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Evolution of the Simian Shelf:
In Maboko Island (1988), a lower jaw of a Kenyapithecus was found, and it provides signs of the simian shelf evolution in modern apes. When there is a Simian Shelf present, the bottom teeth will be leaning forward. This happened to be a unique feature of the Kenyapithecus leading there to be a link between the two. This sign of evolution was identified as lower incisors that lean forward are known to be used to break hard objects. The lower incisors were concluded to be an adaptive feature as it was an adaptation for "hard object feeding including... large mandible... and upper incisors". Although the forward leaning lower incisors began to show up less in apes, the simian shelf still remained: "[it] was retained by most large bodied genera and served as a preadaptation for the evolution of broad lower incisors in living great apes". The simian shelf found in chimpanzees is not found in modern humans. It is hypothesized that the human chin has no true purpose because the simian shelf in chimpanzees is to protect the jaw from the stress of eating and/or chewing. The human speech mechanism also played a role in the evolution of the simian shelf. Changes in the jaw such as "widening of the mandibular arch, loss of the simian shelf, and development of the chin" allowed for the development of the speech mechanism. This may suggest that the simian shelf was no longer found in modern humans due to the changes that occurred due to the development of speech.

Simian Shelf in fetal and infant chimpanzees:
There is a study that targets the study of the mandibular symphysis in fetal and infant growth in chimpanzees. The mandibular symphysis consists of the external portion of the mandible, and the symphysis refers to the line seen between the two bones found in the middle of the mandible that appear during the fetal and infant stages. In the fetal chimpanzees, the study showed that the mandibular symphysis is "anteriorly inclined". It then begins to have "an increasingly vertical orientation up until birth". The study has shown that the shiftings that occur are due to the simian shelf becoming repositioned. The shifting that occurs with the simian shelf may show that "the suprahyoid muscles have a significant influence on the anterior growth of the symphysis". The mandibular symphysis in this case is also compared to that of the fetal and infant humans. Both, the human and chimpanzee, start out with a v-shaped mandible during the fetal stages, but the chimpanzee mandible remains in that v-shape leading to the development of the simian shelf. The forming of the simian shelf occurs due to the "basal symphysis [being] modified" due to the v-shape being maintained past the fetal stage.

Sources:
Benfit, Brenda R., McCrossin, Monte L. "Miocene Hominoids and Hominid Origins" Annual Review of Anthropology.24(1): 237-256. https://www.annualreviews.org/doi/pdf/10.1146/annurev.an.24.100195.001321

Cahart, Raymond. "Evolution of the speech mechanism". Quarterly Journal of Speech. https://www.tandfonline.com/doi/pdf/10.1080/00335633809380408

Coquerelle, Michael., Bookstein, Fred L., Braga, Jose, Halazonetis, Demetrios J., Weber, Gerhard W. "Fetal and infant growth patterns of the mandibular symphysis in modern humans and chimpanzees (Pan troglodytes)". Journal of Anatomy. 217 (5): 507-520. https://doi.org/10.1111/j.1469-7580.2010.01287.x

Hogenboom, Melissa. "Chins are a bit useless so why do we have them?". www.BBC.com. 4 February 2016. http://www.bbc.com/earth/story/20160204-why-do-humans-have-chins Retrieved 1 May 2020.

"Mandibular Symphysis". IMAIOS. Retrieved 10 May 2020.

Other Wikipedia Sources:
Kenyapithecus