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It has been shown that larynx lowering is probably not the key factor leading to the emergence of speech in Humans. Rather, it has been argued that larynx lowering is a process which allows males to produce lower frequency sounds and thus giving females the impression of a bigger (more powerful?) potential sexual partner (Fitch 2000, 2002, 2005; Ohala, 2000). Although the process of larynx lowering is found in a number of mammal species (deer, lions) with apparently a similar function, that of providing males with "impressive" vocalizations exaggerating their size (Fitch and Reby, 2001), it is not found in our closely related non-human primates. Why did our non-human primate cousins not develop a lowered larynx? One possible explanation is that these species possessed air sacs which fulfilled the same function. These laryngeal air sacs have the capacity to produce lower frequency sounds (de Boer, 2008; Gautier, 1971) but they are also able to produce louder sounds.

Our hypothesis is that the male common ancestor of non-human and human primates had laryngeal air sacs which were replaced by a lowered larynx in the line which led to Homo sapiens. Why did air sacs disappear? We propose an ecologically induced explanation. In a forest environment, it is very important to produce loud sounds for two reasons. First, forest environments often render difficult the visual identification of conspecifics. Second, sound propagation is dampened in forest environments. In such an ecological context air sacs were quite appropriate to produce both lower frequency and louder sounds. When our ancestors left a forested environment, the need for loud sounds was no longer necessary. In a savannah type of environment sounds propagate much more efficiently than in a forest environment and, in addition, it is easy to visually perceive other individuals of your group. However, in order to preserve the "exaggerated male size" the disappearance of air sacs had to be replaced by another mechanism. The ancestors of Homo sapiens selected a process used by a number of mammal species: the lowering of the larynx. Hyoid bone fossil data seem to indicate that Australopithecus afarensis had air sacs (Alemseged et al. 2006) but Homo heidelbergensis (Martinez et al. 2008) and Neandertals did not.

We will provide supporting evidence for our hypothesis through an examination of the relationship between the presence versus absence of vocal sacs among related species in contrasting ecological environments. Air sacs are present in species inhabiting forest environments, while they have disappeared in closely related species inhabiting savannah environments. The role of sexual dimorphism will also be discussed.

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