Researchers reevaluated a framework that has shaped language research for 50 years. Until now researchers supposed that changes in the position of the larynx (voice box) about 200,000 years ago enabled the development of language in our ancestors. As the scientists make clear through a review of current studies, the laryngeal descent theory (LDT) lost its basis.
Contrary to previous assumptions, theoretically, apes also possess the anatomical prerequisites for the production of contrasting vowels. According to the researchers' conclusion, the development of speech could have begun more than 20 millions of years ago.
In many ways we are very similar - but there is one important aspect that distinguishes humans from their closest relatives in the animal kingdom: we use complex sound systems to communicate - language is considered a key element in the success of our species. Apes and great apes also make some meaningful sounds, but this form of communication is of a comparatively modest level. As early as the 1930s and 1950s, researchers tested whether it is possible to teach young chimpanzees to speak in a targeted manner, without success. Despite all efforts, no monkey ever said a word.
Why apes allegedly cannot speak?
The researcher Philip Lieberman then seemed to have found the explanation in 1969. He came to the conclusion that there is an anatomical reason why apes cannot produce speech sounds. By comparing the human vocal tract with that of apes, Lieberman discovered that in humans, the larynx is conspicuously low. In contrast, monkeys have a small pharynx with a high position of the larynx. This feature represents an anatomical blockage, according to Lieberman. The larynx position prevents the formation of vowels, according to his explanation.
This theory about the cause of monkey speechlessness subsequently became established and advanced to become the official doctrine. This had further consequences: since the deep-set larynx was considered unique to Homo sapiens, the theory also influenced assumptions about the genesis of language. According to the conclusion, it could only have developed in the last 200,000 years, when the human larynx reached its deepest position.
As part of their review study, an international research team led by Louis-Jean Boe of University of Grenoble have now systematically analysed research data from recent years that have focused on the behaviour of primates, their ability to produce vowels and their communication. Data from the researchers' own investigations and results of acoustic modelling were also included.
As they explain, their evaluations highlight three important aspects that contradict previous theory: First, some studies show that the deep-set larynx among primates is not exclusively found in humans. Secondly, it is not even required to produce contrasting patterns of sound expression. Thirdly, there are non-human primates that can produce vowels with contrasting formant patterns. The bottom line, the scientists sum up, is that this deprives the current doctrine of its basis.
Mental development seems to be responsible for speech ability
One study that the researchers highlight in their review conveys the overall picture in a particularly impressive way: researchers led by Asif Ghazanfar of Princeton University used X-rays to examine changes in the mouth and throat of macaques as they made sounds, ate or even varied their facial expressions. Using these results, the researchers created a computer model of the monkeys' vocal tract. They were able to prove that it is much more flexible and powerful than previously thought.
In the end, it became clear that the macaques actually possess the anatomical prerequisites for the development of thousands of different words. The researchers substantiated this thesis by using computer simulations to generate theoretical utterances of the animals - they let them speak virtually. The blockage that prevents them from speaking does not appear to be in the monkeys' necks, but rather in their minds, they concluded.
As Boë and his colleagues point out, the change in doctrine now has further consequences. If the development of articulated language was not linked to the downward movement of the larynx, which took place around 200,000 years ago, scientists can now imagine a much earlier development of language. According to them, the time frame now extends as far as 20 million years.
Our then ancestor probably also possessed the ability to produce contrasting vowels. However, a question still remains: when does our ancestors produce the mental abilities to develop speech? It remains largely unknown, in which phase of human evolution speech began, and when a sound communication that we would identify as language first emerged.