Humans are talkative animals. We enjoy chatting with family, friends, and colleagues over daily happenings. Verbal communication is one of the major means to express ourselves. Turning to the animal kingdom, animals also use acoustic cues for their communication. We see dogs bark and cats meow to communicate with their owners. Birds sing to mark their territory or attract mates. However, their vocal repertoires are not as extensive as humans. Chimpanzees are the closest phylogenetic relative of humans and are well known for their complex societies and highly developed cognitive abilities. Nonetheless, they might be considered taciturn if they are put in a human society. 

The verbal communications of humans are based on speech, in which acoustic cues are generated through skillful maneuvers of speech structures The vocal folds vibrate to generate sound sources, which are formed into voiced sounds by passing through articulators such as the tongue, jaw, and lips. Interestingly, the main structure of speech is similar between humans and other animals. It is thus an intriguing question to ask why they communicate so differently. It is also a mystery why only humans have achieved complex languages. One apparent answer is the human brain, which has highly developed areas devoted to language processing. This is however not the only point to distinguish humans from other animals. An alternative view is possible by looking into the speech structures. Although humans and animals share similarities in their speech structures, there exist several major differences, and such differences may physically inhibit animals from talking like humans. 

One of the distinguished features of speech organs in animals is the vocal membranes (or vocal lips). As illustrated in Figure, the vocal membranes represent an extra set of protruding tissues, located just above the vocal folds. In a recent study, Nishimura et al.1 investigated the laryngeal physiology of 43 species of primates, ranging from gibbons and siamangs to macaques and chimpanzees. The vocal membranes were found in all the species except humans. Then, they observed the laryngeal movements of chimpanzees and macaques in vivo and found that, during the vocalization, the vocal membranes do vibrate to contribute to the sound production.  

What are the advantages of possessing the vocal membranes and why were they lost in the evolutionary process to humans? In an early study, Mergell et al.2 predicted through mathematical modeling that the vocal membranes may effectively lower the phonation threshold pressure (PTP). The PTP is the minimum lung pressure to induce and sustain the vocal fold oscillations. The lowered PTP is advantageous since it requires less power from the lungs to initiate vocalizations and thus realizes efficient vocalizations. They also suggested that the vocal membranes can interfere with the vocal folds to induce irregular laryngeal vibrations. According to Fitch et al.3, such irregular vocalizations may serve as one of the vocal repertories in animals. The study of Nishimura et al.1 indeed confirmed irregular laryngeal vibrations in the in vivo observation as well as in the excised larynx experiment of macaques.

In speech production, laryngeal vibrations play a key role in producing the source sounds. Periodic laryngeal vibrations that contain strong harmonic components are indispensable for exciting the resonances of the articulators (i.e., formant frequencies), while irregular vibrations without strong harmonics may degrade the clarity of the speech sound. In human speech, distinct phonemes (e.g., vowels and consonants) should be produced clearly and sequentially even in a short duration of exhalation. Irregular laryngeal vibrations induced by the vocal membranes are therefore not favorable. Another potential disadvantage is the high-frequency nature of the vocal membrane vibrations. According to Titze4, high-frequency vibrations of the laryngeal source may interfere with the resonances of the articulators and restrict independent movements of the larynx and articulators. Such a situation should be avoided in human speech.

These disadvantageous aspects may provide a key to answer why the vocal membranes were lost in the evolution to humans. Although speech itself is not the same as language, it has undoubtedly made major contributions to the origin and evolution of human languages. The finding of Nishimura et al.1 provided an important clue for what triggered the emergence of human languages. The exploration of this mysterious tissue has only started, and new findings are yet to come.