Evolutionary changes involve accumulation of mutations.

Distantly-related organisms accumulate a greater number of mutations than closely-related ones.

This divergence is seen among vertebrates in the amino acids of a hemoglobin protein.

Humans are closely related to rhesus monkeys, with 8 differences out of 146; the distantly related lamprey has 125 differences.

Mutations are random and accumulate at a constant rate, so can be used as a molecular clock to date a common ancestor among related species.