Lynley Littlejohn, of Dunedin, asks :-
Why are the people of the world showing such different eye shapes? Slit, round, mongolian fold, etc. I had thought slit eyes might be for protection from the sun but natives of Africa have bold, round eyes.
John Dennison, an anatomist at Otago University's School of Medicine, responded.
Although genetic studies are revealing that humans are not divided by 'race' but all lie along one big line, we still trace back to three stocks: Negroid, Mongoloid, and Caucasoid. Mongoloid is subdivided: northern, (Inuit/Siberian); central, (Chinese/Mongolian/Japanese); both North and South American Indian; and southern (Malay). With that wide distribution across the globe, the 'slit-eye' appearance seems to have more to do with head form than with sunlight level, (though it can be developed by habitual muscle contraction - the Amazonian Xingu Indians actively extract their eye lashes to achieve this).
There are two eye forms: the non-Mongoloid eye lies more deeply in its socket, usually overhung by brow ridges and eyebrows; the Mongoloid eye has a narrow slit, usually, but not always, with an upward outward slant. It is very fatty, with bigger tear glands, and very thick lids. The eye fills the socket and protrudes, with less-developed brow ridges and scanty eyebrows. The lower rim of the socket juts forward; the cheek bone is large and wide, under a thick fat pad; the nose has a low bridge. All contribute to a 'flat' facial appearance.
The small vertical opening causes the slit-like appearance, due to the thick, overhanging upper lid. A skin fold often drops over the free edge of the eyelid, partially obscuring the lashes. The wide-open Caucasoid eye has a wrinkle parallel with the free edge of the upper lid. The reddish caruncle is seen at the inner corner. In the Mongoloid eye, a skin fold may cover the caruncle, (the 'Mongoloid fold'); or the entire upper eyelid; or just the outer part.
The inner fold appears in Caucasoid infants. They lose the Mongoloid appearance as the bridge of the nose elevates, taking up the skin slack. The outer fold may appear in older people due to progressive muscle tension loss.
The Negroids, the root of whose nose is nearly as low as in Mongoloids, generally do not show the Mongoloid fold. Some, the Hottentots and Bushmen especially, may show this. Also, their skin shade is more yellow.
The African great apes do not show the Mongoloid fold. The orangutan, (Asiatic), however, often show it.