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The purpose of the crest was to provide a surface for masticatory muscle attachment or insertion.

Explanation:

This crest is called the sagittal crest. It is present in extinct hominids, such as Australopithecus and Paranthropus, and can also be found in living animals such as gorillas or lions and some reptiles.

The sagittal crest is a bony structure that crosses the hominids´ skull longitudinally, and it is so well-developed that it stands out notoriously.

This crest, like others in the skulls, provides an attachment surface for many muscles. In the particular case of the sagittal crest, it is believed to provide an insertion area to mastication muscles.

It seems that Australopithecus had a special diet composed of tough items. The hominid probably needed to strong bite while eating. The strong jow muscles gave them the possibility to bite but needed a wide area for insertion. The sagittal crest provided that wide surface. There was a direct relationship between the size of the crest and the muscle mass. The bigger the crest was, the more developed the muscle was.

It is believed that in living animals, such as gorillas, the development of the crest in males is for sexual selection and reproductive success. The bigger the crest is, the more attractive the male is, and the more chances to be selected by the female to mate it will have. Some hypothesize that the development of the crest in Australopithecus could also be related to sexual selection.