What Makes Human Voice Different from Chimps? Simplicity of Throat, Scientists Say

Simple is more sophisticated when it comes to producing human speech—clearer and more stable than the voices of other primates. Evolution has gifted humans a much simpler vocal anatomy than our ancestors. Notably, the simplicity of the vocal anatomy acquired by humans in the course of evolution has rendered it producing a stable, comprehensible and evenly toned voice in comparison to the rough and warbling sounds that the other primates can produce.

In recent research published in Science on August 11, scientists have claimed to find these exciting facts about the human voice.

Mammalian vocalisation depends upon air passage through the larynges. The larynges are also known as the voice box, which is an organ mostly towards the top of the throat. Mammals force air through the larynges, and in this process, folds of tissues start oscillating, producing a wide variety of sounds. In the case of humans, the folds of tissues are known as the vocal cords, which are basically a pair of folds.

Vocal cords are responsible for creating the human voice. It is relatively simple when we compare the vocal anatomy of humans to other primates. Non-human primates have an additional feature in their vocal anatomy. Their vocal tracts contain thin flaps or vocal membranes. Among non-human primates, this additional tissue fold is often found near the vocal folds or connected to it.

The vocal membranes, despite being known to scientists, were unclear about their function. William Tecumesh Fitch, an evolutionary biologist at the University of Vienna and an author of the Science research, commented, "We didn’t really know the function, and we really didn’t know how widespread they were.”

William Tecumesh and his colleagues started their research on dead chimpanzees that were donated by local zoos. They removed larynges from the bodies of the apes and linked three of them to a device blowing compressed air through the organs. This process acted to simulate vocalisation. The research group also had high-speed cameras, and the footage obtained from them showed the way the vocal folds along with the membranes vibrated.

The patterns thus obtained by the researchers found matching with those in live animals. The live animals' vocal function was seen in an endoscopic video where a chimpanzee woke up from anaesthesia. Along with this, there were other similar recordings of the larynges of rhesus monkeys and squirrel monkeys.

Fitch and other researchers in the team built a computer model of a larynx with the help of the data found from the experiments on dead and live animals. The researchers simulated their computer model where a virtual vocal box coupled with the vocal folds and the vocal membranes (of a chimpanzee). The resulting sound was chaotic and uneven. The virtual vocal membranes oscillated chaotically, and the soundwaves were unstable. On the other hand, when the simulation was conducted with a humanlike virtual voice box, which has only the vocal folds without the membranes, the resulting sounds were like those from a humanlike voice.

The team also deep scanned the larynges of 44 primate species, including baboons and orangutans and found the vocal membranes in all of them. The results manifest that humans lost the vocal membranes in order to make the voice more stable and intelligible. On it, Fitch said—“I think people have a tendency to think of evolution as always getting more complex,” This is a nice counterexample.”

The study authors say that as the vocal membranes don’t fossilise, it is not possible to say when did our species lost these in the course of evolution. However, if scientists can one day identify the genes responsible, they might be able to show if they disappeared, say, only after humans evolved”—commented Katarzyna Pisanski, the first author of the study.