Cytochrome c is an enzyme found in bacteria, fungi, plants and animals. Here it is given that the human amino acid sequence for cytochrome c differs at 12 locations from the sequence in horses and pigeons. This information might be useful for inferring in preliminary stages that horses and pigeons are closely related to each other since they both show similar amount of difference from humans.
However, this hypothesis is being made with respect to the the comparison between "humans and horses" and "humans and pigeons". To further test this hypothesis pigeons and horses should be directly compared to each other without the use of a third species like humans. Hence, the sequence for cytochrome c in horse and pigeon should be compared with each other rather than comparing to humans.
