An anatomical term, the supraorbital ridge refers to the ridge of bone above each eye. This ridge differs between species, and also tends to look different between genders. They act as part of the structural strength of the skull, among other functions.
Human eyes are located within indentations in the skull. These indentations are bordered on the top by the suprorbital region. The supraorbital ridge is one part of this anatomical area, and refers specifically to the bone that curves around the top of the eye socket and under the eyebrows. Alternate terms for the area include the supraorbital arch and brow ridge. Above each supraorbital ridge is the frontal eminence of the skull, which lies under most of the forehead.
Different species, including that of the ape and monkey have variations in skull shape, and therefore have a supraorbital ridge structure that is distinct to each species. Humans are no exception, and ancient versions of humans also display identifiable types of supraorbital ridge. Even in modern humans, a diversity of ridge types exist.
Evolution means that skull shapes and other physical differences show adaptations to environment and survival pressures. Anthropologists, who study humans, and ancestors of humans, can differentiate fossil skulls into species of human-like creatures through features such as supraorbital ridges. The Neanderthal, for example, has very prominent brow ridges compared to modern humans. This is because present-day humans have a vertical forehead, and the Neanderthal has a forehead that slopes sharply back from above the eyes.
Modern human women and men have subtly different types of brow ridges. Masculine people tend to have thicker and more obvious brows, with a forehead that angles back slightly, while women have flatter brows and a more straight-up-and-down forehead. Even different races of modern humans can show recognizable differences in brow ridge appearance. For example, some native Australians have a single ridge running above the eyes and nose. Most of the world's population have two separate ridges, one over each eye.
The supraorbital ridges also provide protection for the eyes from accidental injury. Even nerves and blood vessels use the brow ridge bone for their own purposes. Above each eye in a modern human is a small hole in the bone, or a little dent in the bone, where nerve bundles and veins can run through from inside the skull to the outside. These features are missing in some ancestral species, and so are another feature that can help anthropologists distinguish between species.