Hank: Let me introduce you to one of the bravest pioneers in the history of life on planet Earth, an organism that blazed the trail for every single vertebra that lives on land today and many that don’t. It is one of your most important ancestors. It, well, doesn’t have a name, and we don’t exactly know what it looked like either, but we do know that about three hundred eighty million years ago this fishy looking thing with big fleshy fins achieved one of the animal kingdoms greatest milestones, breathing air.
Sounds simple enough, but believe me it wasn’t because for billions of years before this fishy ancestor came around, basically all of life evolved in water. From the very beginning, the earliest, simplest forms of life, like bacteria, extracted oxygen they needed right from the water through their membranes, and they did it through simple diffusion, when a material automatically flows from where it is concentrated, to where it is less concentrated so it balances out. Diffusion works really well and it requires zero effort, but it wasn’t gonna cut it in the big leagues. Anything larger than a small worm is simply too big and needs too much oxygen for diffusion to work. So in order to get bigger, early life forms needed a circulatory system that could move bulk amounts of oxygen around faster inside their bodies and a respiratory system to bring more oxygen in contact with their wet membranes. So their respiratory services moved from their outer surfaces to the insides of their bodies. First there were gills, but gills of course still only work inside of water, and a little more than three hundred and eighty million years ago this was starting to lose some of its charm. Earth was getting warmer. The seas were getting shallower, and much of the planet’s surface water had lower concentrations of oxygen than it used to. Finally, a humble little lobe-finned fish got fed up, swam up to the water’s surface, and started breathing air. It could do this because it had evolved a fancy new interface to move gases between the air and it’s cell membranes. I’m talking about lungs, wet lungs. With an efficient new way to take in nearly limitless amounts of oxygen from air, animals were eventually able to get bigger and more diverse over the ages, and now all of us lung-having vertebrae share that common ancestor. For lots of animals, including humans, those lungs come with a bunch of other equipment, like protective ribs, a stiff trachea, and, in mammals, a strong diaphragm, and together they form your respiratory system, which happens to be best friends and business partners with your circulatory system. It’s only by working together and using both the bulk flow and simple diffusion of oxygen that they can make possible the process of cellular respiration, in other words, life itself. So a lot improvements have been made to it over the eons, but the respiratory system that you are using right now is your inheritance from that ancient ambitious fish, leader of one of the most anatomical revolutions in the past half million years.
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