Some features of living cells exhibit
irreducible complexity meaning that they function only if all their parts are present in good working order at the outset. For example, a mousetrap consists of several parts which work together to catch a mouse; if any part is missing or defective, the mousetrap fails to work at all. Similarly, the biochemistry of light detection requires complex interactions among many different molecules, each performing a very specialized job.
According to Lehigh University biochemist Michael Behe there is no imaginable way that the necessary combination of molecules could be built up piecemeal, as the theory of evolution requires; either they are all present, or the process does not work.
See: intelligent design