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This is why expressive brows might have mattered during human evolution

Mobile eyebrows gave us the communication skills to establish large, social networks.

Image: Africa Studio via Shutterstock

HIGHLY MOBILE EYEBROWS that can be used to express a wide range of subtle emotions may have played a crucial role in human survival, new research from the University of York suggests.

Like the antlers on a stag, a pronounced brow ridge was a permanent signal of dominance and aggression in our early ancestors, which modern humans traded in for a smooth forehead with more visible, hairy eyebrows capable of a greater range of movement.

Mobile eyebrows gave us the communication skills to establish large, social networks – in particular, to express more nuanced emotions such as recognition and sympathy, allowing for greater understanding and cooperation between people.

The study contributes to a long-running academic debate about why other hominins, including our immediate ancestors, had gigantic brow ridges while anatomically modern humans evolved flatter foreheads.

“Looking at other animals can offer interesting clues as to what the function of a prominent brow ridge may have been,” senior author Paul O’Higgins, Professor of Anatomy at the University of York, said.

“In mandrills, dominant males have brightly coloured swellings on either side of the 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,” he said.

Sexually dimorphic display and social signalling is a convincing explanation for the jutting brows of our ancestors.
Their conversion to a more vertical in modern humans allowed for the display of friendlier emotions which helped form social bonds between individuals.

The study

Using 3D engineering software, the researchers looked at the iconic brow ridge of a fossilised skull, known as Kabwe 1. It belonged to a species of archaic hominin – Homo heidelbergensis, who lived between 600,000 and 200,000 years ago.

The researchers discounted two theories commonly put forward to explain protruding brow ridges – that they were needed to fill the space where the flat brain cases and eye sockets of archaic hominins met, and that the ridge acted to stabilise their skulls from the force of chewing.

“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,” O’Higgins said.

He explained that the researchers then simulated the forces of biting on different teeth and found that very little strain was placed on the brow ridge. When they 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,” he said.

The ‘eyebrow flash’

Co-author of the paper, Dr Penny Spikins from the Department of Archaeology at the University of York said: “Moderns 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 has 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 in the middle is an expression of sympathy.

Spikins said that tiny movements of the eyebrows are also a key component to identifying trustworthiness and deception.

In contrast, Spikins said 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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