Biology
The First Eye On Planet Earth
Why does do we have eyes? Why is it so dominant in the animal kingdom as the #1 sensory mechanism? How did vision begin? How did eyes evolve?
From when we first saw the world as a baby to when we take that last glance at our grandkids, all of us on Planet Earth perceive the world with our eyes — an organ that we so easily dismiss.
You, as a matter of fact, rely on eyesight for almost everything in your life (including reading this article), how often do you take the existence of your eyes for granted?
You might argue why we shouldn’t take our eyes for granted, after all, they’re something almost every living species has. You’ve gotten used to waking up each morning and checking your phone, or even looking up at the sky every day, we’ve gotten so used to our eyes that we don’t need to give them the credit they deserve.
Perhaps, but, we can’t refuse to ignore how difficult life would be without them.
Why do we have eyes? How did evolution play the cards so well?
How or why did vision evolve on Earth?
Starts with a cell, ends with a cell
It all started with a single cell ~3.7 billion years ago that could replicate.
Which cell and how did it get there?
It’s a really old ancestor of cyanobacteria or more commonly called as blue-green algae.
No one really knows how first cell on Planet Earth originated but my favourite hypothesis is the “primordial soup” hypothesis.
According to this hypothesis, 3.7 years ago, Earth had a relatively warm and shallow ocean that contained a rich mixture of organic compounds, such as amino acids, sugars, and nucleotides which were created fom chemical reactions involving the energy from lightning strikes, volcanic activity, and radiation from the sun.
Over time, these organic compounds combined and recombined in several different ways, eventually forming self-replicating structures that became the first cells (the ancestors of cyanobacteria)
These cells were extremely simple, with a basic membrane that separated the cell from its environment that allowed it to exchange materials with the outside world.
Sunlight — The Goldilocks wavelength
The sun is the source of energy for cells.
They use the light it emits to grow and divide. However, not all types of light are helpful for cells.
Some types of light, like X-rays and gamma rays, can be harmful to cells.
On the other hand, really long wavelengths, like radio waves, don’t have enough energy to be useful for cells.
But, there is a specific range of wavelengths that are just right for cells. These wavelengths are in the visible and near-visible range, which includes colors from violet to red, and even some light that we can’t see, like ultraviolet and infrared.
They have enough energy to be useful for cells, but not so much that they cause damage. This was the sweet spot.
Over time, cells evolved to be able to capture this type of light and use it for energy.
Baby Steps
These first cells had something special called a chloroplast that allowed it to use a specific kind of light from the sun to make energy.
This was really important because it helped make the planet green and allowed for the production of a substance called beta carotene, which is needed to make something called retinal.
Why is Retinal important?
Retinal is important because it is used in a family of proteins called opsins, which help living things detect and respond to light.
The first opsins were probably similar to another type of protein called a G-protein coupled receptor, but they combined with retinal to make a proton pump that produced energy for certain Archaea (a type of microbe).
Later on, this same combination led to the development of opsins that could detect light in more complex creatures, including us.
There are other compounds that can help cells respond to light, but they aren’t as good as retinal and opsins.
What happened Next?
When opsin bonded with retinal, it allowed cells to sense light.
Behold, The first eye.
Instead of being used as a proton pump, this molecule was co-opted for sight, and eventually led to the development of multicellular organisms with light-sensing cells in an “eyespot”.
Next, these eyespots began to sink into the organism’s body, forming a concave shape that could focus light and create a rudimentary image. This primitive eye, while still simple, was capable of detecting the presence and direction of light with greater accuracy than the earlier photoreceptor clusters.
As organisms evolved and became more complex, their eyes also became more sophisticated. The concave shape of the eye gradually deepened, forming a spherical shape that could focus light more precisely. A transparent layer of tissue, the cornea, evolved to protect the eye while still allowing light to enter.
In addition, a lens evolved within the eye to further focus light, allowing for even greater image resolution. The lens is surrounded by ciliary muscles, which allow it to adjust its shape and focus on objects at different distances.
Finally, the human eye evolved to include a layer of specialized cells at the back of the eye, called the retina. The retina contains millions of photoreceptor cells, which detect light and transmit signals to the brain via the optic nerve. The brain interprets these signals to create the images that we see.
So, the evolution of the human eye was a long and complex process, but one that ultimately resulted in a truly remarkable organ capable of perceiving the beauty and complexity of the world around us.
The first eye on Earth was’nt just a random event, but the culmination of billions of years of evolution, of countless mutations and adaptations, of an unrelenting drive towards complexity and sophistication.
As we contemplate the wonder of the first eye on Earth, we cannot help but feel a sense of awe and reverence for the beauty and complexity of the natural world.
The evolution of the eye is a remarkable tale of trial and error, of tiny steps taken over millions of years, and of the slow march of life towards ever greater awareness of the world around us.
