The importance of the eye, in terms of health, is emphasised in the quotation above. St Luke is believed to have been a physician. The eye in its different forms has long attracted interest and attention.
In geological time light-sensitive structures appeared long before the vertebrate eye. Photoreceptors in single-celled organisms permitted the latter to be aware of light and, if necessary, to respond to it by moving or changing behaviour (photoperiodism) but the cells were not sufficiently complex to facilitate detailed navigation nor, indeed, to inter- pret light in any detail. Simple photoreception has, however, enabled many extinct and extant single-celled organisms to take maximum advantage of sunlight for photosynthesis and move towards it. Insofar as complex eyes are concerned, fossils representing the early appearance of such date back 500–550 million years ago, to the period of rapid evolutionary change that is commonly called the ‘Cambrian Explosion’. This resulted in the appearance of a wide range of ocular structures, which differed in acuity, the range of wavelengths that they could detect and whether they could differentiate colours.