Ancient hominins sported prominent eyebrow ridges, unlike modern humans who have flatter foreheads and mobile brows. For decades, scientists have debated both the purpose of large brows in our ancestors and why they went out of facial fashion in human evolution. One leading explanation, the spatial hypothesis, suggests that large brow ridges were needed to support hominins’ eye sockets and brain cases. Another hypothesis focuses on biting mechanics, proposing that brow shape and size is driven by the strain that biting places on the skull.
Researchers at the University of York, UK—Ricardo Miguel Godinho, Penny Spikins, and Paul O’Higgins—tested these hypotheses using a virtual model of a Homo heidelbergensis skull. Based on a computed tomography (CT) scan, they virtually reconstructed Kabwe 1, a skull found in Zambia and dated to between 125,000 to 300,000 years ago. Next, the team manipulated the model skull to create two alternate versions with significantly trimmed-down brow ridges, one with and one without a groove behind the eye sockets.
The results of the scientists’ modeling revealed that neither the spatial nor biting mechanics hypotheses could completely explain the bulging brows in H. heidelbergensis. Shrinking, but not eliminating, the brow ridge did not cause any major issues with the skull’s structural ability to support the eye sockets and brain case. When they simulated the strain that different bites placed on the brows of the three virtual skulls, they found the trimmed-down brow to show little-to-no strain.
Searching for other explanations, the team looked to non-human primates, such as mandrills, that have bony facial features that vary between sexes, are influenced by hormones, and play a role in social communication. Other research has suggested links between social, cooperative behavior and certain changes in cranial anatomy in mammals. Drawing upon such examples, the team advanced an alternate, less established social hypothesis: Brain changes and the shrinking of modern human faces created flatter brows and straighter foreheads, making our eyebrows more visible and flexible—capable of expressing subtle and complex communication—compared with those of our primate cousins. “This is crucial from a social perspective,” says Godinho. (Nature Ecology and Evolution)