(HealthDay News) -- Highly expressive eyebrows likely played a big role in humans' evolutionary success, researchers report.

Early ancestors of humans had a large, bulging brow ridge -- a permanent signal of dominance and aggression, according to the team at the University of York in England.

But over the past 100,000 years, modern humans developed a smooth forehead with more visible, hairy eyebrows. Those brows provided a greater range of movement and aided in expression of subtle emotions such as recognition and sympathy. That, in turn, fostered greater understanding and cooperation and led to the creation of large social groups, the study authors contend.

"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," said Penny Spikins, co-author of the new paper. She is a senior lecturer in the archaeology of human origins.

The research adds to a long-running debate about why other hominins had huge brow ridges, while modern humans evolved flatter foreheads.

"While our sister species the Neanderthals were dying out, we were rapidly colonizing the globe and surviving in extreme environments. This had a lot to do with our ability to create large social networks," Spikins said in a university news release.

For their study, the researchers used 3D engineering software to look at the brow ridge of a fossilized skull, known as Kabwe 1. It belonged to a hominin that lived up to 600,000 years ago.

That analysis discounted two common explanations for the protruding brows -- that they were needed to fill space where flat brain cases and eye sockets met and, that they provided stability from the force of chewing.

"Since the shape of the brow ridge is not driven by spatial and mechanical requirements alone, and other explanations … such as keeping sweat or hair out of the eyes, have already been discounted, we suggest a plausible contributing explanation can be found in social communication," said senior author Paul O'Higgins, a professor of anatomy.

The study authors said the communicative forehead is a side effect of human faces gradually getting smaller over the past 100,000 years.

Eyebrow movements are something of a universal language. Spikins said they allow people to express complex emotions and understand 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," she said.

"On the flip side," Spikins added, "it has been shown that people who have had Botox, which limits eyebrow movement, [appear] less able to empathize and identify with the emotions of others."

The findings were published April 9 in the journal Nature Ecology and Evolution.

More information

The Natural History Museum in the United Kingdom has more on human evolution.

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