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In adult primates the hyoid is a horseshoe shaped bone, located in the neck, just below the mandible and above the thyroid cartilage. In humans, the hyoid body is flat and bar-shaped and lies below the inferior margin of the mandibular body, just below the tongue root. However, in great apes the hyoid is placed superior to the inferior margin of the mandibular body and lies behind the tongue root.
In chimpanzees, bonobos, and gorillas there is a cup-shaped extension of the hyoid bone called the hyoid bulla, which is believed to keep open the connection between the vocal tract and pharyngeal air sacs. Modern humans lack this this hyoid bulla. Fossil evidence tracks the loss to somewhere between Australopithecus afarensis (~3.3 million years old), which shares the Great ape hyoid bulla morphology, and a ~530 thousand year old Homo species ancestral to Neanderthals (sometimes included in Homo heidelbergensis) which shares the modern human hyoid shape. This loss is proposed to be adaptive for human speech development due to pharyngeal air sac impediment of forming easily perceptible speech sounds.
Comparative morphology of the hominin and African ape hyoid bone, a possible marker of the evolution of speech., , Hum Biol, 2013 Oct, Volume 85, Issue 5, p.639-72, (2013)
Loss of air sacs improved hominin speech abilities., , J Hum Evol, 2012 Jan, Volume 62, Issue 1, p.1-6, (2012)
Human hyoid bones from the middle Pleistocene site of the Sima de los Huesos (Sierra de Atapuerca, Spain)., , J Hum Evol, 2008 Jan, Volume 54, Issue 1, p.118-24, (2008)
A juvenile early hominin skeleton from Dikika, Ethiopia., , Nature, 2006 Sep 21, Volume 443, Issue 7109, p.296-301, (2006)