New research from the University of York suggests that highly mobile, expressive eyebrows may have been key to helping humans form social bonds and cooperating with each other as part of their survival.
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The prominent brow ridge that can be seen in archaic hominins who lived between 600,000 and 200,000 years ago signified dominance and aggression. According to the researchers, a smooth forehead with more visible and hairy eyebrows developed as our faces gradually became smaller over the past 100,000 years.
These more mobile eyebrows gave humans the communication skills to establish large social networks by enabling them to express subtle emotions such as sympathy and recognition.
The study contributes to a long-standing debate amongst academics about why other hominins, including our immediate ancestors, had large, pronounced brow ridges, while anatomically modern humans developed flatter foreheads.
Sexually dimorphic display and social signalling is a convincing explanation for the jutting brows of our ancestors. Their conversion to a more vertical brow in modern humans allowed for the display of friendlier emotions which helped form social bonds between individuals."
Paul O'Higgins, Professor of Anatomy at the University of York
O'Higgins and colleagues used 3D engineering software to study the iconic brow ridge of a fossilised skull that belonged to a species of archaic hominin called Homo heidelbergensis. The skull, which is known of as Kabwe 1, is held in the collections of the National History Museum.
After discounting theories that the shape of the brow ridge was driven by spatial and mechanical requirements, the team suggested that a plausible contributing explanation can be found in social communication.
While our sister species the Neanderthals were dying out, we were rapidly colonising the globe and surviving in extreme environments. This had a lot to do with our ability to create large social networks - we know, for example, that prehistoric modern humans avoided inbreeding and went to stay with friends in distant locations during hard times.”
Dr Penny Spikins
Spikins adds that, as well as allowing us to express complex emotions such as sympathy, eyebrow movements enable us to perceive the emotions of others. Tiny eyebrow movements are also key to identifying trustworthiness and deception.
"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.