Look after your eyes – they are at least half a billion years old, and a good deal older than your brain. The eyes are one of our most remarkable and precious organs, yet their origins have been shrouded in mystery until quite recently, explains Professor Trevor Lamb of The Vision Centre and Australian National University, who has devoted more than 30 years to investigating their secrets.
Prof. Lamb has just published a major scientific review of the origin of the vertebrate eye and vision, summarising the findings of hundreds of scientists round the world.
"There are profound questions about the eye which are still not easy to answer because it appeared so very long ago," he says. "Why did the eye develop? Why are there many different kinds of eye, including one for insects and crustaceans – and one for vertebrates like us?
"What kinds of animals needed these incredible seeing machines and how did they use them? How deep into time do the roots of vision go? How has the eye influenced our subsequent development?"
The deep origins of 'sight' go back more than 700 million years when the earth was inhabited only by single-celled amoeba-like animals, algae, corals and bacteria. At this time the first light-sensitive chemicals, known as opsins, made their appearance and were used in rudimentary ways by some organisms to sense day from night.
Ancient cells already had signalling cascades that sensed chemicals in their environment, and the advent of opsins allowed them to sense light. "But these animals were tiny, and had no nervous system to process signals from their light sensors," he explains.
Over the following 200 million years those simple light-sensitive cells and their opsins slowly and progressively became better at detecting light – they became more sensitive, faster, and more reliable – until around 500 million years ago they already closely resembled the cone cells of our present day eyes.
"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," Prof. Lamb says. "Creatures that could see clearly had the jump on those that couldn't.
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