The structural difference between rhodopsin and isorhodopsin is only in the cis-position of the chromophore, but the difference leads to a large discrepancy in photoisomerization period and quantum yield. The photoinduced cis-trans isomerization dynamics of the two chromophores are investigated using a Quantum Mechanics/Molecular Mechanics trajectory surface hopping scheme. Rhodopsin shows a straightforward and fast excited-state dynamics whereas the isorhodopsin dynamics in the excited state is complicated due to differences in retinal motions and space gaps formed by surrounding residues. Consequently, the isorhodopsin --> bathorhodopsin reaction is slower and less efficient. Photoexcitation of rhodopsin gives bathorhodopsin only, whereas isorhodopsin yields an analog with 9,11-di-cis-retinal in addition to bathorhodopsin. These differences explain why life uses rhodopsin rather than isorhodopsin.