Proto-consonants were information-dense via identical bioacoustic tags to proto-vowels

Why did our ancestors combine the first consonant- A nd 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. 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.