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Eyebrows Linked to Evolutionary Adaptation, Study Finds

Unibrow / Photo by Zoohouse via Wikimedia Commons

 

We used to treat our eyebrows nowadays as a simple, usually unnoticed part of our body. While some people make a big deal out of this, using outlandish resources and money to draw the best eyebrows on “fleek,” one can really argue that these small, hairy parts of our head play a very simple role in our system.

As an external part of our body, we learned in elementary and basic education that our eyebrows serve as a barrier to prevent sweat and other debris from entering our eye sockets. In this way, it helps us not to develop any complications from possible contamination that may arise from debris falling into our eyes.

Socially, eyebrows are being used as a sign of expression, and even a signal of our emotions. However, a recent research published in Nature Ecology and Evolution can further improve the way we know how and why eyebrows exist, and it may have played a very large part in the survival of our earliest ancestors.

While it has been silent for the past years, there has actually been a long debate on why earlier hominids had very large brow ridges while we, the modern species, have a flatter forehead. Researchers at the University of York suggest that it played a large part in their survival then.

Penny Spikins, co-author of the study, said that eyebrows are the biggest mysteries of our human evolution. "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 said.

 

Primate skull / Photo by Museum of Veterinary Anatomy FMVZ USP via Wikimedia Commons

 

Before, some studies prior to the York study have posited the possibility of our brow ridges as a protection for skulls from forceful chewing. However, a study in 1991 has partially debunked the hypothesis. “There is no good reason to believe that enlarged brow-ridges in living and/or fossil primates are structural adaptations to counter powerful masticatory forces,” the study said.

Other studies, meanwhile, have indicated the possibility of brow ridges as something to fill the void between the brain and eye sockets. Meanwhile, Spikins and her team created a very different experiment, by examining the brow ridge of a Homo hiedelbergensis, one of the earliest primates which are considered to be one of our direct yet extinct ancestors.

The skull, hoiused in the Natural History Museum in London, was digitally recreated for the study and experimented by manipulating the brow ridge size and applying different biting pressures in the ridge. The results agreed with the 1991 study in the relationship between chewing and ridge sizes, saying that it does very little to relieve pressure on the skull while eating.

They also debunked the hypothesis pertaining to the link between eye sockets and the brain, saying that the brow ridges were much larger to fill the gap of H. heidelbergensis's braincase and its eye sockets.

We use our eyebrows as a form of communication and emotion, while the earliest primates have used their gigantic brow ridges as a form of aggression and dominance compared to other earlier species. According to the research, it played a very significant part at that time. In our extinct ancestors, their prominent eyebrows have signaled status or aggression.

 

 

"Social signalling is a convincing explanation for the jutting brows of our ancestors," said Paul O'Higgins, senior author of the paper and anatomy professor at the University of York. "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."

While ancestors of modern humans used to have a pronounced ridge on their brow, this was also relatively inflexible. As the years passed, we traded in this form of brow ridges with a much flatter surface, allowing us now to use our eyebrows in much more complex and nuanced forms of communication and social interactions.

According to the researchers, our communicative foreheads started off as a side-effect of our faces getting gradually smaller over the past 100,000 years. This process has become particularly rapid in last 20,000 years and more recently after we switched from being hunter-gatherers to agriculturalists -- a lifestyle that meant less variety in both diet and physical effort.

“Clearly during our evolution, we developed more complex and larger social groups with many interdependencies,” Higgins told Newsweek.

The study proved that eyebrow movements, while it helped humans in dominance before, can now be a source for us to express complex emotions and perceive those of others. However, whenever we add modifications, it can change as well.

“On the flip side, it has been shown that people who have had botox which limits eyebrow movement are less able to emphasize and identify with emotions of others,” Higgins told The Hindu.

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