Ernst Haeckel is attributed to be the first to describe the evolutionary relationships among living organisms, a geneology of life, as analogous to a tree. Haeckel, interestingly, described all living things as falling into not just two kingdoms (plants and animals), but also a third kingdom for microorgansims — the Protista. There are several other interesting features of this vision — for example, Haeckel postulated a common origin for all life (plants, animals and microbes). This is still a common assumption, but it makes sense with modern molecular evidence as well. From here.