It is assumed that similarity proves an evolutionary relationship and the degree of similarity shows its proximity in time. This argument has been applied from the gross anatomical level to the molecular level. Creationists argue that similarity is consistent with a common creator and, moreover, is what one might expect from organisms that live in similar environments and metabolize similar foods and gases. Therefore, correspondence between homologous structures cannot be pressed back to similarity of position of the cells of the embryo or parts of the egg out of which these structures are ultimately differentiated. Evolutionists have increasingly despaired of relying on the fossil record to document the evolutionary descent of organisms.
Molecular homology is seen as the virtual savior of evolutionary biology.
There are at least 26 species of the protozoan Tetrahymena, all of which are nearly identical in structure, BUT there are enormous differences between their homologous proteins. The same is true of the more than 2000 species of fruit flies. It is actually quite difficult to compare gene and protein homologies. Calcitonin - lowers blood calcium levels. Certain proteins may be found in for example fish and reptiles but not in amphibians which are supposed to be a phylogenetic link between them.