Why did our ancestors combine the first consonant- and vowel-like utterances to produce the first syllable or word? To answer this question, it is essential to know what constituted the communicative function of proto-consonants and of proto-vowels before their combined use became universal. Almost nothing is known, however, about consonant-like calls in the primate order 1,2 . Here, we investigate a large collection of voiceless consonant-like calls in nonhuman great apes (our closest relatives), namely orangutans (Pongo spp.). We analysed 4,486 kiss-squeaks collected across 48 individuals in four wild populations. Despite idiosyncratic production mechanics, consonant-like calls displayed information-dense content and the same acoustic signatures found in voiced vowel-like calls by nonhuman primates, implying similar biological functions. Selection regimes between proto-consonants and proto-vowels were thus probably indistinguishable at the dawn of spoken language evolution. Our findings suggest that the first proto-syllables or proto-words in our lineage probably constituted message reiterations, instead of messages of increasing intricacy.
We thank the Indonesian Institute of Science (LIPI), the Indonesian Ministry of Research and Technology (RISTEK), the Indonesian Directorate General of Forest Protection and Nature Conservation (PHKA), Gunung Palung National Park Bureau (BTNGP), Gunung Leuser National Park (TNGL) and Leuser Ecosystem Management Authority (BPKEL) for authorization to carry out research in Indonesia. We thank Universitas National (UNAS), Tanjungpura University (UNTAN) and Universitas Sumatera Utara (USU) for supporting the project and acting as counter-partner. Bornean Orangutan Survival (BOS, Palangka Raya, Central Kalimantan), Sumatran Orangutan Conservation Programme (SOCP, Medan, North Sumatra) and Gunung Palung Orangutan Project (GPOCP, Ketapang, West Kalimantan) acted as sponsors. We thank M.-C. Pagano for technical support. R. Mundry and J. Kendal provided input on the design of the generalized linear mixed models, as did H. Colleran and S. Roberts at the First Quantitative Methods Spring School 2016 at the Max Plank Institute for the Science of Human History, Jena, Germany. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.