Eyes Are 500 Million Years Old: Study
MELBOURNE: Our eyes are at least half a billion years old — a good deal older than the brain — a new study has found.
The eyes are one of our most remarkable and precious organs, yet their origins have been shrouded in mystery until quite recently, said Professor Trevor Lamb of the Australian National University and The Vision Centre.
The deep origins of 'sight' go back more than 700 million years when 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 said.
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," Lamb said.
"Our type of eye — a single globe packing in millions of photoreceptors — first starts to emerge between 500 and 600 million years ago. This was the crucial moment for our vision system," Lamb said in a recently published scientific review in ScienceDirect.
"Baby sea squirts have a simple eyespot called an ocellus, which is basically a bundle of photoreceptors. The adult animal loses this, as it becomes immobile, so vision is not important. This organ appears to date back at least 600 million years.
The hagfish has a patch of translucent skin on each side of the head where you'd expect to see its eyes, and buried beneath are a pair of very simple 'eyes' with light sensing cells and a simple optic nerve — but no muscles, lens or iris. Hagfish ancestors go back more than half a billion years, so this crude light sensing organ seems to have been the start of something big," he said.
Lampreys also appeared around 500 million years ago, and have a pair of camera-style eyes similar to our own. These appear to be direct forerunners of the vertebrate eye, which we have inherited through our fish ancestry, said Lamb.
"From this we can say that the vertebrate-style eye has been around at least 500 million years — and although its light-sensors and signalling 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," he said.