How evolution raised an eyebrow to give us expressive faces
Modern humans might never have raised a quizzical eyebrow had Homo sapiens not lost the thick, bony brows of its ancient ancestors in favour of smoother facial features, a new study suggests.
Researchers at the University of York believe early humans bore prominent brow ridges as a mark of physical dominance, and as the human face evolved to become smaller and flatter, it became a canvas on which the eyebrows could portray a much richer range of emotions.
“We traded dominance or aggression for a wider palette of expression,” said Prof Paul O’Higgins, the lead author of the study who lectures in anatomy. The York team stress their conclusions are speculative, but if they are right, the evolution of smaller, flatter faces may have unleashed the social power of the eyebrow, allowing humans to communicate at a distance in more complex and nuanced ways.
“We moved from a position where we wanted to compete, where looking more intimidating was an advantage, to one where it was better to get on with people, to recognise each other from afar with an eyebrow flash, and to sympathise and so on,” said Penny Spikins, a palaeolithic archaeologist at York and co-author on the study, published in the journal Nature Ecology & Evolution.
The scientists set out to investigate why ancient humans had such prominent brow ridges in the first place. Over the years, researchers have put forward a range of hypotheses. One idea states that the ridge simply filled the gap that would otherwise exist between the protruding face and the braincase. Another argues that a prominent brow served as structural reinforcement, ensuring the face could take the stress of powerful chewing.
Working with their colleague Ricardo Godinho, the researchers obtained a 3D x-ray scan of an ancient skull belonging to a human ancestor called Homo heidelbergensis that lived in what is now Zambia between 300,000 and 125,000 years ago.
Known as Kabwe 1, the skull displayed a thick brow ridge that was even more prominent than the ones seen on of Neanderthals.
Using computer models, the scientists performed a series of experiments on the virtual skull. They looked at how much brow bone was needed if its purpose was to plug the gap between the face and the braincase, and “shaved away the bone to get the minimum needed to fill the gap and found we could reduce its size dramatically,” O’Higgins said. “The skull has far more bone than is needed to fill the gap.” The researchers looked at how the stress of chewing spread over the face with and without the brow ridge.
“We fully expected serious consequences for the face, but nothing happened. It’s clear that this is not about resisting bending in the face,” O’Higgins said. “What we are left with is the plausibility of a social explanation.”
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