Highly mobile eyebrows that can be used to express a wide range of subtle emotions helped modern humans in establishing large social networks and played a crucial role in their survival, finds a study.
While a pronounced brow ridge was a permanent signal of dominance and aggression for hominins, modern humans have a forehead with more visible, hairy eyebrows capable of a greater range of movement, the study showed.
Mobile eyebrows gave modern humans the communication skills in particular to express more nuanced emotions such as recognition and sympathy, allowing for greater understanding and cooperation between people.
"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," said Penny Spikins from the University of York in Britain.
"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.
The growth of a prominent brow ridge in our ancestors was triggered by hormonal factors.
The modern humans' communicative foreheads started off as a side-effect of our faces getting gradually smaller over the past 100,000 years.
This process became particularly rapid in last 20,000 years and more recently, as we switched from being hunter gatherers to agriculturalists -- a lifestyle that meant less variety in both diet and physical effort.
"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," Spikins said.
Even tiny movements of the eyebrows are a key component 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," Spikins explained, in a paper detailed in the journal Nature Ecology and Evolution.
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.
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