Long ago primitive sharks had ridges running down their sides from gill to tail. Later, muscles grew into the folds, and eventually, the central portion of each ridge receded while the ends enlarged to form fins both fore and aft. All was well.
Then one day several hundred million years ago, a fish was swimming blissfully in a shallow pool. The tide went out and much to the fish’s surprise, she could use her five-rayed fins to move around a bit on the rocky bottom. The tide came in and she swam away, never to give this event another thought. The world, however, was forever changed.
For many generations thereafter, that fish’s offspring went back to the pool and progressively got better at moving across the rocky surfaces on their fins. Some even started squirming short distances overland from pool to pool. They developed pockets in their throats that could absorb oxygen directly from the air, which was far more efficient than deriving oxygen from moving water through their gills. Not only was breathing more efficient on terra firma, but also plant food was abundant and there were no predators. The fish slowly developed stronger skeletons, waterproof skin, and longer fins. They now walked and hopped and only returned to the water when they laid eggs. Amphibians were born. Birds and reptiles were next; the sequence is under some dispute, but those fins were now assuming various shapes to facilitate flying, running, and even fighting. For now, there were competitors.
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