Theoretical Rules Governing Life in the Universe
On Earth, many animals only have one locomotory organ (such as the tail of a fish or a legless lizard). Some have five limbs (starfish). It just happens that the presently dominant group of large animals has bilateral symmetry (a distinct head and tail, underside and back, and medial and lateral, but usually no left and right distinction, except in internal organs), and hence often has paired anatomical features.