Eyebrows are crucial to human evolution, scientists have discovered, because it is how our predecessors first learnt to communicate.
Dr Penny Spikins, one of the researchers from the University of York, said: “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.
“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.”
Modern mobile eyebrows inherited their social function from the prominent thick brow ridges that were a key feature of primitive human ancestors, the scientists believe.
After conducting a new study of the skull of Homo heidelbergensis, an ancient African hominin dating back 124,000 – 300,000 years, the researchers discounted previous theories about the origin of brow ridges.
Earlier research had suggested the heavy brows may have been needed to protect the skull when chewing, or were a structural feature arising from the conjunction of a flat brain case and large eye sockets.
Instead it was much more likely that the purpose of brow ridges was social, said the scientists, who created a 3D computer simulation of a skull from Zambia known as Kabwe 1 housed at London’s Natural History Museum.
Huge, jutting brow ridges may have signalled dominance and aggression, according to the findings reported in the journal Nature Ecology & Evolution.
In a similar way, dominant male mandrills, the world’s largest monkey, sport brightly coloured muzzle swellings to display their status.
The bones underlying mandrill swellings are pitted in the same way as the brow ridges of ancient hominins, the researchers point out.
As human faces became smaller and smoother over the course of 100,000 years, jutting brow ridges gave way to eye brows capable of more subtle emotional displays, it is claimed.
Dr Spikins added: “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.”