The tree of life is a metaphor describing the relationship of all life on Earth in an evolutionary context. Charles Darwin talks about envisioning evolution as a "tangled bank" in On the Origin of Species; however, the book's sole illustration is of a branched diagram that is very tree-like.
"From the first growth of the tree, many a limb and branch has decayed and dropped off; and these fallen branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only in a fossil state. As we here and there see a thin, straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus (Platypus) or Lepidosiren (South American lungfish), which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever-branching and beautiful ramifications."—Charles Darwin, On the Origin of Species
The evolutionary relationships of the tree of life were refined using genetic data by the American microbiologist Carl Woese, the discoverer of the domain Archaea and a pioneer in molecular (genetic) methods in evolutionary biology. In February 2009, BBC One broadcast an animated, interactive tree of life as part of its "Darwin Season."
The Tree of Life Web Project is an ongoing Internet project containing information about phylogeny and biodiversity, produced by biologists from around the world. Each page contains information about one group of organisms and is organized according to a branched tree-like form, thus showing relationships between organisms and groups of organisms.