Binomial nomenclature provides a two-part naming system for species, with the first word being the genus and the second being the specific epithet. Linnaeus established a seven-level hierarchical system of classification including kingdom, phylum, class, order, family, genus, and species. Cladograms and molecular clocks use DNA comparisons to represent evolutionary relationships and time since divergence between organisms. The three domains of life are Bacteria, Archaea, and Eukarya, with Bacteria and Archaea being prokaryotic and unicellular, and Eukarya containing multicellular eukaryotes including plants, animals, fungi, and most protists.