The eyebrow is an area of thick, delicate hair above the eye that follows the shape of the lower margin of the brow ridges of some mammals. The most recent research suggests that in humans its primary function is to allow for a wider range of ‘non-verbal’ communication. It is common for people to modify their eyebrows by means of hair removal and makeup.
A man's eyebrow and eye
A number of theories have been proposed to explain the function of the eyebrow in humans including that its main function is to prevent moisture, mostly salty sweat and rain, from flowing into the eye, or that clearly visible eyebrows provided safety from predators when early hominid groups started sleeping on the ground 
Recent research, however, suggests that eyebrows in humans developed as a means of communication and that this is their primary function. Humans developed a smooth forehead with visible, hairy eyebrows capable of a wide range of movement which are able to express a wide range of subtle emotions – including recognition and sympathy.  
Cosmetics and modification
Cosmetic methods have been developed to alter the look of one's eyebrows, whether the goal is to add or remove hair, change the color, or change the position of the eyebrow.
Fashion in eyebrow shape has regularly changed throughout the ages but eyebrows have always featured heavily in female fashion.
Eyebrow brushes, shaders, and pencils are often used to define the eyebrow or make it appear fuller. These can create an outline for the brows or mimic hairs where there are sparse areas. Brow gels are also used in creating a thicker brow; they allow for the hairs to be more textured, which gives the appearance of thicker, fuller brows. Lastly, brow powders or even eyeshadows are used for those who want a fuller and more natural look, by placing the brow powder or eyeshadow (closest to the natural hair colour) in areas where there is less hair.
A fairly recent trend in eyebrow modification is in eyebrow tinting: permanent dye, similar to hair colour, is used on the eyebrows, often to darken them.
Several options exist for removing hair to achieve a thinner or smaller eyebrow, or to "correct" a unibrow, including manual and electronic tweezing, waxing, and threading. The most common method is to use tweezers to thin out and shape the eyebrow. Waxing is becoming more popular. Lastly, there is threading eyebrows, where a cotton thread is rolled over hair to pull it out. Small scissors are sometimes used to trim the eyebrows, either with another method of hair removal or alone. All of these methods can be painful for some seconds or minutes due to the sensitivity of the area around the eye but, often, this pain decreases over time as the individual becomes used to it. In time, hair that has been plucked will stop growing back. Some people wax or shave off their eyebrows and leave them bare, stencil or draw them in with eye liner, or tattoo them on. In Western societies, it has become more common for men to pluck part of their eyebrows.
To create a fuller look, eyebrows can be cloned in an eyebrow transplant. Individual strands of the eyebrow are created to mimic a natural-looking eyebrow of the desired shape. The process of eyebrow transplant is quite similar to the process of hair transplant. In this process as well, follicles from an active area are transferred to the area where there are no hairs. Follicles are mostly taken from the back of the head because it is the best harvesting site when it comes to hairs. The follicles are then injected into the skin.
The healing process after an eyebrow transplant is similar to the process of tattoo. In this process, patients may experience light bruising and crusting of the skin. People who have genetically thin eyebrows or who have over-tweezed are considered ideal for the transplant.
An eyebrow lift is a cosmetic surgery to raise the eyebrow, usually to create a more feminine or youthful appearance. It is not a new phenomenon, with the earliest description of brow lifting published in medical literature in 1919 by French surgeon Dr Raymond Passot. Brows can be affected during a face lift or an eye lift. In the seventies, doctors started injecting patients' eyebrows with botox or similar toxins to temporarily paralyse the muscles to raise the eyebrow.
From the 8th century, many Japanese noblewomen practiced hikimayu, shaving or plucking the eyebrow hair and painting smudge-like ones higher on the forehead. Shaving lines in eyebrows is another cosmetic alteration, more common among younger people in the 1990s and 2000s.
A new trend is eyebrow tattoos. This is seen as a way to achieve and maintain an eyebrow shape, achieved by using a coil machine, rotary machine and linear machine. This process, also called cosmetic tattooing, microblading, or blade & shade brows, involves an eyebrow artist making small, precise cuts that mimic the look of hair. Several celebrities have undergone the procedure, including Lorde, Lena Dunham, Adele, and Bella Thorne.
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- Anti-eyebrow, a body piercing placed below the eyebrow
- Eyebrow piercing, a body piercing done through the eyebrow
- Eyebrow restoration, surgical procedure to change the look of one's eyebrows
- Trichotillomania, a disorder where sufferers have an impulse to pull out body hair, including eyebrows
- Unibrow, the presence of abundant hair between the eyebrows
- Joseph Jordania, Why do People Sing? Music in Human Evolution, chapter "I Can See you! Eyespots in Humans." Logos, 2011, pg. 96
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- Bruce Ely, The Oregonian (28 April 2008). "Teens shaving lines in eyebrows raise gang concerns in schools". OregonLive.com. Retrieved 23 June 2012.
- ""Microfeathering" is L.A.'s Latest Brow Trend — & The Results Are SO Natural". Retrieved 20 September 2018.
- IEEE Xplore: About the relationship between eyebrow movements and Fo variations
Media related to Eyebrows at Wikimedia Commons