Words may be the way modern humans form new relationships – but long before speech it was our eyebrows which did the talking, a British study suggests.
The evolution of highly mobile eyebrows to express emotions helped early humans convey nuanced messages of recognition or sympathy, the research found.
Scientists believe this was crucial to survival because it created large social networks forged by a greater understanding between our ancestors.
And it is probably why modern humans developed a smooth forehead with more visible brows, compared to the pronounced brow of early hominins, they said.
Senior author Professor of Anatomy Paul O’Higgins at the University of York, 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."
The researchers compared the pronounced brow of early humans to the antlers on a stag – with more prominent features deemed a signal of dominance and aggression.
But as relationship building became more important to survival, humans shifted towards a more expressive brow.
Prof O’Higgins added: "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."
The team used 3D engineering software to study the iconic brow ridge of a fossilised skull known as Kabwe 1, which lived between 600,000 and 200,000 years ago.
They then discounted two theories commonly put forward to explain its protruding brow ridges.
The first was that they were needed to fill the space where the flat brain cases and eye sockets of archaic hominins met.
The second was that the ridge acted to stabilise their skulls from the force of chewing.
By ruling out the theories, the team look into the possibility the changes were linked to social factors.
Prof O’Higgins said: "We used modelling software to shave back Kabwe’s huge brow ridge and found that the heavy brow offered no spatial advantage as it could be greatly reduced without causing a problem.
"Then we simulated the forces of biting on different teeth and found that very little strain was placed on the brow ridge.
"When we took the ridge away there was no effect on the rest of the face when biting.
"Since the shape of the brow ridge is not driven by spatial and mechanical requirements alone, and other explanations for brow ridges such as keeping sweat or hair out of eyes have already been discounted, we suggest a plausible contributing explanation can be found in social communication."
The researchers explained that communicative foreheads started off as a side-effect of our faces getting gradually smaller over the past 100,000 years.
They said the process sped up in last 20,000 years as we switched from hunter gatherers to agriculturalists – a lifestyle with less variety in both diet and physical effort.
Co-author Dr Penny Spikins at York’s Department of Archaeology said: "Modern humans are the last surviving hominin.
"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.
"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.
"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."
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