External ears were likely a development from when we lived under the feet of the dinosaurs as small, mainly-nocturnal creatures and as a means of temperature regulation. Dinosaurs, birds, reptiles, they all mainly rely on vision and smell. Mammals rely on their hearing, and the external ear allows a more focused sound waves to come in, mainly at night. Same principle with the origin of general lack of color vision in mammals (except for humans and other apes that redeveloped advanced colored vision). You don’t need crazy color vision if you are nocturnal. But you do need smell and good hearing to avoid big dinos.
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