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It turns out we owe a lot to fish! We recently talked about how our tooth enamel evolved from fish scales. And, according to a 2006 study published in Nature, we got our auditory ability from fish gills. How did a gill become an ear? About 370 million years ago, a fish called Eusthenopteron had a small bone which had a kink that blocked the gill opening. Its descendant, Panderichthys, had a slightly wider version of that bone, called a spiracle. Spiracles allowed fish to breathe air while remaining underwater, and scientists think the spiracle is the missing link between fish gills and land-animal's ability to hear. There were a number of evolutionary steps in-between: for example, the hyomandibular bone (which became the stapes) is believed to have evolved from a bone that used to brace the lower jaw of sharks and early jawed fish. According a study from the Journal of the Acoustical Society of America, mammals have larger audible fields than birds and reptiles mostly because of our ability to hear higher frequencies due to adaptations of the middle ear that evolved around 40 million years ago. Humans don't have the best hearing in the natural world, but we don't have the worst either. Because our ears are relatively close-set, we can hear high-frequency sounds, like the rustling of a nearby predator for example.
A 2015 study in the journal Science Advances scanned the skulls of two hominins, Australopithecus Africanus and Paranthropus robustus and created computer models of early human ears. Because of physiological differences between them and us, they could hear very differently than modern humans. Back then, they were more sensitive to T's, K's, F's and S's in the 1 to 3 kHz range. The researchers think they would have been able to hear these hard consonant sounds in short-range across open areas, which makes sense because they lived in wide-open savannahs. On top of that, they were better at 1 to 3kHz than humans and our primate cousins are now. By comparison, modern humans today speak in the 1kHz to 6kHz range, something Australopithecus and Paranthropus would not have been good at. A 2011 study in Science used a mathematical model to trace DNA evidence of human migration and language development, and found verbal communication did likely originate in Africa, (and it was probably in the form of a click language).
Ancient Human Ancestors Heard Differently (Scientific American)
"Now a study finds that early human species may have had sharper hearing in certain frequencies than we enjoy."
Hearing Ability of Early Humans Revealed in New Study (New Historian)
"A new research study has revealed that human ancestors as they existed around two million years in the past had an ability to hear that has much in common with the modern chimpanzee - but with several indications that evolution was already having an effect."