Eyebrow evolution link to social signalling

The loss of the pronounced eyebrow ridge seen in human ancestors may have more to do with how modern humans get along with each other than it does with biomechanics, suggests new research published in Nature Ecology & Evolution.

It has long been thought the prominent browridge seen in early hominins evolved for purely mechanical and structural reasons. Various researchers have hypothesised, for example, that it served to protect the head from blows, that it helped shade the eyes from sunlight or prevented hair from blocking vision. These seemingly common-sense hypotheses are generally poorly regarded due to lack of supporting evidence.

Scientists have settled on two candidate hypotheses that had more empirical backing. The first is that the shape of the browridge is a result of the way the orbits of the eyes and the brain case relate to each other spatially – the browridge is thought to fill a necessary structural role. The other is that its shape is the result of the biomechanics associated with biting and chewing: in particular, that its shape helps to protect the face from mechanical stresses caused by mastication.

Of course, really testing these hypotheses is a bit tricky in the absence of actual hominins to observe while they’re sitting down to lunch – even assuming they’d let you hook them up to the necessary monitoring equipment.

Ricardo Godinho, an archaeologist from the University of York in the UK, and colleagues have now found a way around this problem: “virtual functional simulation”. The team took a mid-pleistocene (125,000 to 300,000 years old) Homo heidelbergensis skull known as Kabwe 1, found originally in Zambia, and scanned it to produce a virtual model.

As Kabwe 1 is without a jawbone, the researchers substituted one from the closely related Neanderthal. While this is a clever fix, it should make us cautious about the results says Spanish paleobiologist Markus Bastir in an editorial in the same issue of Nature Ecology & Evolution.

Having produced a whole virtual skull the scientists carried out experiments to test the two widely entertained hypotheses, only to discover that both seem unsupported. “These results demonstrate that the browridge is significantly larger than required to simply bridge the gap between the orbits and the frontal bone,” they write, and that change in the shape of the browridge “does not impact in any substantial way the mode or magnitude of deformation of the face during biting.”

This has opened the way for thinking about more complex and interesting causes for the evolutionary loss of heavy brows: social ones.

Godinho and team hypothesise that the loss of a large, low browridge allowed let the eyebrows become more mobile and play a part in our repertoire of non-verbal signals.

In modern humans, the eyebrows play a strong role in social signalling. There are cross cultural eyebrow signals for recognition, surprise and sympathy, among others. Mandrills (Mandrillus sphinx) also have browridge features that are not caused by spatial or mechanical factors, but are a strong reflection of social behaviour.

While there is still work to do to truly confirm these results, the research points to a way to rule in or out mechanical or structural causes for anatomical features of interest in evolutionary studies. In doing so, it opens such research up to the possibility of a more holistic approach that takes hominin sociality seriously as a causal factor in our evolutionary history.