State what the general theory of evolution is.
Different kinds of organism have evolved different structures containing light receptors. Eyes are organs that include light receptors. Fig. 6.1 outlines the light receptors of several types of organism.
Using the information in Fig. 6.1, suggest a sequence by which a complex eye such as a mammal's could have evolved through successive stages.
Octopuses are molluscs with eyes that are very similar to mammalian eyes. Octopuses and mammals are not closely related. Their lenses can alter shape so they can focus on objects at different distances. Suggest reasons why octopuses and mammals have evolved similar eye structures.
Molecular evidence is used to study evolution. One investigation focused on a marine worm, Platynereis dumerilii, which still shows features similar to those of its ancestors from 600 million years ago. Scientists sequenced all the proteins in light receptor cells of $P.\ dumerilii$ and humans. The results showed many similarities between the protein sequences of $P.\ dumerilii$ and humans, especially in the light-detecting protein opsin. State what this molecular evidence shows about the evolutionary origins of $P.\ dumerilii$ and humans.
Explain how amino acid sequences show how closely related two species are.