How evo­lu­tion raised an eye­brow to give us ex­pres­sive faces

The Guardian Weekly - - Discovery - Ian Sam­ple

Mod­ern hu­mans might never have raised a quizzi­cal eye­brow had Homo sapi­ens not lost the thick, bony brows of its an­cient an­ces­tors in favour of smoother fa­cial fea­tures, a new study sug­gests.

Re­searchers at the Univer­sity of York be­lieve early hu­mans bore promi­nent brow ridges as a mark of phys­i­cal dom­i­nance, and as the hu­man face evolved to be­come smaller and flat­ter, it be­came a can­vas on which the eye­brows could por­tray a much richer range of emo­tions.

“We traded dom­i­nance or ag­gres­sion for a wider pal­ette of ex­pres­sion,” said Prof Paul O’Hig­gins, the lead au­thor of the study who lec­tures in anatomy. The York team stress their con­clu­sions are spec­u­la­tive, but if they are right, the evo­lu­tion of smaller, flat­ter faces may have un­leashed the so­cial power of the eye­brow, al­low­ing hu­mans to com­mu­ni­cate at a dis­tance in more com­plex and nu­anced ways.

“We moved from a po­si­tion where we wanted to com­pete, where look­ing more in­tim­i­dat­ing was an ad­van­tage, to one where it was bet­ter to get on with peo­ple, to recog­nise each other from afar with an eye­brow flash, and to sym­pa­thise and so on,” said Penny Spikins, a palae­olithic ar­chae­ol­o­gist at York and co-au­thor on the study, pub­lished in the jour­nal Na­ture Ecol­ogy & Evo­lu­tion.

The sci­en­tists set out to in­ves­ti­gate why an­cient hu­mans had such promi­nent brow ridges in the first place. Over the years, re­searchers have put for­ward a range of hy­pothe­ses. One idea states that the ridge sim­ply filled the gap that would oth­er­wise ex­ist be­tween the pro­trud­ing face and the brain­case. An­other ar­gues that a promi­nent brow served as struc­tural re­in­force­ment, en­sur­ing the face could take the stress of pow­er­ful chew­ing.

Work­ing with their col­league Ri­cardo God­inho, the re­searchers ob­tained a 3D x-ray scan of an an­cient skull be­long­ing to a hu­man an­ces­tor called Homo hei­del­ber­gen­sis that lived in what is now Zam­bia be­tween 300,000 and 125,000 years ago.

Known as Kabwe 1, the skull dis­played a thick brow ridge that was even more promi­nent than the ones seen on of Ne­an­derthals.

Us­ing com­puter models, the sci­en­tists per­formed a se­ries of ex­per­i­ments on the vir­tual skull. They looked at how much brow bone was needed if its pur­pose was to plug the gap be­tween the face and the brain­case, and “shaved away the bone to get the min­i­mum needed to fill the gap and found we could re­duce its size dra­mat­i­cally,” O’Hig­gins said. “The skull has far more bone than is needed to fill the gap.” The re­searchers looked at how the stress of chew­ing spread over the face with and with­out the brow ridge.

“We fully ex­pected se­ri­ous con­se­quences for the face, but noth­ing hap­pened. It’s clear that this is not about re­sist­ing bend­ing in the face,” O’Hig­gins said. “What we are left with is the plau­si­bil­ity of a so­cial ex­pla­na­tion.”

United Artists/Al­ls­tar

Moore’s law … James Bond ac­tor Roger shows that the eye­brows have it

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