A method of tracing evolutionary lines based on the changes in the protein structure and DNA of living organisms that take place over long periods of time. By establishing the degree of difference between the proteins of two species, it is possible to calculate how long ago they shared a common ancestor. Studies of molecular evolution rates have led to the proposition that macromolecules may serve as evolutionary clocks. If the rate of evolution of a protein or gene were approximately the same in the evolutionary lineages leading to different species, proteins and DNA sequences would provide a molecular clock of evolution. The sequences could then be used not only to reconstruct the topology of a phylogeny (the sequence of branching events) but also the time when the various events occurred.