The Evolution of Sight

Added: 24/09/21

Every morning when you blink your eyes open, light floods in and instantly pictures begin to form. We don’t distinguish these pictures from reality. You don’t look out of your window at a rendering of the skyline; it is the skyline. Mostly we don’t think about how we see at all, we just start looking about. 

In the backroom of our body, our visual system - eyes and brain - work together to form these perceptions of the outside world. A whole region of our brain is dedicated to detangling and deciphering the thousands of wavelengths of light our retina detects every second. This complicated and advanced system is the product of millenia of adaptation and evolution. Its first iteration, way back when, was as rudimentary as it was sublime.
 

Our Bacteria Ancestors

Cyanobacteria, the first creatures to govern this planet, harnessed light’s raw power to survive. They photosynthesised, using energy from sunlight to create energy for movement and growth. From Cyanobacteria, all plant life evolved photosensitive chloroplast to produce energy for their growth. Indeed, the chloroplast with which plants make food for themselves is actually a modern day cyanobacterium living within the plant's cells. 

A by-product of photosynthesis is Oxygen. Photosynthesis in Cyanobacteria led to the rise of oxygen on Earth ~2.3 billion years ago, facilitating the development of aerobic respiration, which is what allowed human beings – and all animals – to evolve.

But photosynthesis didn’t just produce oxygen for us to breathe, it is also the ancestor of our vision. 

 

Growing an Eye

The earliest ‘eye’ was not much more than an eyespot, a patch of photoreceptor proteins that were sensitive to light in a single bacteria cell. This eyespot couldn’t pick out shapes but it was able to determine whether the bacterium was facing the light or not, and whether it was day or night triggering its circadian rhythm. 

From here all animals’ vision evolved. Our vision has the same origin as an insect’s, as your cat’s, as fish underwater. Different species have evolved to have differently specialised eyes for their environments and hunting strategies. For example, predators generally have eyes on the front of their heads for better depth perception to focus on prey. Prey animals' eyes tend to be on the side of the head giving a wide field of view to detect predators from any direction. But all of our eyes have the same evolutionary history. 

 

The single light sensing eyespot expanded to form a platform of light sensing cells. This then formed a bowl, helping it to identify different directions of light. After a while, instead of deepening the bowl, adaptation began to reduce the size of the opening instead, closing the bowl in. This effect is somewhat like a pinhole camera. It allowed organisms to see rough images.

A membrane then grew over this to prevent debris entering the eye, and, this way, little by little, our eyes evolved to create a gland that produced liquid with the same salinity as seawater. It is a common misconception that tear ducts make tears. In fact, the lacrimal glands of the eye make tears. The tears then flow over the eye and drain into the tear ducts. Tears prevent excessive drying of the surfaces of the eye and provide some nutrition and oxygen to surface structures. These tear ducts in our eyes today are a fossil of our evolution and act as a reminder of how long light has been crucial to life on Earth.

 

Did you know that Newborn babies don’t cry tears? For the first several weeks of a newborn’s life, they can scream and wail but they will not shed tears. When their lacrimal glands are fully developed, they will start crying real tears. 

 

There are other indications of our photosensitive ancestors in our vision. For example, it is likely that eyes specialise in detecting the range of wavelengths on the visible light spectrum as the earliest species (cyanobacteria) to develop light sensitivity were aquatic, and water filters out a lot of wavelengths of radiation bar those we refer to as blue, through to longer wavelengths we identify as red. 

So far we can see the shape of the eye developing, but the evolution of the human eye has some way to go from here. The development of the lens, the flexing muscular iris for focus, and of colour vision. We’ll explore those developments in the coming weeks. 

The story of the earliest roots of the eye illustrate how crucial light has been to the development of life on earth, and how light sensitivity (if not sight) is something most living creatures share. Take a moment to think about that next time you look up at a tree, or at the houseplant bending towards the sun on your windowsill. 

This is an extract from the next chapter in our Light Tribe Shedding Light series. If you’d like to learn more about the science, history and psychology of light, sign up to our Light Tribe members area to receive exclusive access to our monthly, downloadable chapters. 


Chapter 6: A Sense of Light will be released next week. Sign Up Now. 

 

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PHOS

PHOS

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Power of Light

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