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Diagram of a human eye; note that not all eyes have the same anatomy as a human eye. The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment. [10] The human eye is not a plain sphere but is like two spheres combined, a smaller, more sharply curved one and a larger lesser curved sphere.
The size of the eyes in mammals is relatively small; in humans, eye weight is 1% of the mass of the head, while in a starling it reaches 15%. Nocturnal animals (for example, tarsiers) and animals that live in open landscapes have larger eyes.
Almost all mammals have brown or darkly-pigmented irises. [34] In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it. [35] Brown eyes result from a relatively high concentration of melanin in the stroma of the iris, which causes light of both shorter and longer wavelengths to be absorbed. [36]
This type of eye is common in mammals, including humans. The simplest eyes are pit eyes. They are eye-spots which may be set into a pit to reduce the angle of light that enters and affects the eye-spot, to allow the organism to deduce the angle of incoming light. [1] Eyes enable several photo response functions that are independent of vision.
The rate of eye evolution is difficult to estimate because the fossil record, particularly of the lower Cambrian, is poor. How fast a circular patch of photoreceptor cells can evolve into a fully functional vertebrate eye has been estimated based on rates of mutation, relative advantage to the organism, and natural selection.
The functioning of a camera is often compared with the workings of the eye, mostly since both focus light from external objects in the field of view onto a light-sensitive medium. In the case of the camera, this medium is film or an electronic sensor; in the case of the eye, it is an array of visual receptors.
Image credits: historycoolkids The History Cool Kids Instagram account has amassed an impressive 1.5 million followers since its creation in 2016. But the page’s success will come as no surprise ...
The cooperative eye hypothesis is not the only one that has been proposed to explain the appearance of the human eye. Other hypotheses include the proposal that white sclerae are a sign of good health, useful in mate selection, or that eye visibility promotes altruistic behaviour by letting people know they are being watched. A study by the Max ...