The two animals that exhibit the most linear relationship based on the provided information are birds and reptiles.
Both belong to the class of animals known as amniotes and share several characteristics, such as the development of scales (in reptiles) and feathers (in birds, which are modified scales). Additionally, they have similar features in their evolutionary lineage. In contrast, the relationships among amphibians and fish, as well as humans and birds, are less direct due to significant differences in developmental stages and classifications.