Tree of life (biology)

The tree of life image that appeared in Darwin's On the Origin of Species, 1859. It was the book's only illustration

Charles Darwin (1809–1882) used the metaphor of a "tree of life" to conceptualize his theory of evolution. In On the Origin of Species (1859) he presented an abstract diagram of a theoretical tree of life for species of an unnamed large genus (see figure). On the horizontal base line hypothetical species within this genus are labelled A – L and are spaced irregularly to indicate how distinct they are from each other, and are above broken lines at various angles suggesting that they have diverged from one or more common ancestors. On the vertical axis divisions labelled I – XIV each represent a thousand generations. From A, diverging lines show branching descent producing new varieties, some of which become extinct, so that after ten thousand generations descendants of A have become distinct new varieties or even sub-species a¹⁰, f¹⁰, and m¹⁰. Similarly, the descendants of I have diversified to become the new varieties w¹⁰ and z¹⁰. The process is extrapolated for a further four thousand generations so that the descendants of A and I become fourteen new species labelled a¹⁴ to z¹⁴. While F has continued for fourteen thousand generations relatively unchanged, species B,C,D,E,G,H,K and L have gone extinct. In Darwin's own words: "Thus the small differences distinguishing varieties of the same species, will steadily tend to increase till they come to equal the greater differences between species of the same genus, or even of distinct genera.".[17] This is a branching pattern with no names given to species, unlike the more linear tree Ernst Haeckel made years later (figure below) which includes the names of species and shows a more linear development from "lower" to "higher" species. In his summary to the section, Darwin put his concept in terms of the metaphor of the tree of life:

Page from Darwin's notebooks around July 1837, showing his first sketch of an evolutionary tree, with the words "I think" at the top

The affinities of all the beings of the same class have sometimes been represented by a great tree. I believe this simile largely speaks the truth. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost 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 from having been found 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 or Lepidosiren, 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.

— Darwin, 1859.[18]

The meaning and importance of Darwin's use of the tree of life metaphor have been extensively discussed by scientists and scholars. Stephen Jay Gould, for one, has argued that Darwin placed the famous passage quoted above "at a crucial spot in his text", where it marked the conclusion of his argument for natural selection, illustrating both the interconnectedness by descent of organisms as well as their success and failure in the history of life.[19] David Penny has written that Darwin did not use the tree of life to describe the relationship between groups of organisms, but to suggest that, as with branches in a living tree, lineages of species competed with and supplanted one another.[20] Petter Hellström has argued that Darwin consciously named his tree after the biblical Tree of Life, as described in Genesis, thus relating his theory to the religious tradition.[21]

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  Haeckel's Stammbaum der Primaten (1860s)
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  Haeckel's tree of life in Generelle Morphologie der Organismen (1866)
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  The tree of life as seen by Haeckel in The Evolution of Man (1879)

Ernst Haeckel (1834–1919) constructed several trees of life. His first sketch (in the 1860s) of his famous tree of life shows "Pithecanthropus alalus" as the ancestor of Homo sapiens. His 1866 tree of life from Generelle Morphologie der Organismen shows three kingdoms: Plantae, Protista and Animalia. His 1879 'Pedigree of Man' was published in The Evolution of Man.