Only mammals chew their food. No other vertebrates use elaborate, repetitious jaw movements to process food before swallowing.
Other animals do have ears. Mammals started out with their ears on their lower jaws.
Versatile chewing movements evolved in mammals to efficiently break down food. Teeth and chewing adapted to diets to optimize reducing food to bits, thus facilitating digestion. The smaller the food particles, the larger the surface area for gut microbes to eat into. Diet, dentistry and chewing form an evolutionary nexus for mammalian nutrition.
The modern mammalian lower jaw is a single bone from which teeth extend. In cynodonts – mammalian ancestors – the lower jaw was composed of multiple bones. Some of those jaw bones transmitted sound via bone conduction through a so-called mandibular ear. The jaw served double duty, as it also functioned to snag food and chew. As the middle ear bones were on the jaw, feeding disrupted audition.
As an adaptive optimization, the ear bones migrated away from the jaw and distinct ears emerged. The lower jaw became a single bone.
Modularization is a common evolutionary technique. Grass is another example of modularity.
Grass blades grow to catch the Sun’s rays. Herbivores (or lawn mowers) chewing grass leaves do not kill the grass because the roots retain all functionality except photosynthesis.
Ishi Nobu, The Elements of Evolution, BookBaby (2019).
Julia A. Schultz, “Eat and listen – how chewing and hearing evolved,” Science (17 January 2020).
Fangyuan Mao et al, “Integrated hearing and chewing modules decoupled in a Cretaceous stem therian mammal,” Science (17 January 2020).