Do you see what I see? | Bangkok Post: learning

THE CHINGCHOK Hunter

The eye is an exceptional organ although the effectiveness of an animal's eye depends on how much it relies on sight and the environment in which it lives. Arguably, birds of prey, such as eagles and falcons, as well as scavenging vultures, have the most advanced eyesight, and they are able to spot tiny prey scurrying among the bushes or a carcass from kilometres away.

Spy eye Illustration NATTAYA SRISAWANG

But at night, their vision is nothing spectacular, whereas an owl's vision is superior. But neither would be good underwater, where a squid's or fish's eye would be the most useful. And if you compare all these to the virtually non-existent eye of the Mexican blind cave fish (blind tetra), whose eyes degenerate over its life, you will realise that the eyes of the animal suit the animal.

This is because all animals with eyes have evolved eyes and eyesight to suit their particular living requirements. In the case of the blind tetra, which lives in pitch-dark caves, eyes serve no purpose and are expensive organs to grow and maintain, so in essence, even this creature has the perfect eyes for its environment.

Separate evolution

The complexity of eyes used to be an argument for creation, with the creationists' point being how can something so intricate and complex arise without a designer?

They extended this by asking what the point was in "half an eye" if eyes had gradually evolved over time and also likened it to there being no point in "half a watch" as something as complex as a watch is obviously designed and created.

But from an evolutionary point of view, there is every point in half an eye, or even a tenth of an eye. Having photoreceptive cells that may alert an animal to danger, leading it to survive better than its photoreceptive-lacking competitors, will allow it to reproduce and pass on its genes, which will spread the benefiting photoreceptive cells throughout the species.

After millions of years, the first mutation led to photoreceptive cells, and the eye became more and more specialised, as evidenced by the eyes of today's modern animals.

Biological research and DNA (deoxyribonucleic acid) evidence have led to the discovery that the complex structures of different animals' eyes have evolved separately in separate species through evolution. When we live on a planet bathed in light for half of each day, it is not that surprising that light-sensing organs would evolve, but it is still fascinating.

Although all creatures with eyes have a very distant common ancestor about 540 million years ago that would have had basic light receptive cells, mammalian eyes, insect eyes, squid eyes and many others have evolved on a very separate path and are all unique.

The human eye

The human eye is nothing short of remarkable, although to be honest, if it were "designed" the "designer" could have done a much better job! Our eye, similar to cephalopod eyes, is called a camera eye and this is the basic structure.

The black hole in the middle of the eye is the pupil and is there to let light in, which is the essence of sight. The colourful (in my case blue) ring around the eye is made of a muscular tissue, and is called an iris.

The iris controls the amount of light that enters the eye. When it is bright, the iris expands, thereby contracting the pupil and reducing the amount of light allowed in. In darker conditions, the iris recedes, causing the pupil to dilate, which can allow up to 30 times more light in than when the pupil is at its smallest.

The clear membrane in front of the iris and the pupil is called the cornea. It serves to bend light as it enters the eye, which enables us to focus on images. The white part of the eye - the outer, tough layer - is called the sclera and gives the eye its structure.

When light reflects off an object, it refracts as it passes through the cornea and then the pupil. It is further bent and focused by the lens, which is a clear blob-like structure that changes shape by means of the ciliary body in order to finely focus the light entering the eye.

The focused image rests upon the back of the eye known as the retina. It is the area sensitive to light and has two very specialised types of cells. Rod cells allow you to see in low light and cone cells allow you to see details and in colour.

Light is converted into electrical impulses on your retina, which joins to the optic nerve, which sends the message to your brain for interpretation. Interestingly, the image that forms on your retina is upside down, so your brain has to turn it to the right way up.

The sclera is connected to three pairs of muscles that allow the eyes to rotate in their sockets, allowing you to move them to things of interest, such as this article. Otherwise, you'd have to move your head from side to side as you read. The light they allow in is focused by using the ciliary body so that you can discern each word printed here.

Super senses

The largest eye in the animal kingdom belongs to the giant squid (or maybe the colossal squid) and it can grow to the size of a beach ball! The largest land animal's eye belongs to the ostrich. Its eye is actually bigger than its brain!

Cats and other nocturnal predators generally have elliptically-shaped pupils that can allow more light in, resulting in better night vision. They also have a reflective membrane in their eyes that reflects light twice onto the retina, again allowing for better night vision. Although cats cannot see in pitch darkness, the slightest moonlight can mean clear vision and a good night's hunting.

Arthropods have unusual compound eyes, which is like having lots of eyes that make up one big eye. It allows them to see movement quite well. That is why it is so difficult to successfully swat a fly.

Honeybees can see in colour. Their eyes are sensitive to the blue end of the spectrum; they can even see in ultraviolet (UV) light. You'd be amazed how different many bee-pollinated flowers look under UV than under normal light, attracting bees with patterns we don't even know are there because our eyes aren't sensitive to that frequency.

The most amazing example belongs to a fish called the Bathylychnops, It has two normal eyes, but it has also evolved two smaller eyes that point downwards to alert it to predators underneath it! Four eyes are better than two.

Eyes really are incredible, and I hope you see what I mean!

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