“Although modern humans are considered to be morphologically distinct from other living primates because of our large brains, dexterous hands, and bipedal gait, all of these features are found among extinct hominins. The chin, however, appears to be a uniquely modern human trait. Probably because of the chin's exclusivity, many evolutionary scenarios have been proposed to explain its origins. To date, researchers have developed adaptive hypotheses relating chins to speech, mastication, and sexual selection; still others see it as a structural artifact tangentially related to complex processes involving evolutionary retraction of the midfacial skeleton. Consensus has remained elusive, partly because hypotheses purporting to explain how this feature developed uniquely in modern humans are all fraught with theoretical and/or empirical shortcomings.”
“Chins, probably because of their status as a modern human autopomorphy, continue to generate a great deal of interest among evolutionary anthropologists and biologists. Yet none of the proposals purporting to explain why this unusual feature occurs in modern humans (and only modern humans) is without problems. […] This should serve as motivation, not discouragement, for researchers to continue investigating this modern human peculiarity. Despite the vast amount written on the chin, it is deserving of continued attention, for perhaps understanding the chin will reveal some unexpected insight into what it means to be human.”
Source: The enduring puzzle of the human chin Evolutionary Anthropology: Issues, News, and Reviews, Jan. 2016.