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5 DEC 2023

The Curiosity of Sight

How eyes work and how we discovered it

This episode Anton and I discover how animals see. From the Cambrian Explosion and mass of new creatures that evolved to neutral implants via some unfortunate German rabbits we uncover the history and the science.

Early eyes and the Cambrian Explosion

The Cambrian Explosion was a pivotal event in Earth’s history about 541 million years ago. It marked a period of rapid and remarkable evolutionary changes. Over a relatively short geological time frame of about 20-25 million years, there was an unprecedented diversification of life forms in the oceans.

This era saw the emergence of most major animal groups, along with the development of complex structures like shells, limbs, and notably, eyes. The Cambrian Explosion is crucial in understanding the evolution of the eye, as many organisms developed varied forms of vision. From basic light-sensitive spots to more complex structures like compound and pinhole eyes, this period was a hotbed for evolutionary experimentation in sensory organs. The sudden increase in predatory behaviours and environmental complexities likely drove this rapid evolution, making the Cambrian Explosion a cornerstone in the study of life's history on Earth.

Above: The evolution of eyes from basic photoreceptors to complex, mammalian eyes.

Below: The development of compound eyes.

Notice the two different paths taken and the differences between enclosed eyes, with a single lens focusing light on many photoreceptor cells, and compound eyes having many lenses.

Insects don’t see many copies of the same image like in films. Their vision is more similar to a low resolution screen on pixels.

Each tiny lens captures an individual image, and the mosquito's brain puts all of the images together to achieve peripheral vision without the insect having to move its eyes or head.

Encyclopedia Britannica

Pioneers of vision

Greek concepts for vision included those of Empedocles who believed the eyes emitted rays that reached out and touched items. The Atomist Leucippus thought objects gave off a thin film of particles that entered out eye.

It would take until the Islamic golden age in the 10-11 centuries for the scholar Inb al-Haytham to understand how light enters, and is focused by, the eye.

Diagram of the structure of the human eye according to Ibn al-Haytham.

Little would for several hundred years until a new era of scientists equipped with a knowledge of biochemistry and instruments allowing them to see at a smaller scale began to uncover the secrets of the eye.

The key discovery was rhodopsin, the light sensitive chemical in the eyes of all living creatures. It was the key to how the eye captured images.

It was believed that the dying eye would capture a final image and a series of grisly experiments went on to prove this was true. However the image quickly fades.

Wilhelm Kühne would capture images from the eyes of rabbits and people!

Mach bands - the phenomena where solid colours appear to gradate when placed next to others of high contrast - would be an important clue in how eyes adapt what they see before passing the information to the brain.

There are records from Egypt dating back to 2,500 BCE to remove cataracts. Poppy seeds would be chewed up and spat into the patient’s eye to reduce pain.

A ‘couching’ needle would then be used eye to dislodge the cataract until it came loose and could be pushed into the eye.

Today treats include advanced medical practices such as neutral implants that bypass the eyes altogether and feed signals directly from a camera to the brain!

To learn more listen to our episode.

Further reading

Transcript

This transcript is automatically generated so may contain errors.