Why fish have vertical fins, and dolphins and whales not?

Have you ever wondered why a dolphin and a whale have a horizontal fin and the rest of the fish family not? No, it is not because they are mammals or bigger or even smarter. Here is why...

The explanation is actually simple. These animals just evolved from different kinds of creatures.

Fish evolved more from something that slithers, like a snake,worm or a lizard, whatever you want to call it. Basically from something that moves from left to right, or right to left. I guess this wormy-creature wiggled it's way to a swimming fish. Which is why fish have vertical tails.

Dolphins and whales had it a bit different. Evolving from a land creature, who ran with limbs under their bodies, they have spines flexing up and down. 

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