When counseling patients about their hearing test results, I’ll often describe the mechanism of how we hear, how sound in our environment is converted into electrical impulses for the brain to interpret. They almost universally remark about what an amazing and complex system it is, which is absolutely true. What they are likely not aware of is that it is also a remarkable illustration of evolution at work!
Think about how things sound when you’re under water at the pool, with kids yelling and laughing above the surface. You hear sound, but it doesn’t have any distinction or clarity. This has to do with the physics of sound. As you know, sound is literally a wave. This wave can be in the air, or a wave in fluid (think of whale ‘songs’). The issue arises when sound in one medium (air) tries to propagate to another medium (fluid). The water is a thicker/denser medium than the air, so that almost all of the sound energy is impeded from transferring through. In physics parlance, this is referred to as an impedance mismatch. So how does this relate to the ear and human evolution? Stay with me now…
The mammalian peripheral auditory system is made up of the outer & middle ear (ear lobe, canal, ear drum and 3 tiny middle ear bones called ossicles) and the fluid-filled inner ear. The outer and middle ear, called the “conductive system”, conducts sound energy to the inner ear where it gets converted into electrical energy for the brain to process. Reptiles and birds all have only one middle-ear bone. The bones that would otherwise form their “ossicles” are fused with the joints of their jaw, meaning hearing and chewing are intertwined. Like air based creatures, creatures in the sea also have a fluid-filled inner ear, called the lateral line organ. But since they only have to ‘hear’ vibrations in fluid, they have no need for our conductive system, as there is no impedance mismatch between the vibrations in their fluid environment and their lateral line organ.
The fossil record indicates that around 350 to 400 million years ago, the first creatures (“tetrapods”) climbed out of the sea. Though we obviously don’t know why they decided to venture out of the water, they had evolved for life in the sea, which means that they weren’t able to ‘hear’ sound in their new (air based) environment well. But as they spent more time out of the water, it became imperative from an evolutionary perspective to be able to overcome this ‘hearing loss’. The solution was the evolution of the conductive system, particularly the ear drum and ossicles, which overcomes the impedance mismatch between sound in air and our fluid filled inner ear.
A relatively recent fossil discovery dating back ~120 million years ago, unearthed a previously unknown species of early ancestor to mammals that provided a missing link to our unique auditory system. This newly discovered fossil revealed that the middle ear bones of this species were ‘decoupled’ from the lower jaw. So prior to this step in mammalian evolution, like reptiles and birds today, hearing and chewing were tied together. This evolutionary step, while not only sharply improving hearing acuity for our early ancestors, also allowed them to adapt to a wider variety of food sources. Apparently, this unique adaptation was such a potent evolutionary step, that it appears to have independently emerged multiple times in early mammals.
It’s not an overstatement to say that the development of the middle ear bones, the smallest in the human body, likely offered the evolutionary advantages necessary for our early ancestors to thrive, greatly increasing the odds of us making it to where we are today! Now if we can only evolve past our current (self-inflicted) hurdles…
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