Mobile, expressive eyebrows give humans the ability to express a broader range of feelings and emotions, adding more subtlety and nuance to our interpersonal relations.

Eyebrows on fleek: Model of a modern human skull next to Kabwe 1. Image credits: Paul O’Higgins, University of York.

If you think about it, Homo sapiens don’t really have much going for them as a dominant species. We’re not the strongest or most agile creatures around, and we weren’t necessarily the smartest. But, humans did shine in one particular aspect: communication. Humans established large social networks which functioned with unprecedented efficiency — and apparently, eyebrows also played a part in that.

Modern humans have smooth, vertical foreheads with communicative eyebrows. Early humans, in contrast, sported thick, bony brow ridges. Previous studies have argued that these features protected against bites or scratches, but a new study suggests that, like antlers on a stag, pronounced brow ridges were a permanent signal of dominance and aggression in our early ancestors’ times.

Ricardo Godinho and colleagues digitally recreated a fossil Homo heidelbergensis skull thought to be between 125,000 and 300,000 years old. They found that the brow ridge is much larger than what would be needed to account for the disjunction between the eye sockets and braincase, and it also does little to protect the skull when eating.

Instead, Godinho and his colleagues suggest that the brows had a social purpose, especially since similar features are used for signaling in other primates. For instance, the baboon-like mandrills have bony, colorful muzzles that signal dominance in males and reproductive status in females. Paul O’Higgins, Professor of Anatomy at the University of York, and another lead author of the study, said:

“Looking at other animals can offer interesting clues as to what the function of a prominent brow ridge may have been. In mandrills, dominant males have brightly coloured swellings on either side of their muzzles to display their status. The growth of these lumps is triggered by hormonal factors and the bones underlying them are pitted with microscopic craters – a feature that can also be seen in the brow bones of archaic hominins.”

Previous research suggests that our faces have gotten progressively smaller, over the past 100,000 years, with the process accelerating in the past 20,000 years — especially as we switched from hunter-gatherers to farmers, a process that involved less effort and less variety in the foods we ate.

As all other hominins were fading away, humans were rapidly colonizing the globe, surviving in varied, and sometimes extreme environments. This was largely owed to our complex social networks. For instance, as Dr. Penny Spikins from the Department of Archaeology at the University of York explains, we do know that prehistoric modern humans avoided inbreeding and went to stay with friends in distant locations during hard times. Eyebrows could have played a surprisingly big role, allowing humans to send elaborate social signals in an instant — eyebrows might be the communication missing puzzle piece, Spikins says.

“Eyebrow movements allow us to express complex emotions as well as perceive the emotions of others. A rapid “eyebrow flash” is a cross-cultural sign of recognition and openness to social interaction and pulling our eyebrows up at the middle is an expression of sympathy. Tiny movements of the eyebrows are also a key component to identifying trustworthiness and deception. On the flip side it has been shown that people who have had botox which limits eyebrow movement are less able to empathise and identify with the emotions of others,” Spiking adds.

“Eyebrows are the missing part of the puzzle of how modern humans managed to get on so much better with each other than other now-extinct hominins,” she concludes.

Journal Reference: Supraorbital morphology and social dynamics in human evolution. DOI 10.1038/s41559-018-0528-0