Editorial: The Evolution of Rhythm Cognition: Timing in Music and Speech

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OVERVIEW OF THIS PAPER
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.

TIMING IN MUSIC AND SPEECH
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: (i) develop over time and have a temporal dimension crucial to physically characterize music and speech, (ii) rely on timing as one of their most conspicuous perceptual dimensions, (iii) can be tokenized as sequences of temporal intervals, which are perceived as a "rhythm, " (iv) are composed by temporal intervals that possibly differ in duration and acoustic marking by different spectral properties, generating metrical expectations.
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, 1981Fraisse, , 1982Fraisse, , 1984Lee, 1982, 1984;Povel, 1984Povel, , 1985Essens 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?

RHYTHM: A MULTIDISCIPLINARY FIELD
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: (1) developmental studies of rhythm that are useful in understanding whether rhythm perception and production involve critical acquisition periods, or instead result mostly from enculturation during the whole lifespan (Hannon and Trehub, 2005), (2) comparative and cross-cultural studies of rhythm that serve to explain whether musical enculturation or exposure to specific languages can affect which specific rhythmic patterns can be produced/perceived and how frequently (Greenberg et al., 1978;Rzeszutek et al., 2012), (3) comparisons of rhythm processing in music and speech, at both behavioral and neural levels  that help understanding whether common music-speech networks exist and similar behavioral patterns can be observed when humans engage in music and speech production, (4) evidence and comparison of rhythm processing across modalities and domains that are used to understand whether, for instance, metrical expectation in speech is strictly bound to the speech domain or instead recruits the same capacities for metricality available in music, or even in dance and vision (Iversen et al., 2015), (5) studies of rhythm in interaction and context (Yu and Tomonaga, 2015), explaining how social, affective, and other factors affect the emergence of rhythmic patterns, (6) archaeological findings trying to reconstruct rhythmrelated behavior and cognition in our early hominid ancestors (Morley, 2003), (7) mathematical and computational models (e.g., connectionist, symbolic) of the mechanisms underlying perception and production of rhythmic behavior (Desain and Honing, 1989, 1991, 2003, (8) mathematical and computational models of rhythmic capacities as evolved behaviors (Miranda et al., 2003) in line with a long tradition in evolutionary and theoretical biology, (9) evidence of spontaneous rhythmic behavior in other animals (Fuhrmann et al., 2014;Ravignani et al., 2014a) showing how similar rhythmic traits can evolve via similar pressures in phylogenetically distant species, (10) controlled experiments in non-human animals (Cook et al., 2013) probing the potential for producing/perceiving rhythm (even though these are not usually part of these species' natural behavior); these experiments can show the existence of basic, evolutionary conserved cognitive processes that may have been exapted in humans for rhythmic purposes.
The cognitive and neurobiological bases of rhythm are increasingly well understood 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. . 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 corpusbased approach investigated rhythmic regularities in children's songs and finds a connection between rhythms in song and nonsong 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.;

DEVELOPMENT EVIDENCE
Cumming et al.

EEG AND FREQUENCY TAGGING
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 (

MUSIC, SPEECH, AND SYNTAX
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 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.).

RHYTHM IN OTHER JOURNALS SINCE LATE 2015
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.

Evolutionary Hypotheses for Rhythm Origins
Some review papers have properly focused on the evolutionary origins of musical rhythm and animal species showing humanlike rhythmic traits (Bannan, 2016;Iversen, 2016;Wilson and Cook, 2016). Bannan (2016) provided a recount of Charles Darwin's thoughts on music and how he thought human musicality may have emerged via sexual selection. Iversen (2016) summarized and compared many evolutionary hypotheses on the origins of rhythm in humans. Wilson and Cook (2016) discussed which animal species are capable of synchronizing to a beat, either spontaneously or after being trained, and how this evidence relates to evolutionary hypotheses. Some of these evolutionary hypotheses on music and rhythm have been tested via genetics (Mosing et al., 2015), behavioral experiments (Miani, 2016), electrophysiology (Bouwer et al., 2016) or animal comparative work (ten Cate et al.; van der Aa et al., 2015).

Speech Rhythm and Comparative Anatomy of Vocal Tracts
In the evolution of speech, several studies have shown how vocal tracts in non-human primates are more flexible than previously thought. Other primates' vocal tracts are capable of producing a human-like range of vowels Boë et al., 2017) and consonants (Lameira et al., 2015(Lameira et al., , 2016(Lameira et al., , 2017. The overall conclusion is that the complexity of human speech, including its rhythmical nuances, must have neural, rather than morphological, bases (Ravignani et al., 2014b;Fitch et al., 2016;Belyk and Brown, 2017).

Cultural Evolution and Cognitive Biases
Rhythm seems to be slowly overcoming the classical naturenurture debate that actually is built on a false dichotomy. Along these lines, recent research has focused on the cultural evolution of musical rhythm and perceptual priors (Savage et al., 2015;Trehub, 2015;Hansen et al., 2016;Le Bomin et al., 2016;Fitch, 2017;Jacoby and McDermott, 2017). Statistical universals found in musical rhythms all over the world (Savage et al., 2015) can emerge via the combined effect of human cognitive biases and cultural transmission . Interestingly, these biases seem at least partly modulated by enculturation (Jacoby and McDermott, 2017).

The Evolution of Dance
The field of musical rhythm is increasingly expanding to encompass the scientific study of dance (Ellamil et al., 2016;Fitch, 2016;Laland et al., 2016;Ravignani and Cook, 2016;Su, 2016;Fink and Shackelford, 2017). Only in 2016, three papers have introduced conceptual frameworks for the evolutionary study of dance (Fitch, 2016;Laland et al., 2016;Ravignani and Cook, 2016). We believe the field would benefit from connecting these theoretical frameworks with recent empirical findings on dance (Ellamil et al., 2016;Su, 2016).

Timing and Time Perception
The science of timing and time perception has been a major research area in the last century. After a less active period, this field is again experiencing an increase in research efforts. A whole special issue of Current Opinion in Behavioral Sciences was recently devoted to "Time in perception and action" (Meck and Ivry, 2016). In addition, a "Timing Research Forum" was established in 2016, to spur and connect research on timing and time perception across disciplines.

FINAL CONSIDERATIONS
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.

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

FUNDING
AR was supported by funding from the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No 665501 with the research Foundation Flanders (FWO) (Pegasus 2 Marie Curie fellowship 12N5517N awarded to AR), a visiting fellowship in Language Evolution from the Max Planck Society (awarded to AR), and ERC grant 283435 ABACUS (awarded to Bart de Boer). HH was supported by a Distinguished Lorentz fellowship granted by the Lorentz Center for the Sciences and the Netherlands Institute for Advanced Study in the Humanities and Social Sciences (NIAS), and a Horizon grant (317-70-10) of the Netherlands Organization for Scientific Research (NWO). SK was supported by a BBSRC award (BB/M009742/1), a British Academy Skills Acquisition Award (SQ140010), and a Marie Skłodowska Curie Action (MSCA: 707727) under the Seventh Framework Programme (FP7).

ACKNOWLEDGMENTS
We are grateful to the editors of Frontiers in Human Neuroscience, Frontiers in Psychology -Auditory Cognitive Neuroscience, Frontiers in Neuroscience -Auditory Cognitive Neuroscience (Isabelle Peretz, Robert J. Zatorre, Hauke R. Heekeren, and Srikantan S. Nagarajan), all members of the editorial teams, and the guest editors of some of the papers in this issue: Virginia Penhune, Mikhail Lebedev, Huan Luo, Angela D. Friederici. We would like to thank all referees who have offered their time and expertise in reviewing the papers.