The eyes are one of our most remarkable and precious organs, yet their origins have been shrouded in mystery until quite recently. Professor Trevor Lamb of The Vision Centre and Australian National University, has completed a major scientific review of the origin of the eye. The article summarizes the findings of hundreds of scientists around the world and was recently published in the journal Progress in Retinal and Eye Research.
The origins of ‘sight’ date back more than 700 million years when the earth was inhabited only by single-celled amoeba-like animals such as algae, corals and bacteria. During this period, the first light-sensitive chemicals, known as opsins appeared. They 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,” Lamb 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. “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.”
An example was Anomalocaris, a metre-long predator like a giant scorpion that Lamb referred to as the “Jaws” of its day. It had eyes the size of marbles to help it navigate the ancient seas and locate its prey. This beast, which employed the ‘insect eye’ model with many facets, had at least 16,000 facets containing vision cells in each eye.
“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.” Lamb says. “For the first time animals begin to ‘see’ the complex landscape which they inhabit.”
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, Prof. Lamb contends. Lampreys have a pair of camera-style eyes remarkably similar to our own. These appear to be direct forerunners of the vertebrate eye, which we have inherited through our fish ancestry, says Prof. Lamb.
“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.”
Paired eyes also appear to feature in the strange ‘crest animals’ found by Chinese scientists in rocks that are around 500 million years old. From then on the basic plan of the vertebrate eye became more settled, gradually evolving from fish, amphibians, reptiles, birds and then to us.
“The advent of spatial vision provided immense survival value to the creature that had it – but the process occurred slowly, over countless steps, with the transition from a simple eye spot to the vertebrate-style camera eye possibly taking as long as 100 million years,” Lamb concludes.