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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. [33] In humans, brown is by far the most common eye color, with approximately 79% of people in the world having it. [34] 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. [35]
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.
Moles are small, subterranean mammals. They have cylindrical bodies, velvety fur, very small, inconspicuous eyes and ears, [1] reduced hindlimbs, and short, powerful forelimbs with large paws adapted for digging. The word "mole" most commonly refers to many species in the family Talpidae (which are named after the Latin word for mole, talpa). [2]
The human eye, showing the iris and pupil. In 1802, philosopher William Paley called it a miracle of "design."In 1859, Charles Darwin himself wrote in his Origin of Species, that the evolution of the eye by natural selection seemed at first glance "absurd in the highest possible degree". [3]
Statements that certain species of mammals are "born blind" refer to them being born with their eyes closed and their eyelids fused together; the eyes open later. One example is the rabbit . In humans the eyelids are fused for a while before birth, but open again before the normal birth time, but very premature babies are sometimes born with ...
The cooperative eye hypothesis is a proposed explanation for the appearance of the human eye. It suggests that the eye's distinctive visible characteristics evolved to make it easier for humans to follow another's gaze while communicating or while working together on tasks.