hole that light would enter, much like a pinhole camera. From there, a retina would develop, as well as a lens at the front of the eye. Over millions of years, small changes that created a slight advantage for survival would change a simple light-sensitive structure to the complex eyes we have now.
Scientists make these assumptions about how the eye evolved because eyes in corresponding stages to this sequence have been found in species that exist today.
A Single Source
As eyes were evolving from crude, light-sensitive cups to more complex systems, the Earth was also undergoing a dramatic shift. A complex interplay of environmental changes were setting the stage for large, active creatures to evolve. And they did just that, with an outburst of speciation that is now known as the Cambrian explosion. It was during this time that some eyes became more complex and specialized, and began to take on different shapes and colours.
Due to the diversity of eye types around the world, scientists used to believe that eyes had many independent origins. Advances in technology helped us learn more about the molecular structure of eye, and showed that proteins known as opsins are the foundation of all eyes in all creatures. This commonality confirms that all organisms with eyes, at one point, shared a common ancestor.
In the blink of an eye
The first organisms with a modification resembling an eye lived around 550 million years ago. According to one scientist’s calculations, if they eye improved just 0.005 percent each generation, it would take 364,000 years from eyes to evolve from a patch of light sensitive cells to the complex eyes we have today. And in a geological timeframe, that’s just a blink of an eye.