Answer: Life on Earth began in the water. So when the first animals moved onto land, they had to trade their fins for limbs, and their gills for lungs, the better to adapt to their new terrestrial environment. ... In air, eyes can see much farther than they can under water. Tetrapods evolved from a group of organisms that, if they were alive today, we would call fish. They were aquatic and had scales and fleshy fins. ... Between 390 and 360 million years ago, the descendents of these organisms began to live in shallower waters, and eventually moved to land.
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