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Who created the first Tree of Life?
Comparing trees of Hitchcock, Darwin and Haeckel
by Gert Korthof first published 12 February 2017
IntroductionDarwin introduced the Tree of Life as metaphor for evolution in The Origin of Species in 1859. Surprisingly, Darwin did not produce a Tree of Life with species names or geological strata in later works. Geologist Edward Hitchcock produced a 'paleontological chart' with names of fossil plant an animal groups against a background of geological strata in 1840. Ernst Haeckel produced many trees of Life with and without geological and paleontological data. Who produced the first real tree of life? Is a tree a good metaphor for evolution anyway?
DarwinApart from the famous sketch in his notebook, the only graphic Darwin produced in the Origin of Species is an abstract schema:
This drawing is an abstract illustration of the origin of species, subspecies and varieties, including extinction and stasis. On the vertical axis generations of organisms are indicated by horizontal lines (I - XIV). They are not geological periods, because there are no names, but they certainly are chronological. On the bottom the oldest, on top the most recent, just as a tree grows bottom up. One could call this a Tree of Life, but Darwin's intention is not to show specific plant and animal groups, but only the principle of speciation.
German historians of science Uwe Hossfeld & Georgy S. Levit [1,2] claim that 150 years ago the German biologist Ernst Haeckel (1834–1919) published the first real tree of life. To prove their point they presented this illustration:
It shows three kingdoms Plantae, Protista and Animalia being derived from a common origin (monophyletic tree). Additionally, all other main groups within those kingdoms are included in the tree. Remarkably, Darwin never produced such a tree, although it is the core of his Descent with Modification theory. Haeckel produced many Trees of Life. The most famous is perhaps the 'oak tree':
This figure shows Haeckel's famous pedigree of the descent of man. The human species is immodestly situated at the top and is descended via many intermediate forms from single-celled organisms (Monera) at the base of the tree. In the right margin there are no geological strata as expected, but the names of animal groups. Because of this, the branches of fish, birds, insects and single-celled organisms do not end at the top: as if they were all extinct. This is very similar to the non-evolutionary Great chain of being, a ladder on which life 'climbs up' from primitive to highly complex. The human species at the top. The rest of life stuck somewhere halfway. In Haeckel's tree the 'rungs of the ladder' consist of primitive Protozoa, Invertebrates, Vertebrates, Mammals, Apes and Man.
Surprisingly, the famous Tree of Life of Ernst Haeckel looks suspiciously similar to the non-evolutionary Medieval ladder, despite Haeckel's intention to show an evolutionary tree! The main reason that his tree looks like the Great Chain of Being is that geological strata are absent and life forms are presented as a hierarchy on top of each other without overlap. They are sharply separated by horizontal lines.
The original Great Chain of Being is a static ladder showing a fixed hierarchy of life forms. No species really climbs up. Haeckel transformed a static ladder into a hierarchical tree to portray a dynamic process in time. Furthermore, he has given his 'ladder' branches. Not surprisingly, those branches stay within the borders of the group (within the horizontal lines). The pedigree of Haeckel is actually a transitional form of 'The Great Chain of Being' and an evolutionary tree.
Aside: interesting is that in this figure birds are a branch of reptiles. So birds descended from reptiles. A modern insight!
Such a lifelike (botanical) tree cannot –by definition– be chronologically correct. Because then all branches should end at the top.
However, Haeckel did produce a tree with a geological time scale (shown left in Figure 5). In this case, a family tree of vertebrates:
In this figure, the whole idea that it should look like a tree has been abandoned. We see fish, amphibians, reptiles, birds and mammals ending at the top. Just as it should be for extant (that is living) species. Although it is a major improvement, this figure is not consistent in geological dating. For example, Marsupials (marsupials: kangaroo, etc) are not extinct. This figure suggest they are extinct. Well, that we condone. In fact, Darwin never made this kind of pedigrees. Not even in later editions of his Origin or other works as far as I know. This is actually quite striking. Darwin, had no drawing skills? or no interest? Haeckel produced thus both types of 'trees'.
There is one exception to my claim that a life-like tree can never be a proper geological tree:
The horizontal lines (from Fig. 2) are transformed into curved lines (fig. 6). Here's how: when you connect the ends of all branches of a real tree with a line you get a semi-circle. If you do that for several subsequent years, you get a number of concentric circles. This way you can combine a real tree figure with proper geological periods such as: Pleistocene, Eocene, Jurassic, Permian. All species on top are extant species. The root of the tree is the oldest. Extant and extinct species and branching are indicated correctly. So both family relationships as chronological geological information is present. In this illustration for lizards, but that can be done for all life. It provides an attractive and informative figure. Regrettably, one encouters it rarely.
Edward HitchcockBack to 'the first real tree of life'. Nineteen years before the publication of the first edition of the Origin of Species, the American geologist Edward Hitchcock (1793-1864) published the first 'tree of life' in his Elementary Geology (1840) . Hitchcock produced a figure with two trees: one tree for plants and one tree for animals. The trees contain names of plant and animal groups. Furthermore, he divided the vertical axis in 7 geological periods. This tree of life is the earliest known version that incorporates paleontological and geological information.
Hitchcock's Tree is based on fossils in geological strata. No wonder: he was a geologist! Furthermore, there is continuity of the oldest to contemporary life forms; there are (some) branches and extinctions. New side branches represent new species. All of this is remarkable considering that Hitchcock published this in 1840 and could not know Darwin's theory of evolution! What he did was just plugging in fossils in a geological time scale. And then something like a Tree of Life appears. No theory could influence him. Just data.
When comparing Hitchcock's (1840) Tree with that of Haeckel (1866) (we can't compare it with Darwin because he did not publish a detailed tree), we see differences:
ConclusionThe good thing about the tree metaphor is that it is very good at illustrating branching, and family relationships, and in a rough way the relative times of origin of groups. But the disadvantage is that it is visually difficult to combine with the geological time scale. You cannot easily fit in both extinct and living plants and animals without distorting the tree image. The tree does not give complete freedom to let many branches end at the top. For example, bacteria originate early in evolution and still exist, but are difficult to incorporate into the tree metaphor because the branch should be necessary running from the base to the top. All so-called 'primitive' groups like sponges, worms, algae started early in evolution but are not extinct. So, they all must end at the top in the geological time-line. It is crowded at the top anyway. And all extinct dinosaurs (for example Tyrannosaurus Rex), must end somewhere halfway the time-line.
The modern 'trees of life' in figure 9 do not look like real trees. They include the three main groups of life on earth. In the revised version, you see something never seen in real trees: branches grow together (blue lines). Note that animals (in red) a unremarkable, small twig forms at the end of a long branch. This is a stark contrast with the anthropocentric trees of the late 19th and early 20th century!
So, there are actually three types of Tree of Life, and four if you count the hybrid form:
"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. ... " .Especially intriguing is the expression "have sometimes been represented by a great tree" because it implies that Darwin knows of others who used the tree metaphore. Amazingly, he does not tell us who!
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