Eyes are amazing and technical organs, precious to each individual. But how did the human eye develop? According to a review of the evidence, it stems back to fish more than 500 million years ago.

Professor Trevor Lamb of The Vision Centre and Australian National University conducted a major scientific review of the origin of the eye, in which he analyzes findings of hundreds of scientists worldwide.

From this review, published in the journal Progress in Retinal and Eye Research, the most significant finding is how the eye has evolved to become the organs we have today.

Prof. Lamb says that the origins of sight date back to more than 700 million years ago. At this time, the earth was only inhabited by “single-celled amoeba like animals,” such as bacteria, algae and corals.

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Researchers say that human eyes, as we know them, may have taken 100 million years to fully develop

Prof. Lamb explains that during this period, the first light-sensitive chemicals, known as opsins, appeared. He says that opsins were used by some organisms as a way of sensing day from night.

He explains that these organisms already possessed “signaling cascades” that could sense chemicals in their environment, and while opsins allowed them to see light, “these animals were tiny, and had no nervous system to process signals from their light sensors.”

The review says that over the next 200 million years, the opsins evolved to become better at detecting light, and they became more sensitive, faster and reliable.

But Prof. Lamb says it was around 500 million years ago when the human eye began to develop.

He says:

The first true eyes, consisting of clumps of light-sensing cells, only start to show up in the Cambrian, about 500 million years ago – and represent a huge leap in the evolutionary arms race. Creatures that could see clearly had the jump on those that couldn’t.”

He uses the example of an Anomalocaris. This was a meter-long shrimp-like predator, which Prof. Lamb refers to as the “jaws” of its time. Researchers believe the creature had eyes the size of marbles that it used to hunt its prey and for navigation in the sea.

It is also thought that this creature possessed more than 16,000 facets in each eye which contained “vision cells.”

Prof Lamb says:

This generates an avalanche of information, known as optic flow, running from the eyes and along the creature’s nervous system. This all has to be processed, so we also begin to see the rapid development of a central nervous system able to cope with such immense amounts of data, continually provided by the eyes and other sensory organs from the world around the animal.

For the first time animals begin to ‘see’ the complex landscape which they inhabit.”

He says that Lampreys, a jawless fish that is part of the eel family, first appeared during this period. These fish possess “camera-style” eyes very similar to human eyes. He adds that Lampreys may be the “direct forerunners” of the vertebrate eye.

“From this we can say that the vertebrate-style eye has been around at least 500 million years,” says Prof. Lamb. “Although its light-sensors and signaling systems are very similar to those of insects and other invertebrates, its optical system evolved quite independently from the insect-style eye with its many facets.”

Prof. Lamb notes that from 500 million years ago, the basic plan of the vertebrate eye became more settled, gradually evolving from fish, amphibians, reptiles, birds and then to us.

He adds that the development of the human eye as we know it today may have taken as long as 100 million years to fully develop.

Prof. Lamb concludes that the purpose of this review is to understand how the evolution of the human eye happened, in order to see how the human photoreceptors may evolve in the future.