This editorial serves a number of purposes. First, it aims at summarizing and discussing 33 accepted contributions to the special issue “The evolution of rhythm cognition: Timing in music and speech.” The major focus of the issue is the cognitive neuroscience of rhythm, intended as a neurobehavioral trait undergoing an evolutionary process. Second, this editorial provides the interested reader with a guide to navigate the interdisciplinary contributions to this special issue. For this purpose, we have compiled Table 1, where methods, topics, and study species are summarized and related across contributions. Third, we also briefly highlight research relevant to the evolution of rhythm that has appeared in other journals while this special issue was compiled. Altogether, this editorial constitutes a summary of rhythm research in music and speech spanning two years, from mid-2015 until mid-2017.

Human speech and music differ in many respects but also share similarities. One of the main similarities lies in their temporal nature. In fact, both music and speech:

Humans seem to be particularly rhythmic animals. Decades of research have shown that human brains are tuned-in to the fine degrees of rhythmic information in music and speech (Bolton, 1894; Fraisse, 1981, 1982, 1984; Longuet-Higgins and Lee, 1982, 1984; Povel, 1984, 1985; Essens and Povel, 1985; Povel and Essens, 1985; Shmulevich and Povel, 2000). This human propensity to perceive, produce, and process rhythm is increasingly well understood, though its evolutionary origins remain a bit of a mystery. Let's compare this to what we know about the eye. This organ has evolved in animals as a complex photoreceptor to supply the need of sensing light (Fitch, 2015a). In addition, color vision in humans and many other species appears particularly useful to assess the ripeness of food or the quality of a potential mate, hence conferring an evolutionary advantage. Unfortunately, we are still far from providing similar answers for a complex neurobehavioral trait such as rhythm. However, we firmly believe that rhythm needs to be anchored in an evolutionary perspective.

A number of critical questions spurred this special issue. When did the sensitivity for rhythm arise in human evolutionary history? How did rhythm cognition develop in human evolution? How does this evolutionary path relate to rhythm ontogeny? What is the biological function of rhythm in the millisecond to second range? Do environmental rhythms affect the evolution of brain rhythms, and how? Do speech and music share rhythm-specific neural circuits and cognitive modules? Are these circuits shared with other domains and even across species?

In general, the long-time scales involved in evolutionary processes prevent direct observation. Sometimes the evolutionary dynamics of simple traits can be replicated in the lab: For instance, the evolution of learning in fruit flies can be directly observed (Mery and Kawecki, 2002). Instead, the evolution of human behavior and neurobiology requires a more indirect scientific method. This is why understanding the cognitive neuroscience of rhythm and its evolution calls for a tight integration of different perspectives (Fitch, 2015b; Honing et al., 2015; Ravignani, 2017a). In particular, complementary approaches include but are not limited to:

The cognitive and neurobiological bases of rhythm are increasingly well understood (Honing et al., 2015; Merchant et al., 2015). While researchers in many fields are interested in rhythm, there is little awareness of how related and potentially converging their research strands are. This special issue builds a bridge across a large number of scientific disciplines; the focus lies in the cognitive neurosciences of rhythm, conceptualizing rhythm as a neurobehavioral trait undergoing an evolutionary process.

A good proportion of the papers in this issue deals with developmental aspects of rhythm (Abboub et al.; Bedoin et al.; Cirelli et al.; Cumming et al.; Hannon et al.; Lense and Dykens; Teie). Among those, one theoretical contribution raised the intriguing possibility that an individual's fetal environment may already affect his future rhythmic repertoire (Teie). Two contributions tested rhythmic abilities in infants ranging from 7 to 15 months of age, with a focus either on beat perception (Cirelli et al.) or speech meter and grouping (Abboub et al.). A corpus-based approach investigated rhythmic regularities in children's songs and finds a connection between rhythms in song and non-song speech features (Hannon et al.). Two contributions focused on the interaction between musical beat and language in 9 year old children with specific language impairments (Bedoin et al.; Cumming et al.). Finally, Lense and Dykens tracked rhythmic abilities over the lifespan in a sample of 74 children and adults affected by Williams syndrome.

The study of rhythm in speech and music is increasingly adopting a global, cross-cultural perspective (Abboub et al.; Bekius et al.; Polak et al.; Roncaglia-Denissen et al.; Teie). The field seems to be expanding beyond learners of English as first language or musically-enculturated Westerners. In speech, three contributions explored the relationship between different languages or learning a foreign language, and rhythmic capacities (Abboub et al.; Bekius et al.; Hannon et al.; Roncaglia-Denissen et al.). In music, the focus is on biologically-driven rhythmic universals (Teie) and experiments involving cross-cultural comparisons (Polak et al.).

Along a methodological dimension, empirical papers adopted three alternative approaches: corpus analyses, behavioral experiments or brain imaging/electrophysiology. It is interesting to note that all experimental papers in this issue that employed EEG also adopted a frequency-tagging approach (Celma-Miralles et al.; Cirelli et al.; Teki and Kononowicz), rather than a grand-average ERP method (but see Henry et al., 2017 for a note of caution).

The relationship between music, language, and speech continues being of great interest in the scientific community. This continued interest is found also in the papers in this issue (Bedoin et al.; Bekius et al.; Cumming et al.; Geambaşu et al.; Norton and Scharff; Ravignani et al.; Roncaglia-Denissen et al.). In particular, one paper investigated how beat keeping and phonological patterning are related (Bekius et al.). Another study focused on recursion, a topic of great debate in linguistics and showed how human adults are sensitive to recursive structures in rhythmic patterns (Geambaşu et al.).

Modality-specificity and domain-specificity were also explored in this issue (Celma-Miralles et al.; Matthews et al.; Richter and Ostovar; Su). Findings about rhythm in vision (Celma-Miralles et al.; Su) and movement (Su) suggested that some circuits for rhythmic timing may coincide across modalities.

Complementary to meticulously controlled individual experiments, rhythm can be investigated by taking a more holistic approach, and probing rhythmic behaviors in interaction (Benichov et al.; Gamba et al.; Hartbauer and Römer; Lense and Dykens; Richter and Ostovar; Woolhouse et al.). Vocal coordination behavior in groups of primates (Gamba et al.), songbirds (Benichov et al.), and insects (Hartbauer and Römer) can offer insights for human interactional timing. Connections between internal rhythms and group behaviors can be investigated in healthy adults (Richter and Ostovar; Woolhouse et al.) and individuals with specific syndromes affecting musicality and sociality (Lense and Dykens).

Three papers discussed rhythm from the perspective of dance (Richter and Ostovar; Su; Woolhouse et al.). Rhythm and dance should be thought as a tightly connected pair (Richter and Ostovar), which can be empirically investigated (Su) and shed light on other aspects of cognition (Woolhouse et al.).

Two papers in the issue were devoted to mathematical and computational modeling (Forth et al.; Jadoul et al.). These approaches are complementary. On the one hand, rhythm and timing can be investigated using top-down abstract models (Forth et al.). On the other hand, different aspects of speech timing can be statistically modeled with different degrees of precisions and assumptions made (Jadoul et al.).

This issue also contains ample evidence on rhythm from a comparative approach (Benichov et al.; Dufour et al.; Gamba et al.; Hartbauer and Römer; Hoeschele and Bowling; Norton and Scharff; Ravignani et al.; Rouse et al.; Spierings and ten Cate; ten Cate et al.). Songbirds continue to be a particularly often-used model species in the study of rhythm (Benichov et al.; Hoeschele and Bowling; Norton and Scharff; Spierings and ten Cate; ten Cate et al.). For instance, important advances have been made by confirming how the subjective “feeling of rhythm” experienced when listening to a songbird has a quantitative, isochronous counterpart in the animal's song (Norton and Scharff). Rhythmic behaviors in two primate species were also explored in this issue. These works examined either the closest primate to humans, the chimpanzee (Dufour et al.) or one of the phylogenetically farthest group, the lemur (Gamba et al.). This suggests that some components of human rhythmicity may be due to evolutionary homology (common descent from our last common ancestor with chimpanzees) while other traits to analogy (convergent evolution in man and singing lemurs). A taxonomic group emerging as particularly promising for future rhythm research is the pinnipeds, which features harbor seals, sea lions, and walruses (Ravignani et al.; Rouse et al.).

Other papers discussed general issues related to timing and time perception (Rajendran et al.; Sameiro-Barbosa and Geiser; Teki; Teki and Griffiths). Two contributions tested general aspects of the relationship among timing, rhythm and cognitive functions (Rajendran et al.; Teki and Griffiths). A theoretical paper reviewed neural entrainment mechanisms (Sameiro-Barbosa and Geiser). Finally, Teki provided a useful overview of timing papers since 2000, ordering them by number of citations, so to identify community trends and overall research interests.

Since the launch of this Frontiers Research Topic, a number of publications on rhythm have appeared in other journals. Among those, some strands are particularly relevant to research in the evolution of rhythm. Far from attempting a comprehensive overview, we mention these papers and summarize some of them below.

Similarly to other fields, the study of the evolution of rhythm must build on a tight integration of experiments, theory, and modeling. Ideally, empirical observations of rhythm in music and speech are first recorded in the field. Observations are then contrasted to generate testable hypotheses. Based on these hypotheses, experiments on linguistic and musical rhythm are performed. Experimental factors and variations can encompass sensory modalities, ages, and animal species, to name a few, in order to address questions about domain-specificity, development, and evolutionary phylogeny. Finally, experimental insights should be integrated via synthetic modeling. The advantage of models is that they generate predictions that are quantitatively testable. Following these predictions, new empirical observations should be collected and compared, continuing the incremental loop of scientific investigation.

This journal issue contains novel empirical findings and state of the art reviews of hot topics in each discipline. We hope it will be useful as a reference volume on the evolution of rhythm cognition. Combining well-established findings and novel results on the evolution of rhythm, it should serve as an introductory reference for newcomers, a source of novel findings for researchers more familiar with one of the areas, and an interdisciplinary overview of progress in neighboring disciplines.

All contributions discussed so far show the many sides of rhythm. From this volume, rhythm emerges not as a monolithic concept, but as a multifaceted phenomenon for research. We hope that exciting future research will be ignited by this multifaceted display of rhythm across domains and species.

All authors listed, have made substantial, direct, and intellectual contribution to the work, and approved it for publication.