Archaeologists at York University have published an article in Nature Ecology and Evolution about the role of brow ridges in human evolution
A team of researchers consisting of experts in the field of anatomy and archeology decided to put to the test core theories about the role of prominent brow ridges of our early ancestors. Unexpected result of the study was published in Nature Ecology and Evolution journal. It turned out that eyebrows have played a significant role in human evolution and made people change their appearances.
Researchers say that strongly-pronounced brow ridges, like large horns of a deer, served as a signal of dominance and aggression in our early ancestors, which modern humans “exchanged for” smooth forehead with more visible, “hairy” eyebrows capable of a greater range of movement, able to express a very wide range of emotions. Researchers believe that this gave us additional tool for effective communication to establish large social networks.
Strongly-pronounced brow ridges, like large horns of a deer, served as a signal of dominance and aggression in our early ancestors
“Sexually dimorphic display and social signalling is a convincing explanation for the jutting brows of our ancestors. Their conversion to a more vertical brow in modern humans allowed for the display of friendlier emotions which helped form social bonds between individuals,” – scientists reported.
Be social or die off
“Looking at other animals can offer interesting clues as to what the function of a prominent brow ridge may have been. In mandrills, dominant males have brightly colored swellings on either side of their muzzles to display their status. – Paul O’Higgins, Professor of Anatomy and Senior author of the paper, said. – 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.”
To figure out what was function was performed by ancient peoples’ brow ridge, researchers decided to create 3D model of skull of Homo heidelbergensis. Development of this 3D model was based on the very similar skull of Homo rhodesiensis, who lived some 600,000-200,000 years ago and was found in Kabwe Cave, Zambia.
Image credit: Professor Paul O’Higgins, University of York
According to one of the theories, prominent brow ridges were needed to ease the pressure on jaws while nibbling and chewing food.
Another hypothesis suggested that brow ridges were needed to fill the space where the flat brain cases and eye sockets of archaic hominins met. Some researchers believed that brow ridges decreased in size for evolutionary reasons as a consequence of “self-domestication” of a human, since this allowed for the selection and better survival of less aggressive and socially adapted individuals.
To put these theories to the test, authors of the study created two additional 3D models of skull of Homo heidelbergensis – with enlarged and reduced brow ridges. When this was done, the forces of biting on different teeth and brow ridges tension were simulated.
Results obtained knocked the bottom out of both main theories stating that mechanical and spatial factors were the main reasons for the prominent brow ridges of our forefathers. It turned out that very little strain was placed on the brow ridges and was independent of both biting force and size of the ridges.
“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,” – Professor Paul O’Higgins said.
Display of emotions as a recipe for survival
Authors of the study state that some 100,000 years ago brow ridges started to decrease, and 20,000 years ago, when people switched from gathering products to farming and livestock breeding, this process has accelerated considerably.
“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. – Dr. Penny Spikins from the Department of Archaeology at the University of York, Co-author of the paper, said. – 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”.
100,000 years ago brow ridges started to decrease, and 20,000 years ago this process has accelerated considerably
A team of researchers believes that eyebrow movements allow us to express complex emotions – they help us to demonstrate the most delicate emotional nuances. Eyebrows show the emotional state of a person: confusion, amazement, joy, lie, truth, etc. 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.
Authors of the study optimistically conclude that 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.