The supraorbital torus (or brow ridge) is a very distinctive morphological trait in most of our hominin ancestors. What purpose does this feature serve? A few hypotheses around this topic are:

• Dissipation of heavy chewing forces, produced by the jaw muscles and transmitted around the nose and the eye sockets.
• Evolutionary result of the fulfilment of spatial demands between the orbits and the brain case.
• Capacity to dynamically express affiliative prosocial emotions through highly mobile eyebrows.
• Reinforcement of the frontal bone which was weaker in all the hominin species before Homo sapiens. This is a similar idea to explain the development of the chin in modern humans, as a reinforcement of a weaker jaw.
• Protection of the skull and the eyes against blows.
• A signaling effect, accentuating aggressive stares, thus its large size could have been sexually selected through generations. Is the lack of brow ridges related to self-domestication in modern humans?

However, many huge supraorbital tori are hollowed inside with large sinuses (for example: Petralona), suggesting that they did not bear or transmit physical forces from blows to the head or heavy chewing. Another iconic skull, Kabwe 1, has a much larger browridge than the minimum required to fulfil spatial demands, and its size has little impact on mechanical performance during biting.

I like the idea to think about a combination of several factors which made evolution work for a few million years. This post describes the supraorbital tori of 22 iconic hominins:

Australopitecines

Al 444-2: The largest Australopithecus afarensis skull yet discovered has an expansive supraorbital torus, thickened laterally and continuous superiorly-posteriorly with no interruption.

Sts 5 (Mrs. Ples) has a relatively small supraorbital torus, double arched in the front and projecting glabella. Another Au. africanus skull with many similarities is Sts 71, with a less broad torus in comparison to Sts 5, but with a similar expanded glabella.

Supraorbital torus: Sts 5 (centre)-credit Wikipedia, AL 444-2 (left) and Sts 71 (right)-credit Roberto Sáez

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