The argument put forth by Wrangham is the mandible became a truly pussy organ as soon as we learned to cook. The argument put forth by Harari is that group formation is what led to dominance of Homo Sapiens over Homo Neanderthalensis. Chins visually emphasize the smile; as muscle groups gave over to communication from mastication, chins were inevitable.
Even before you consider that they're covered with a secondary sex characteristic in males. the ONE place males humans grow hair that females don't includes the chin.