By Jean Balchin 11/04/2018

There are a number of things about my physical appearance I’m not 100% happy with. I’m pale, covered in freckles and I burn like a crisp on the odd sunny day. But perhaps worst of all is the fact that my eyebrows are virtually non-existent. Unless I carefully pencil them in each morning, my friends and coworkers struggle to ascertain whether I am happy, sad, angry or confused. As anyone who can waggle their eyebrows will tell you, brows are a useful tool for expressing emotion.

New research published in Nature Ecology & Evolution suggests that social signals may have contributed significantly to the evolution of prominent brow ridges in early humans, as opposed to physical drivers. Modern humans tend to possess smooth and vertical foreheads, with communicative eyebrows – even if they have to be drawn on at times. However early humans sported thick and bony browridges that protruded out from the face. Previously, the general consensus held that these ridges offered some protection against the stress of biting and chewing, or helped join the eye sockets and brain case.

However, a digital reconstruction of a Homo heidelbergensis skull aged between 125,000-300,000 years old found the brow ridge was much larger than was needed for either physical use. Ricardo Godinho and colleagues experimented with changing the browridge size and applying different biting pressures to the skull. It was discovered that the browridge in the fossil is much larger than needed to account for the disjunction between the eye sockets and braincase. Moreover, Godhino and his colleagues also discovered that a larger browridge does little to protect the skull when eating.

The researchers came to the conclusion that these browridges played a social role, perhaps as a signal of social dominance or aggression. In other primates, such as the baboon-like mandrills, similar skull growths are used for signalling. With mandrills, bony and colourful muzzles signal dominance in males and reproductive status in females. As humans became a more social, expressive species, brow flattening allowed instead for the development of more visible and mobile eyebrows, capable of subtler, changing displays of emotion.

According to Markus Bastir in the associated editorial,

“The inferential leap made by Godinho and colleagues is worth exploring, as it reflects the difficulties of causal argumentation in craniofacial evolutionary biology.”

Eyebrows are very important in subtle signalling behaviours, as anyone who has misbehaved in public in front of one’s mother can attest. The pronounced eyebrow lift can indicate indignation, while the rapid eyebrow flash, “lasting around one-sixth of a second, found cross-culturally as a sign of contact readiness and recognition.” Sympathy may be indicated by pulling eyebrows up at the middle, while anger may be indicated by pulling the brows together and downwards. Any constraints on muscle movements in one’s forehead – such as botox, for example – affect emotional expressions, and by extension social relationships, as people are less able to empathise with and identify other’s emotions.

You can read more about these expressive and impressive eyebrows here.