Perhaps this question will seem childish, but in fact, the largest scientific figures have been racking their heads over it for many decades. No one expects other primates to instantly compose and read a poem, but why does the informational level of their sound expressions at least not approach that of a human?
Or, rather, why the sounds of our closest relatives are not at all closer to human speech than the methods of communication of a whole series of species that are much more distant from us?
Some creatures, including those as exotic as parrots and elephants, can learn to imitate human speech. Primates, on the other hand, even hominids, after many years of training, are capable of only a few whispered attempts.
Over time, two competing theories have taken hold that explain the "non-speaking primate" paradox. Either their brain is to blame, which is not so developed to cope with complex communication close to human, or the structure of the vocal cords, larynx and oral cavity of monkeys does not allow reproducing the wide range of sounds that the human throat is capable of.
Of course, the explanation could be somewhere in between. Perhaps primates have excellent organs for making sounds, but their brains cannot control them. Let's simplify things to two extremes for the purposes of this article.
Charles Darwin was an ardent supporter of the first theory. Most scientists were also confident in this theory until the end of the 60s of the last century. The other side prevailed after the publication of a famous article by the American researcher F. H. Lieberman and his colleagues, who carefully examined the mouth and larynx of a dead macaque and made a plaster model of it. It was measured and the resulting data entered into a computer to find out how wide a range of sounds a monkey can make.
The result surprised everyone: the sonic capabilities of macaques are extremely limited and do not even come close to human ones. Even if the monkey's brain were sufficient for complex speech, for purely mechanical reasons, macaques would not cope with it. Experiments with other methods of communication, such as signs, have also shown that hominids can communicate quite well. This is how the second theory entered the textbooks.
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There are mechanical capabilities, the problem lies elsewhere
But the most recent studies by a group of European and American biologists prove that F. H. Lieberman was significantly "miscalculated." A team led by a professor named William Tecumseh Fitch (if his name seems strange to you, then know that his full name is William Tecumseh Sherman Fitch III and he is a direct descendant of the famous general during the war of the North and South) instead of a corpse of a monkey and a plaster model studied a live macaque and used an advanced x-ray.
First, scientists taught a monkey to sit during a scan, and then, using X-rays, photographed its throat during various activities: when screaming, while eating, and a variety of facial expressions. A total of 99 different positions of the vocal cords, muscles and tissues were identified in the macaque. The resulting spectrum of sounds corresponding to human vowels practically did not differ from human capabilities. The computer even gave scientists the ability to synthesize certain sentences the way a monkey's throat would pronounce them.
By clicking on this link, for example, you can listen to the sentence "Will you marry me?" ("Will you marry me?") And independently assess how the hypothetical monkey speech is understandable. According to most English-speaking observers, the sentence is as understandable as if it were spoken by a person with a foreign accent.
In a somewhat more detailed experiment, scientists asked a computer model of a monkey's throat to synthesize five of the most different vowels it can only (they approximately corresponded to our i, e, a, u, o). Then independent observers were asked which of these sounds seemed to them to be different vowels.
The results did not differ from the perception of human languages. Moreover, five different vowels are the norm, and in some languages three are enough. The consonants pronounced by the macaque turned out to be more difficult. However, she could almost 100% pronounce p, b, k and g, as well as h, m and v.
So it has practically been proven that from a purely mechanical point of view, nothing prevents hominids from correctly imitating human speech. Let their abilities not completely coincide with ours (for example, macaques could not pronounce i), but people would understand their speech without problems. Moreover, the assessment of the abilities of macaques is very conservative, and if training was carried out, they would easily reach human "heights". So the explanation for the absence of at least some form of speech in monkeys is connected, apparently, with their brain.