Ackermann et al. compile a manual guide to the neurology of acoustic communication in primates and humans that should be read by any student and scholar interested in one of the oldest questions in evolutionary biology – speech evolution. The authors fail, however, to integrate this important information with critical evidence from comparative primate research, (a recurring pitfall in neurology-based hypotheses for language evolution; Arbib Reference Arbib2005; Seyfarth Reference Seyfarth2005) and so the proposed evolutionary model falters on central heuristic pillars.

In agreement with the currently dominant view of speech evolution (Fitch et al. Reference Fitch, Huber and Bugnyar2010; Janik & Slater Reference Janik, Slater, Slater, Rosenblatt, Snowdon and Milinski1997), Ackermann et al. place a pronounced, but unwarranted importance on vocal learning, underlined primarily by vocal fold control. Because nonhuman primates, including great apes, are assumed to be incapable of vocal learning (Janik & Slater Reference Janik, Slater, Slater, Rosenblatt, Snowdon and Milinski1997), the authors logically presume that “motor mechanisms of articulate speech appear to lack significant vocal antecedents within the primate lineage” (sect. 1.1, para. 2). Paradoxically, Ackermann et al. argue then for the existence of vocal continuity at the motor level within the primate lineage and pursue an evolutionary model which addresses speech features that primarily relate to nonhuman primate voiced calls, or “vocalizations,” and vowels.

Interestingly, Ackermann et al. describe and depict in a clear way that vocal fold control is obligatorily involved solely in the production of vowels, while consonants are often voiceless and may be produced via supra-laryngeal articulation alone (with or without simultaneous airflow). The authors recognize that “virtually all languages of the world differentiate between voiced and voiceless sounds” (sect. 4.1, para. 1), and the diagrams provided by the authors illustrate well that supra-laryngeal articulation is versatile, multidimensional, multicomponent, and arguably, in some occasions, at least as complex as vocal fold control, both in motor control and acoustics. This fact is well illustrated, for instance, by the size of the consonant repertoire across all the world's spoken languages, which is three-fold larger than that of vowels (Maddieson Reference Maddieson1984). Any suitable account of speech evolution must thus account for the evolution of both speech building blocks in our lineage.

Like human consonants, some great apes calls do not obligatorily require the control or action of the vocal folds. Great ape voiceless calls, such as clicks, raspberries, smacks, kiss sounds, and whistles, are underlined by voluntary control and maneuvering of supra-laryngeal articulators (i.e., tongue, lips, and jaw) in apparent homology to the articulatory movements of voiceless consonants (Lameira et al. Reference Lameira, Maddieson and Zuberbühler2013c). These calls rely on social learning for their acquisition and fine sensory-motor feedback for proper production (Hardus et al. Reference Hardus, Lameira, van Schaik and Wich2009b; Lameira et al. Reference Lameira, Hardus, Kowalsky, de Vries, Spruijt, Sterck, Shumaker and Wich2013a; Reference Lameira, Hardus, Nouwen, Topelberg, Delgado, Spruijt, Sterck, Knott and Wich2013b; Marshall et al. Reference Marshall, Wrangham and Arcadi1999; Wich et al. Reference Wich, Swartz, Hardus, Lameira, Stromberg and Shumaker2009; Reference Wich, Krützen, Lameira, Nater, Arora, Bastian, Meulman, Morrogh-Bernard, Atmoko, Pamungkas, Perwitasari-Farajallah, Hardus, van Noordwijk and van Schaik2012). Apart from some rare cases across different taxa (e.g., storks, deer, macaques), great apes produce multiple voiceless calls. With the exception of humans, it is yet unclear whether any other animal species have explored the acoustic space of their supra-laryngeal vocal tract to an extent similar to great apes. For instance, in some wild orangutan populations, voiceless calls can account for half of the repertoire of an individual who produces more than ten different call types (Hardus et al. Reference Hardus, Lameira, van Schaik and Wich2009b). Unfortunately, Ackermann et al. neglect the importance of the homology in articulation, acoustics, and acquisition between great ape voiceless calls and human voiceless consonants.

Additionally, the authors tentatively suggest that factors like mother–infant interactions, grooming, social prestige, and communal dancing indirectly supported the emergence of vocal learning. Such suggestions find no ground in primate literature. These factors are either shared between most of nonhuman primates or are only known in humans, obscuring possible phylogenetic approaches to relevant primate communicative traits. As described in the target article, any significant and unique role the mentioned factors may have played in the earliest stages of speech evolution remains at least ambiguous and vague. Cooperative breeding, for instance, is also left out, though this is a promising factor capable of prompting a shift in the fundamental way ancestral primate individuals may have communicated with each other (Burkart et al. Reference Burkart, Fehr, Efferson and van Schaik2007; Reference Burkart, Hrdy and van Schaik2009a; Reference Burkart, Strasser and Foglia2009b; Burkart & van Schaik Reference Burkart and van Schaik2010; Isler & van Schaik Reference Isler and van Schaik2012; van Schaik & Burkart Reference van Schaik, Burkart, Kappeler and Silk2010).

In sum, Ackermann et al. present an evolutionary model inferred virtually from neurology alone, lacking concrete and/or realistic primate behaviors and selective drivers that may have prompted the neural transformations described. Great ape voiceless calls provide one such potent behavioral model and resolve the conflicting notions of motor continuity within the primate lineage. Although further research is needed (Lameira et al. Reference Lameira, Maddieson and Zuberbühler2013c), evidence suggests that a call repertoire composed of innate vocalizations together with a minority of learned voiceless calls represents a shared feature among all great apes (Lameira et al. Reference Lameira, Maddieson and Zuberbühler2013c), dating back thus to our ape ancestor. Such an extended repertoire would have offered direct communicative benefits for the transmission of more (detailed) information, disclosing an advanced primate cognition into acoustic communication (Seyfarth & Cheney Reference Seyfarth and Cheney2003a; Reference Seyfarth and Cheney2008; Reference Seyfarth and Cheney2010; Seyfarth et al. Reference Seyfarth, Cheney and Bergman2005) across whatever contexts. Such benefits would have predictively triggered selective pressures towards increased motor control over call production, even though in the absence of vocal fold control. In other words, it is possible that vocal learning did not trigger the emergence of a primate open-ended call repertoire, but represented sequentially a “secondary” evolutionary step (Lameira et al. Reference Lameira, Hardus, Kowalsky, de Vries, Spruijt, Sterck, Shumaker and Wich2013a). Flexible (e.g., Clay et al. Reference Clay, Pika, Gruber and Zuberbühler2011; Koda et al. Reference Koda, Lemasson, Oyakawa, Rizaldi and Masataka2013; Lemasson et al. Reference Lemasson, Ouattara, Petic and Zuberbühler2011; Ouattara et al. Reference Ouattara, Lemasson and Zuberbühler2009; Slocombe & Zuberbuhler Reference Slocombe and Zuberbuhler2007; Townsend et al. Reference Townsend, Deschner and Zuberbühler2008) and intentional (Gruber & Zuberbühler Reference Gruber and Zuberbühler2013; Schel et al. Reference Schel, Townsend, Machanda, Zuberbühler and Slocombe2013) use of innate vocalizations by nonhuman primates may have then provided the basis for the expansion of motor control over the vocal folds sufficient to allow individuals to learn to produce new voiced calls.

Overall, great ape voiceless calls beg for a reconsideration of the premises of the model proposed by Ackermann et al. The homology between great ape voiceless calls and human consonants warrants serious consideration of the former in any historical account of speech evolution. Great ape voiceless calls, for instance, also show fascinating features in that they may be produced simultaneously with “musical” instruments (Hardus et al. Reference Hardus, Lameira, van Schaik and Wich2009b; Lameira et al. Reference Lameira, Hardus and Wich2012), and their cultural transmission within separate populations leads to the emergence of functional arbitrariness in primate acoustic communication (Lameira et al. Reference Lameira, Hardus, Nouwen, Topelberg, Delgado, Spruijt, Sterck, Knott and Wich2013b). These features are probably based on neurological interactions that are yet to be documented and/or investigated, but that pose intriguing possibilities for our comprehension of speech evolution.

Understanding speech evolution will require integrating evidence collected across multiple levels and disciplines (Christiansen & Kirby Reference Christiansen and Kirby2003). Neurological studies and approaches to the question of speech evolution will be of invaluable importance, but there should be a committed effort to “anchor” neurological data to comparative primate research, mimicking the synergies that likely played out between the primate brain and primate communicative behavior in the course of speech evolution.