The origin of tone, known as tonogenesis, has fascinated researchers studying language evolution and human cognition for a considerable period. Linguistic investigations of tonal languages have proposed various hypotheses regarding the origin of tone[1-3], but these hypotheses have not been quantitatively tested in an evolutionary framework. In this study, we evaluated these tonogenetic hypotheses by conducting phylogenetic comparative methods[4, 5] with a Sino-Tibetan language dataset including a large-scale phylogeny and several tonogenetic potentials. Our results revealed a strong phylogenetic pattern in the distribution of tones and suggested that the Proto-Sino-Tibetan languages should be likely non-tonal. Moreover, we identified specific phonological structures, such as the loss of syllable-final consonants and voice quality on vowels, that were closely associated with the origin of tone. These associations were in line with the linguistic suggestion that phonological simplification could induce the origin of tonal contrast. Interestingly, we also found that the presence of several tonal features did not significantly affect the diversification rate of Sino-Tibetan languages. Collectively, our findings shed light on the nature of tone, revealing that it emerged as a compensatory mechanism to facilitate the structural organization and evolution of languages[6, 7]. This research contributes to a quantitative understanding of the origins and functions of tonal features within the broader context of language evolution and human cognition.

References

1. Thurgood G. Vietnamese and tonogenesis: Revising the model and the analysis. Diachronica. 2002;19(2):333-63.

2. Michaud A, Sands B. Tonogenesis. Oxford Research Encyclopedia of Linguistics2020.

3. Kingston J. Tonogenesis. The Blackwell Companion to Phonology2011. p. 1-30.

4. Pagel M. Detecting correlated evolution on phylogenies: a general method for the comparative analysis of discrete characters. Proceedings of the Royal Society of London Series B: Biological Sciences. 1994;255(1342):37-45.

5. Pagel M, Meade A. Bayesian analysis of correlated evolution of discrete characters by reversible-jump Markov chain Monte Carlo. The American Naturalist. 2006;167(6):808-25.

6. Matisoff JA, editor Tibeto-Burman tonology in an areal context. Proceedings of the symposium “Crosslinguistic studies of tonal phenomena: Tonogenesis, Japanese Accentology, and Other Topics; 1999: Tokyo: Tokyo University of Foreign Studies, Institute for the Study of Languages and Cultures of Asia and Africa.

7. Matisoff JA. Tonogenesis in Southeast Asia. Consonant types and tone. 1973;1:71-96.

Recent research provides strong support for the hypothesis that humidity influences the tonal system of language, and encourages a more comprehensive exploration of the connection between climate and tone. Based on a substantial database of 1,525 language varieties in China, as well as 41 years of monthly climate data, we conduct a thorough investigation into the relationships between multiple climatic factors and the number of tones, while also investigating voice quality and intra-word pitch variation as intermediary factors. The findings reveal that climatic factors affecting the pitch manifestation, voice quality, and the number of tones are multifaceted, with specific humidity, precipitation, and temperature as essential factors. Furthermore, better voice quality and smaller pitch fluctuations, both related to humid and warm regions, are often associated with languages with more tones. Our results offer additional evidence for interactions between ecology and human behavior mediated through physiological mechanisms.

In previous work some correlations were established which are tentatively fitted together in a causal chain in the present paper. Wichmann et al. (2011) and Wichmann and Holman (2023) found that word length is inversely correlated with speaker populations of languages. This relationship resonates with observations in Lupyan and Dale (2010) suggesting an inverse relation between language complexity in general and population size. Subsequently, Wichmann (2023) observed that the number of tones tends to increase as word length decreases across the world's languages. This would ultimately be an outcome of processes of tonogenesis (Wu et al. 2023, Micheaud and Sands 2020). Thus, both processes are inherently plausible and in this sense unexpected. A difference between them, however, is that the correlation between word length and population size found by Wichmann and Holman (2023), although strong (r = -0.92), only emerges across macro-areas, not within areas or families, which suggests that the relationship takes several thousand years to develop, whereas a phylogenetic correlation analysis showed support for the inverse correlation between word length and the number of tones in as many as half of the families for which the size and presence of tones allow for this type of analysis. Thus, it seems to be a relationship that can emerge relatively quickly, perhaps within centuries. The two processes can be tied together in a causal chain: population size > word length > tone, where the greater-than sign means "influences" without necessarily implying a strong, deterministic force. In addition, population size, which thus seems to be an important player in the patterning of linguistic diversity, is itself influenced by a variety of factors, where climate seems to be of central importance (e.g., Tallavaara et al. 2015). Therefore, in addition to exploring the causal chain just mentioned, including the typical tempo of changes along the chain, this paper will inquire into climatic conditions on language speaker populations drawing upon recent resources such as Beyer et al. (2020).

References

Beyer, R. M., Krapp, M. and Manica, A. 2020. High-resolution terrestrial climate, bioclimate and vegetation for the last 120 000 years. Scientific Data 7, 236.

Lupyan, G. and Dale, R. 2010. Language structure is partly determined by social structure. PLoS One 5, e8559.

Michaud, A. and Sands, B. 2020 Tonogenesis. In Oxford Research Encyclopedia of Linguistics (ed. M. Aronoff). Oxford, UK: Oxford University Press. doi:10.1093/acrefore/9780199384655.013.748

Tallavaara, M., Luoto, M., Korhonen, N., and Seppä, H. 2015. Human population dynamics in Europe over the Last Glacial Maximum. Proceedings of the National Academy of Sciences of the U.S.A. 112(27), 8232-8237.

Wichmann, S. and Holman, E.W., 2023. Cross-linguistic conditions on word length. PLoS ONE, 18(1), e0281041.

Wichmann S., Rama, T., and Holman, E.W.. 2011. Phonological diversity, word length, and population sizes across languages: The ASJP evidence. Linguistic Typology 15, 177–197.

Wu, B., Zhang, H. and Zhang, M. 2023. Phylogenetic insight into the origin of tones. Proceedings of the Royal Society B, 290(2002), 20230606.

Languages without tone occur widely distributed throughout the world with the notable exception of sub-Saharan Africa, whereas languages with tone are concentrated in regions relatively close to the equator where generally warm and humid conditions prevail, apart from (primarily western) North America and some marginal cases in Europe. Everett et al (2015, 2016) suggest that low ambient humidity leads to loss of tone distinctions, since precise control of phonation is impeded by dry air. An implicit assumption here is that tonality is the ‘natural’ state of language, and its absence is derived (as is claimed by Brown 2017, among others). Comparative evidence suggests that tone is not original, it can nearly always be shown to be derived through documentable phonological change (Maddieson 2023). Hence, if the claimed correlation between the distribution of tone and humidity is valid (see Roberts 2018 for discussion) motivation to encourage tonality in warm wet climates rather than to discourage it in cold dry ones is required as the explanation. Our analyses suggest that it is worth further pursuing the hypothesis that differences in transmission conditions, rather than concerns with production, influence such design features of languages. Tone contrasts concern steady to slow-moving changes in the acoustic signal, which are less disrupted by ambient factors affecting fidelity of transmission, such as dense vegetation and high temperature, as found in tropical regions, compared to distinctions between consonants, especially obstruents. Thus, for example, transferring contrasts between consonant types to distinctions of tone on the following vowel — the most frequent route to tonogenesis — provides a more robust acoustic identity for the intended message under the prevailing conditions.

Previous work has proposed various mechanisms by which the environment may affect the emergence of linguistic features. For example, dry air may cause careful control of pitch to be more effortful, and so affect the emergence of linguistic distinctions that rely on pitch such as lexical tone or vowel inven- tories. Criticisms of these proposals point out that there are both historical and geographic confounds that need to be controlled for. We take a causal inference approach to this problem to design the most detailed test of the theory to date. We analyse languages from the Bantu language family, using prior geographic-phylogenetic tree of relationships to establish where and when languages were spoken. This is combined with estimates of humidity for those times and places, taken from historical climate models. We then estimate the strength of causal relationships in a causal path model, controlling for various influences of inheritance and borrowing. We find no evidence to support the previous claims that humidity affects the emergence of lexical tone. This study shows how using causal inference approaches lets us test complex causal claims about the cultural evolution of language.

A number of recent studies have examined potential effects of environmental factors on the use of sounds in speech. Work on this topic now includes acoustic data, in the form of over one million audio files recorded across China, which show an association between extreme ambient aridity and patterns in the fundamental frequency of vocal cord vibration (Liang et al. 2023). The effects in question – increased jitter and shimmer in very dry regions – are not expected to be transmitted via language contact or inheritance. Other higher-level patterns in sound systems that are potentially impacted by environmental factors are definitely influenced by genealogy and/or contact. Recent work aims to control for the latter confounding variables in various ways. With respect to language contact, some work has incorporated tests for spatial autocorrelation between languages rather than, for instance, including geographic regions as random factors in a linear mixed model (e.g., Bromhan & Yaxley 2023). Such recent work advances the discussion, but as we suggest here, could be improved by: a) more careful considerations of the causal mechanisms proposed in the literature, and b) contextualizing the autocorrelation results with “sanity checks”, i.e. tests of well-known environmental influences on human behavior. We aim to enhance work on this topic by addressing these two points. We do so while incorporating new and more detailed data on ambient water vapor, obtained via NASA’s AQUA MODIS satellite data repository. These water vapor data are plotted in Figure 1. We test the potential association between aridity and phonation patterns in speech, finding some support for the association even while controlling for spatial autocorrelation. We contextualize this association with results from tests of better established environmental influences on human cultures.

Figure 1. Map of ambient water vapor data worldwide, gathered from NASA satellite data. Brightness corresponds to the amount of water vapor in the air.

References

Bromham, L., & Yaxley, K. J. (2023). Neighbours and relatives: Accounting for spatial distribution when testing causal hypotheses in cultural evolution. Evolutionary Human Sciences, 5, e27.

Liang, Y., Wang, L., Wichmann, S., Xia, Q., Wang, S., Ding, J., ... & Ran, Q. (2023). Languages in China link climate, voice quality, and tone in a causal chain. Humanities and Social Sciences Communications, 10(1), 1-10.

Discussion and future directions.