An evolutionary tree is the pictorial and schematic representation of the evolutionary relationship between species of living organisms. Its trunk (or a preceding branch) represents the common ancestors of a species, and groups of living organisms are distributed along its branches according to the hypothesis on their origin. For example, today we acknowledge that birds and mammals are two distinct branches of the same preceding reptile branch.
Biology cannot provide a definitive evolutionary tree (phylogeny) for all species of living organisms that live or have lived on our planet. There is still much information to be discovered and many gaps in knowledge must be filled. One of the most promising methods of studying phylogeny is the comparison of DNA molecules from different groups of living organisms, researching the similarities and differences in nucleotide sequences that may more or less indicate relationships between species.