The evolution of human speech is a mystery to all of us, and there is little evidence in non-human primates. In recent years, some scientists have proposed that human speech should come from less primal vocalizations and more rhythmic facial expressions.
No matter what language we speak, humans around the world are known to open their mouth 2 to 7 times a second while speaking (2 to 7 hertz), each open-close cycle corresponding to one syllable.
But while the universal rhythms of human speech, or the rapid cycles of opening and closing the mouth, are also found in the gestures of orangutans and macaques, it is the first time that such a rhythm is identified in African monkeys – chimpanzees.
By comparing the recordings of four populations of chimpanzees, wild and captive, the researchers have now discovered that chimpanzees also produce lip smacking at an average speech rate of 4 hertz.
What this can really tell us about our own evolutionary history is limited, but since it is one of the characteristics of human speech, it could help us to link the song of primates and human speech on the chronology of evolution.
A study published last year, for example, found that when 2,137 chimp gestures were classified into groups and their duration was average, they obeyed some of the same basic mathematical principles as human speech.
The authors of this new study, led by researchers at the University of St Andrews in the United Kingdom, conclude that their “results support the hypothesis that speech has recruited ancient rhythm signals from primates.”
“However, this possibility remains provisional until new, more detailed data becomes available from both non-hominid and hominid primates,” they add.
While smearing of the lips of macaques and gibbons is believed to be an innate skill, for example, there is evidence that the voices of orangutans may have been learned.
And that could very well be the case with chimpanzees as well, which generally produce sounds when they groom each other, potentially as a means of initiating and prolonging social interaction.
“In our own analyzes, there appears to be a variation in the frequency with which individual chimpanzees produce lip smacking, some never or very rarely observed to produce lip smacking despite hours of observation similar to that of members of their group, ”write the authors.
By comparing video recordings of zoo chimpanzees in Edinburgh, UK and Leipzig, Germany, to wild chimpanzees in Uganda, the team found a level of variation in the frequency of their lips that they believed has never been reported before, sometimes up to 2 hertz between populations.
In great apes, however, the fastest oral rhythms tend to keep a regular rhythm around a single hertz, so the authors believe that the variability of lip-snapping frequencies in chimpanzee populations may imply social factors rather than cable signals.
Unfortunately, the statistical comparison between individual chimpanzees was limited in this research, and few previous studies have revealed or analyzed the levels of variation found between one chimpanzee or another.
However, between captive and wild populations, the authors found no systematic difference in buccal signals, possibly due to “substantial overlap in the range of rhythms present” between individuals in different groups.
Of course, four populations are not a huge sample size, and additional data will have to be collected between individuals and populations so that we can determine where this strangely speech-like rhythm comes from.
Based on their results, the team is calling for future research on primate species to find out how these human rhythms occur in both individuals and populations. Knowledge could perhaps tell us more about the evolution of our own language.
The study was published in Biology letters.