@dapengliu25
What is the evolutionary reason that no animal species normally have three legs?
It’s down to hydrodynamics.
Limbs evolved from fins. The first fishes had only midline fins, dorsal & ventral, useful if swimming by side to side undulation, but not much use for maneouvering vertically.
As agility makes the difference between catching prey or becoming prey, they later evolved pectoral & later still pelvic fins, two pairs fore and aft being optimum for controlling pitch without causing too much drag (a feature which we copy when designing submarines, with bow planes as well as horizontal rudders).
To add a third pair would contribute extra drag but no additional agility, as the middle pair would be constantly neutral. Conversely, to omit one limb of a pair would cause assymetrical drag and lose agility.
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