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.