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.

African Homo erectus/ergaster

KNM ER 3883 has a double bar that is fairly slight and gracile. However KNM ER 3733 has a slightly less expanded brow ridge relative to ER 3883’s. This is one of the features suggesting that ER 3733 is a female whilst ER 3883 a male.

In KNM WT 15000 (Turkana Boy), the brow ridge is not fully preserved but suggests it was well-developed and comparable to ER 3733.

OH 16 and OH 9 both have a double-arched supraorbital torus that extends continuously into a long sloping frontal. OH 9 torus is a bit larger and it also presents a supraorbital sulcus.

The Buia skull (UA 31) from Eritrea has a very thick torus, also as a continuous bar across the front of the face.

Daka (BOU-VP-2/66) shows a double-arched torus but continuous with no interruption in the middle. The glabella region is very projecting.

Eurasian Homo erectus

Skull 5 from Dmanisi has a fairly prominent supraorbital torus and very projecting glabella.

Sangiran 17 from Java presents a big supraorbital torus as a continuous bar across the front.

Sambungmachan 3 also from Java is a female erectus with a slighter brow ridge than Sangiran 17: similarly, it is a continuous bar with some arching over each orbit, extending as a shelf across the front.

The reconstruction of Peking Man from Zhoukoudian shows a projecting, moderately robust brow ridge. It is not a continuous bar, but rather two arches over each orbit.

African Middle Pleistocene

Bodo has a very thick supraorbital torus, but with a little gap in the glabella region, thus not a continuous bar across the top but rather a double-arched supraorbital torus.

Kabwe may be the largest supraorbital torus in the Pleistocene record. It thins slightly as it goes laterally. Like Bodo, it presents a gap in the middle.

Ndutu’s torus is similar to Kabwe’s, a bit more projecting but less thick.

European Middle Pleistocene

Ceprano skull presents a massive supraorbital torus and a small supraorbital sulcus.

Skull 5 from Sima de los Huesos (Atapuerca) has a well-developed torus but narrower when compared with Ceprano. It is more projecting and thicker in the center than in the lateral margins. The supraorbital sulcus is very slight.

Petralona’s torus is very thick but thinner in the laterals. It is double-arched and disconnected in the middle.

Bonus!

Sahelanthropus tchadensis: the earliest-known hominin presents a very nice, large supraorbital torus occupying prominently the area above the orbits.

Homo naledi presents an anteriorly projecting, well-developed supraorbital torus that is relatively thick mid-orbit and clearly delineated by a moderately deep supraorbital sulcus.

References:

• Godinho, R.M., Spikins, P. & O’Higgins, P. Supraorbital morphology and social dynamics in human evolution. Nat Ecol Evol 2, 956–961 (2018)
• Van Arsdale, A. Introduction to Human Evolution, Wellesley College/edX (2015)
• Stringer, C. The Origin of Our Species, Penguin (2011)
• Dixon, A.D., et al. Fundamentals of craniofacial growth, CRC Press (1997)
• Laird, M.F., et al. The skull of Homo naledi, Journal of Human Evolution (2016)