Evolution of the Larynx

Primates already had bigger and more diverse larynxes, before humans came on the scene.

While oxygen was originally a photosynthetic waste product and toxic to all life forms, we gradually got used to it. Some, especially the eukaryotes, made a virtue of oxygen's great electronegativity to adopt a new and more efficient metabolism with oxygen as the final electron acceptor. This allowed the evolution of large animals, which breathe oxygen. All this happened in the oceans. But it turns out that it is far easier to get oxygen from air than from water, leading air breathing to evolve independently dozens of times among fishes of all sorts of lineages. Lungs developed from many tissues, but rarely from the swim bladder, which had critical roles revolving around constant and controllable pressure, pretty much the opposite of what one needs in a lung. So in our lineage, the lung developed as an adjunct to the digestive system, where the fish could gulp air when gill breathing didn't cut it. 

Overview of the atmosphere of earth. Lungs were only possible when the level of oxygen in air rose sufficiently, and respiration of any kind only when oxygen had vanquished the originally reducing chemical environment.

This in turn naturally led to the need to keep food from going into the nascent lung, (air going into the stomach is less of a problem.. we still do that part), thus the primitive larynx, which just a bit of muscle constricting the passage to the lung. As this breathing system became more important, regulating access to the lung became more important as well, and the larynx developed more muscles to open as well as close the air passage, then a progessively more stable and complex surrounding structure made of cartilage to anchor all these muscles. 

But that was not the end of the story, since animals decided that they wanted to express themselves. Birds developed an entirely different organ, the syrinx, separate from the larynx and positioned at the first tracheal branch, which allows them to sing with great power and sometimes two different notes at once. But mammals developed vocal cords right in the center of the larynx, making use of the cartilaginous structure and the passing air to send one fluttering note out to the world. The tension with which these cords are held, the air velocity going past them, and the shapes used in the upper amplifying structures of the throat, mouth, and sinuses all affect the final sound, giving mammals their expressive range, such as it is.

So why are humans the only animals to fully develop these capacities, into music and speech? Virtually all other mammals have communicative abilities, often quite rich, like purrs, barks, squealing, mewling, rasping, and the like. But none of this approaches the facility we have evolved. A lot can be laid to the evolution of our brains and cognitive capacities, but some involves evolution of the larynx itself. A recent paper discussed its trajectory in the primate lineages.

Comparison of representative larynxes, showing a typical size difference.

The authors accumulate a large database of larynx anatomy and function- sizes, frequency patterns, evolutionary divergence times- and use this to show that on average, the primate lineage has larger larynxes than other mammals, and has experienced faster larynx evolution, to a larger spread of sizes and functions, than other mammals. The largest larynxes of all belong to black howler monkeys, who are notorious for their loudness. "The call can be heard up to 5 km away." They also claim that among primates, larynx size is less closely related to body size than it is among other mammals, suggesting again that there has been more direct selection for larynx characteristics in this lineage.

Primates (blue) show greater larynx size and variability than other carnivores.

This all indicates that in the runup to human evolution, there had already been a lot of evolutionary development of the larynx among primates, probably due to their social complexity and tendency to live in dense forested areas where communication is difficult by other means. Yet do primates have vocal languages, in any respect? No- their vocalizations are hardly more complex than those of birds. Their increased evolutionary flexibility at most laid the physical groundwork for the rapid development of human speech, which included a permanently descended larynx and more importantly, cognitive and motor changes to enable fine voluntary control in line with a more powerful conceptual apparatus.

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