(a) State the general theory of evolution. (b) Different types of organism have evolved different structures containing light receptors. Eyes are organs containing light receptors. Fig. 6.1 describes the light receptors of several types of organism. [Figure 6.1] Using the information in Fig. 6.1, suggest how a complex eye such as that of mammals could have evolved in successive stages. (c) Octopuses are molluscs that have eyes very similar to those of mammals. Octopuses and mammals are not closely related. Octopuses and mammals have lenses that can change shape to focus on objects at different distances. Suggest reasons why octopuses and mammals have evolved similar eye structures. (d) Molecular evidence is used to investigate evolution. One study involved a marine worm, Platynereis dumerilii, that still has characteristics similar to its ancestors from 600 million years ago. Researchers sequenced all the proteins in light receptor cells of P. dumerilii and humans. The results showed that there are many similarities between the protein sequences of P. dumerilii and humans, particularly in the light-detecting protein opsin.