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In early humans, “jaws of steel”

Feb. 11, 2009
Courtesy Arizona State University
and World Science staff

Your moth­er told you not to use your teeth as tools to open some­thing hard, and she was right. Hu­man skulls have small faces and teeth and aren’t well-e­quipped to bite down force­fully on hard ob­jects. Not so of our ear­li­est an­ces­tors, say sci­en­tists.

Com­pres­sive stress in the cra­ni­um of Aus­tra­lo­pi­the­cus afri­can­us, an ex­tinct ear­ly hu­man, im­posed by bit­ing on the pre­mo­lar teeth. Bright col­ors cor­re­spond to high stresses, and in­di­cate that a bony pil­lar run­ning along­side the open­ing of the na­sal cav­i­ty acts as a strut that struc­tur­al­ly re­in­forces the face against pre­mo­lar loads. (Cour­te­sy of Ar­i­zo­na State Uni­ver­si­ty; Hom­i­nid Feed­ing Biome­chan­ics re­search group)


Re­search pub­lished in last week’s on­line is­sue of the re­search jour­nal Pro­ceed­ings of the Na­tional Acad­e­my of Sci­ences points to nut-crack­ing abil­i­ties in our 2.5-mil­lion-year-old rel­a­tives that let them al­ter their di­et to adapt to chang­ing cir­cum­stances.

Us­ing com­put­er mod­el­ing and sim­ula­t­ion – the same tech­niques en­gi­neers use to sim­ulate how a car re­acts to forc­es in a front-end col­li­sion – ev­o­lu­tion­ary sci­en­tists built a vir­tu­al mod­el of the skull of the hu­man an­ces­tor Aus­tra­lo­pi­the­cus afri­can­us. The plan was to see how the jaw op­er­ated and what forc­es it could pro­duce.

“We started with a CT scan of a skull that is one of the most com­plete spec­i­mens of A. afri­can­us that we have,” said Mark Spen­cer, an Ar­i­zo­na State Uni­ver­s­ity as­sis­tant pro­fes­sor, who with doc­tor­al stu­dent Cait­lin Schrein at the uni­ver­s­ity is part of an in­terna­t­ional re­search team be­hind the stu­dy. 

The skull un­der in­ves­ti­ga­t­ion is be­lieved to come from a lat­er an­ces­tor of the popula­t­ions that in­clud­ed “Lu­cy,” the most fa­mous fos­sil ske­l­e­ton in an­thro­po­l­ogy. Dat­ed as about three mil­lion years old, Lu­cy is the most com­plete known fos­sil of a ge­nus of pre­his­tor­ic Af­ri­can hu­man an­ces­tors known as aus­tralo­p­ithecines.

The Lu­cy an­ces­tor in­ves­t­i­gated in the new work was a fos­sil dubbed STS5 and af­fec­tion­ately known as “Mrs. Ples.” The skull, dis­cov­ered in 1947, has struts on the side of the nose, but no teeth. 

“We me­shed those da­ta with an­oth­er spec­i­men with teeth to make the vir­tu­al mod­el of the bone and tooth struc­ture,” Spen­cer said. “Then we looked at chim­panzees, who share com­mon fea­tures with Aus­tra­lo­pi­the­cus, and took mea­sure­ments of how their mus­cles work and added that to the mod­el. We were able to val­i­date this mod­el by com­par­ing it to a si­m­i­lar mod­el built for a spe­cies of mon­key called macaques.” 

The re­sult: a rain­bow-col­ored vir­tu­al skull that il­lus­trates forc­es ab­sorbed by the cra­ni­al struc­ture in sim­ulated bites and how un­usu­al fa­cial fea­tures were suit­ed to crack­ing hard nuts. 

“It was like watch­ing ‘Mrs. Ples’ come to life,” Spen­cer said.

“This reinforc­es the body of re­search in­di­cat­ing that fa­cial spe­cial­iz­a­tions in spe­cies of early hu­mans are adapta­t­ions due to a spe­cialized di­et,” he went on. “The en­large­ment of the pre­mo­lars, the heavy tooth enam­el and the ev­i­dence now that they were load­ing force­fully on the mo­lars sug­gest the size of the ob­jects were larg­er than the pre­vi­ously hy­poth­e­sized small seeds and nuts. These fall-back foods – hard nuts and seeds – were im­por­tant sur­viv­al strate­gies dur­ing a per­i­od of chang­ing cli­mates and food scarcity.”


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Your mother always told you not to use your teeth as tools to open something hard, and she was right. Human skulls have small faces and teeth and aren’t well-equipped to bite down forcefully on hard objects. Not so of our earliest ancestors, say scientists. Research published in last week’s online issue of the research journal Proceedings of the National Academy of Sciences suggests nut-cracking abilities in our 2.5-million-year-old relatives that let them alter their diet to adapt to changing circumstances. Using computer modeling and simulation – the same techniques engineers use to simulate how a car reacts to forces in a front-end collision – evolutionary scientists built a virtual model of the skull of the human ancestor Australopithecus africanus. The plan was to see how the jaw operated and what forces it could produce. “We started with a CT scan of a skull that is one of the most complete specimens of A. africanus that we have,” said Mark Spencer, an Arizona State University assistant professor, who with doctoral student Caitlin Schrein at the university is part of an international research team behind the study. The skull under investigation is believed to come from a later ancestor of the populations that included “Lucy,” the most famous fossil skeleton in anthropology. Dated as about three million years old, Lucy is the most complete known fossil of a genus of prehistoric African human ancestors known as australopithecines. The Lucy ancestor investigated in the new work was a fossil dubbed STS5 and affectionately known as “Mrs. Ples.” The skull, discovered in 1947, has struts on the side of the nose, but no teeth. “We meshed those data with another specimen with teeth to make the virtual model of the bone and tooth structure,” Spencer said. “Then we looked at chimpanzees, who share common features with Australopithecus, and took measurements of how their muscles work and added that to the model. We were able to validate this model by comparing it to a similar model built for a species of monkey called macaques.” The result – a rainbow colored virtual skull that illustrates forces absorbed by the cranial structure in simulated bite scenarios and how their unusual facial features were ideally suited to support the heavy loads of cracking hard nuts. “It was like watching ‘Mrs. Ples’ come to life,” Spencer said. “This reinforces the body of research indicating that facial specializations in species of early humans are adaptations due to a specialized diet,” said Spencer. “The enlargement of the premolars, the heavy tooth enamel and the evidence now that they were loading forcefully on the molars suggest the size of the objects were larger than the previously hypothesized small seeds and nuts. These fall-back foods – hard nuts and seeds – were important survival strategies during a period of changing climates and food scarcity.”