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I wasn't quite sure I had heard correctly. My friend, a fellow geologist, and I were standing in the swimming lanes of a lap pool where we had stopped to give each other greetings of the season.
"My hearing in this ear is a whole lot better," he had said.  
Or so I thought.  
"The surgery replaced the tiny, tiny third bone — the innermost bone — of the ear," he went on. "It had become ossified, sort of cemented to the rest of my head over the years, so it couldn't vibrate like it should."
From high school biology I vaguely remembered three tiny bones in a little chain in the ear, bones that have the task of amplifying sound waves as they enter the ear. Sound waves in air don't pack nearly the "oomph" as pressure-waves in water, so if you want to hear in air (and most of us do), you need little mechanical amplifliers - which is what the three ear-bones are.
The third and final tiny bone gives the last boost of amplification and also separates the air we live in from the water that fills and conveys sounds within the inner ear.   
It's in that fluid that tiny, tiny hairs respond to pressure-waves and translate them into electrical signals that flow to our brains. That's the whole goal of an ear, from my point of view as a physical scientist, to translate sound waves into electrical signals.  
But the system doesn't work if that last, and most tiny of all, bone cannot flex and move.
"The implanted piston goes in and out in place of that last bone. And it works!" my friend said with evident pleasure. "I can hear sopranos again."  
Now, in truth, my own hearing standing in the swimming pool was problematic because I had misplaced my good earplugs that day. My outer ears — the part you can reach with a Q-tip although you are not supposed to do so — had water in them and shaking my head wasn't doing much to get the water out.  
All of that got me thinking about air and fluid in different parts of my ear. But only when I talked to my friend Ken Kardong, biology professor here at Washington State University, did I start to understand that my almost random questions about the matter were unearthing a bit of the long history of life on Earth.  
Ears are nothing new. Many fish have pretty complicated ears, including fluid-filled inner ears. Fish go back to the Paleozoic Era, the oldest era in the history of life that has complex vertebrate fossils (proper animals with backbones). There was one part of the Paleozoic in which there were many, many fish species in the seas - as we know from the fossil record — but still no complex species at all on land. That's how early and simple was what we geologists mean by "the early Paleozoic."
When land-loving vertebrates first show up in the fossil record they are amphibians — animals that move from the water to land and back. Reptiles follow amphibians near the end of the Paleozoic, again a fact we know from the fossil record.
It's no surprise that the inner ears of fish would be filled with fluid. And since the inner ear is separated from the other parts of the ear by a bone and seal, you can see how the inner ears of amphibians and then reptiles would likely remain fluid-filled while the outer parts of the ears started to become air-filled.
When us fully land-loving mammals come on the scene in the Mesozoic Era - the era dominated by the dinosaurs — we naturally enough have air-filled outer ears and fluid-filled inner ears. We still do. That's why swimmers need to drain our ears so the outer parts are filled with air. But we also display each day that fluid-filled inner ear that suggests our ancient lineage with earlier animals in the long chain of vertebrates that have lived on Earth for so very long.
May your holiday season be filled with nothing but good sounds — which you can well and truly hear.
— Dr. E. Kirsten Peters is a native of the rural Northwest, but was trained as a geologist at Princeton and Harvard. This column is a service of the College of Sciences at Washington State University.