If you look at a picture of some of the earliest branches of a human family tree, such as Neanderthals or Homo erectus, you probably notice that Homo sapiens he left relatively light, wise for eyes. Most early hominins had thick bony ridges rather than the flat breasts of modern humans. For years, researchers have been arguing about the existence of these thicker ridges – and why modern people developed thinner brows. A new study suggests that brown ridges had a social need that was more important than the psychological function.
Previous research has suggested that thick-skinned spines help connect early eye sockets of hominins with their brain cavities, or protect the skull from the physical stress exerted by chewing jaws, or even helping early hominins to bring punches to the face.
The new study by York University researchers, published in the journal Ecology of Nature & evolution, used a digital model of a fossil skull, estimated to be between 125,000 and 300,000 years old, of an extinct species known as Homo heidelbergensis that surfaced sometime between 300,000 and 600,000 years ago in modern day Zambia. The researchers manipulated the model, resized the size of the eyebrow spine and observed what happened when they applied different bite weights. They found that the back of the eyebrow was much larger than it needed to be if it were for the direct purpose of attaching the eye sockets to the brain, and did not appear to protect the skull from feeding force.
Instead, the researchers suggest that the back of the eyebrow played a social role. Brown mounds are similar to other primates that serve a social purpose rather than a mechanical one, like male mandrills, which have colorful, heavy-bodied mugs as displays of dominance. Heavy brow ridges may have made the same place in early human species.
Mar Homo sapiens evolution, more subtle communication may have prioritized the lasting social sign of a large brown ridge. As foreheads became more vertical, eyes could move cheaper and thinner, leading to important social signals in today’s people, such as expressing surprise or anger.
An accompanying study in the same journal, by Spanish paleontologist Markus Bastir, warns that the results of the new study are attractive, but should be taken with a grain of salt. The sample used for the digital model was mandible, and the researchers subdivided into a mandible from Neanderthal, a related but still distinct species from Homo heidelbergensis. This may have analyzed the model change and bite pressures. However, the study offers “interesting prospects for future research,” he writes.