Sexual selection and the loss of laryngeal air sacs during the evolution of speech

One noteworthy, but unexplained aspect of the evolution of human speech is the loss of laryngeal air sacs during hominin evolution. Very little is known about the adaptive significance of this curious trait, or the selection pressures that may have driven their evolution among primates, and later loss in Homo. Here, I review the literature on the loss of laryngeal air sacs during the evolution of speech, and argue that sexual selection may have been a key factor. Although air sacs do not fossilize, the presence or absence of air sacs appears to be correlated with the anatomy of the hyoid bone, and fossil hyoid evidence suggests that air sacs were lost in hominins between 3.3 million and 530 thousand years ago. Air sacs are hypothesized to have an acoustic function, and some authors have argued that hominins may have lost their air sacs because they would make speech less clear. In other primates, such as gorillas and howler monkeys, air sacs appear to play a role in acoustic size exaggeration and may be linked to reproductive competition. I explore the hypothesis that changes in social organization and mating system towards reduced male-male competition may have relaxed the selection pressure maintaining loud, low frequency calls in hominins, making air sacs obsolete. While much of the above will remain hypothetical until more concrete data are gathered, we can speculate by saying that air sacs may not have been necessary for the type of quiet vocal interaction that typifies human communication. Perhaps more recent Homo species, with lower levels of sexual dimorphism and increased social tolerance and complexity, began to communicate in a more complex way, eventually leading to spoken language.