Evolutionary tree is the pictorial and schematic representation of evolutionary relations among species of living beings in which the trunk (or a preceding branch) represents common ascendants of species and groups of living beings that are distributed along its branches according to hypothesis on their origin. For example, today it is admitted that birds and mammals are two distinct branches of the same preceding reptile branch.
Biology cannot assures a definite evolutionary tree (phylogeny) on the species of living beings that live or have lived in our planet. There are many data to be discovered and many knowledge gaps to be filled. One of the most promising methods to study phylogeny is the comparison of DNA molecules from different groups of living beings researching similarities and differences in nucleotide sequences that may indicate more or less relatedness among species.
