 The heavier musculature of the jaws—those muscles operating the intensive masticatory process for chewing plant material—similarly would also limit development, here of the skull (braincase).During evolution of the human lineage these muscles seem to have weakened with the loss of the myosin gene MYH16, a two base-pair deletion that occurred about 2.4 million years ago.A study of the mandible across a number of specimens of "A. afarensis" indicated that Lucy's jaw was rather unlike other hominins, having a more gorilla-like appearance.