Image-forming eyes with lenses and visual pigments have evolved independently three times in the history of life: in insects, in octopi and molluscs, and in vertebrates. Together these eyes provide a remarkable example of parallel evolution. Not only are the general optical principles of these eyes similar, they have identical chemical compounds - opsin and retinal -at their photoreceptors.

It may be difficult to explain why the particular retinal molecule was adopted three different times as the trigger for light, but it is not hard to see why a visible light receptor should involve cis-trans isomerization about a double bond: The energy required to carry out this isomerization falls squarely in the visible spectrum. In Chapter 12 we saw that the energy of a C=C double bond is 147 kcal. Mole^-1, and that of a single bond is 83 kcal. Mole^-1.

Twisting a cis double bond over into a trans configuration requires enough energy to go through a temporary single-bond state, or 147 - 83 = 64 kcal mole^-1. This energy corresponds to a wavelength of light in the blue region of the spectrum. It would not take much strain in the cis-retinal molecule, as it was bound to the opsin protein, to reduce the isomerization energy to 40 kcal mole^-1 and make the transformation possible with all visible wavelengths down to the edge of the infrared.