"Long before it's in the papers"
April 14, 2015


Study looks at why we have chins

April 14, 2015
Courtesy of the University of Iowa
and World Science staff

Look at the skull of a Ne­an­der­thal per­son, a chimp or a mon­key. No­tice an­y­thing mis­sing? We have some­thing that nei­ther they, nor any oth­er spe­cies have: a chi­n.

“In some way, it seems triv­i­al, but a rea­son why chi­ns are so in­ter­est­ing is we’re the only ones who have them,” said Na­than Hol­ton, who stud­ies cran­io­fa­cial fea­tures and me­chan­ics at the Uni­vers­ity of Io­wa.

A modern hu­man skull (left) com­pared with a skull from the Nean­derthal era (right.) (Cour­tesy Tim Schoon, U. of Iowa)

New re­search led by Hol­ton and col­leagues pro­poses that our chi­ns emerged in ev­o­lu­tion simply be­cause our faces be­came smaller com­pared to those of our an­ces­tors, ex­pos­ing a bony prom­i­nence at the bot­tom. The find­ing, if cor­rect, may help set­tle a de­bate that’s gone on in­ter­mit­tently for more than a cen­tu­ry.

The re­search­ers con­ducted tests with al­most 40 peo­ple to find out wheth­er chew­ing and oth­er forc­es might lead to new bone form­ing in the chi­n ar­ea. That ex­plana­t­ion does­n’t hold up, they claim. Rath­er, they write in a pa­per pub­lished on­line in the Jour­nal of Anat­o­my, it seems the chi­n emerged be­cause our faces shrank. Our faces are roughly 15 per­cent shorter than those of Ne­an­der­thal peo­ple.

Be­hind this modifica­t­ion, Uni­vers­ity of Io­wa an­thro­po­l­o­gists led by Rob­ert Fran­cis­cus ar­gue, was a grad­u­al lifestyle change, start­ing about 80,000 years ago and pick­ing up great steam with mod­ern hu­mans’ migra­t­ion from Af­ri­ca about 20,000 years lat­er. Peo­ple evolved from iso­lat­ed hunter-gatherer groups to in­creas­ingly co­op­er­a­tive groups with so­cial net­works, and a grad­u­al in­crease in ar­tis­tic and oth­er sym­bol­ic ex­pres­sion. Males in par­tic­u­lar be­came calmer and more co­op­er­a­tive. In tan­dem with this, lev­els of the hor­mone tes­tos­ter­one dropped, re­sult­ing in changes to the head, in­clud­ing a smaller face.

“Mod­ern hu­mans had an ad­van­tage at some point to have a well-con­nect­ed so­cial net­work, they can ex­change in­forma­t­ion,” said Fran­cis­cus, who was on the team that first laid out the the­o­ry in a pa­per pub­lished last Au­gust in the jour­nal Cur­rent An­thro­po­l­ogy. He is al­so a con­tri­but­ing au­thor on the cur­rent pa­per. “For that to hap­pen, males have to tol­er­ate each oth­er. There had to be more cu­ri­os­ity and in­quis­i­tive­ness than ag­gres­sion, and the ev­i­dence of that lies in fa­cial ar­chi­tec­ture.”

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Look at the skull of a Neanderthal person, a chimp or a monkey. Notice anything missing? We have something that neither they, nor any other species have: a chin. “In some way, it seems trivial, but a reason why chins are so interesting is we’re the only ones who have them,” said Nathan Holton, who studies craniofacial features and mechanics at the University of Iowa. “It’s unique to us.” New research led by Holton and colleagues proposes that our chins emerged in evolution simply because our faces became smaller compared to those of our ancestors, exposing a bony prominence at the bottom. The finding, if correct, may help settle a debate that’s gone on intermittently for more than a century. The researchers conducted tests with almost 40 people to find out whether chewing and other forces might lead to new bone forming in the chin area. That explanation doesn’t hold up, they claim. Rather, they write in a paper published online in the Journal of Anatomy, it seems the chin emerged as our faces became smaller in our evolution. Our faces are roughly 15 percent shorter than those of Neanderthal people. Behind this modification, University of Iowa anthropologists led by Robert Franciscus argue, was a gradual lifestyle change, starting about 80,000 years ago and picking up great steam with modern humans’ migration from Africa about 20,000 years later. People evolved from isolated hunter-gatherer groups to increasingly cooperative groups with social networks, and a gradual increase in artistic and other symbolic expression. Males in particular became calmer and more cooperative. In tandem with this, levels of the hormone testosterone dropped, resulting in changes to the head, including a smaller face. “Modern humans had an advantage at some point to have a well-connected social network, they can exchange information, and mates, more readily, there’s innovation,” said Franciscus, who was on the team that first laid out the theory in a paper published last August in the journal Current Anthropology. He is also a contributing author on the current paper. “For that to happen, males have to tolerate each other. There had to be more curiosity and inquisitiveness than aggression, and the evidence of that lies in facial architecture.”