I’ve defended Charles Darwin’s metaphorical Tree of Life previously; now I shall celebrate his vision by showing how it developed over time and why it is still relevant today.
Darwin’s first inkling (1837)
Charles Darwin had been back less than a year from his tour on the HMS Beagle and he had been thinking furiously about the paradoxical diversity and commonality of life when he had a flash of insight. He reached for his “B” notebook and scribbled this sketch showing species branching and sub-branching from each other. This is one of the most famous images in science and a popular tattoo among scientists. It does not, however, look a great deal like a tree; it looks like a leftover grape stalk. Darwin does not use the word “tree” or “branch” in his notebook; instead he refers to “gradation” and “greater distinction.” What is most important about this sketch is often edited out of images and tattoos. The large letters at the very top read, “I think”: simultaneously a recognition of his own uncertainty and an expression of his way of working through problems. Darwin was 28 years old.
The Origin of Species (1859)
By the time Darwin came to publish The Origin of Species, he had been thinking about evolution for a further 22 years. The concept of speciation was more than just a cluster of splitting lines, it was a fully grown Tree Metaphor. The image above is the only diagram in the first edition of Origin. Instead of being a map of branches, Darwin has now added time as an axis to his map so that the early life forms begin at the bottom and move through time towards the top, branching as they go. Each horizontal line represents “a thousand generations; but it would have been better if each had represented ten thousand generations.” The diagram has also expanded its ambition to include extinctions and evolutionary cul-de-sacs. The diagram still does not look much like a tree. River weeds, maybe.
In the text, though, Darwin waxes prolix on the concept of the Tree of Life: “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.” What follows is one of his most memorable passages.
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.
Beautiful ramifications, indeed. Gorgeous writing, and also very precise. From the diagram and from the text Darwin was very careful to neither imply nor refute a universal common ancestor. The Tree of Life referred to “all the beings of the same class” and not to all life on Earth. There might be one Tree or several. Darwin could not know and does not proffer an answer. Darwin wanted the reader to be very, very clear that the Tree of Life is a metaphor. In one sentence he calls it a “representation” and in the next a “simile.”
Ernst Haeckel’s flamboyant Tree of Life (1866)
Darwin wrote Origin of Species for other naturalists. Popularising the theory of evolution fell to others, possibly because Darwin’s gentle disposition did not lend itself to the rough-and-tumble of public debate where more combative personalities such as Thomas Huxley were to shine. Among the great popularisers was Ernst Haeckel, a German biologist who is best remembered today for his artworks. Haeckel’s illustrations are still considered among the most beautiful scientific drawings in history. Wikipedia hosts a small sample of his prolific output. In 1866, Haeckel wrote the first great popular book explaining evolution, and it includes this image of the Tree of Life:
Unlike Darwin’s stremlined diagram, Haeckel created a work of art. In the process, Haeckel lost two important features of Darwin’s Tree. First, Haeckel implied a universal common ancestor. Today we have good reason to hypothesise a universal common ancestor or at least a set of universal common ancestors, but most of the evidence upon which this rests was unimaginable in Haeckel’s time. Haeckel guessed right, but it was still a guess that he should not have dressed up as a certainty. Second, Haeckel abandoned the depiction of extinctions. Where Darwin laid out a number of dead branches on his evolutionary Tree, Haeckel ignores them altogether. What appear to be dead ends are not extinctions at all — those dead ends include molluscs and lichens and amphibia, creatures very much alive today. In fact, the dead ends don’t represent anything at all, they are simply the artist’s way of squeezing a lot of branches inside a rectangular border.
Despite Haeckel’s great contributions to art and science, his legacy is controversial today. He is the creator of a famous illustration of embryological development that his detractors consider fraudulent and even his most admiring apologists consider sloppy. Even harder to swallow, Haeckel came to be one of the foremost exponents of polygenism, a particularly revolting pseudoscientific formulation of racism that was directly at odds with everything Darwin stood for.
Haeckel’s Grand Oak (1879)
Thirteen years later, Haeckel returned to the Tree of Life in a book called The Evolution of Man. The Tree, as you can see, has flourished. It has grown from a bush into a massive oak that could have come from the deepest glades of the great forests of Europe. As art it is more awe-inspiring than Haeckel’s straggly bush of 1866, but as science it is even more flawed. The extinctions are still missing from the picture and the unwarranted assumption of a universal common ancestor persists; compounding his errors Haeckel has implied a heirarchical structure to the tree with humans, sorry (CAPS-ON) “MAN,” at the top. We even get a box around us to remind us how important we are.
According to Haeckel, the mammalian section of the tree grew out of the trunk marked Amphibia while the reptiles branch off to the side. Actually, mammals and reptiles diverged from common ancestors called the mammal-like reptiles. Humans, snakes, ostriches, as well as the long-gone dinosauria: we are all amniota, but amphibians are not. As errors go this is forgiveable given the lack of information available to Haeckel at the time, but the way he has gone about his error shows his bias towards humanity. Amphibia get their own massive-girthed trunk while the reptiles and birds get a scrawny branch, all because Haeckel didn’t intuit the correct sequence of divergences that lead to nature’s crowning glory (Western European human males, in case you hadn’t guessed).
Against the Tree of Life
The problem with Darwin’s metaphorical Tree of Life is that he derived it from the small subset of living creatures that were available to his observations. For animals, plants, fungi, and other macroscopic creatures, the Tree of Life is a very good representation of the sequence of divergences and extinctions that make up their history. But life goes back a long way before animals and plants and fungi. The oldest fossils known in Darwin’s time came from the Cambrian era, so Darwin only had access to the last sixth of life’s history.
When we look at our smaller cousins, the ones we need microscopes to observe, we notice that they do not reproduce as we do. Bacteria freely swap clusters of genes between each other. Scientists call this sexual transmission, but it is nothing like sexual reproduction in the animal and plant kingdoms. Bacteria even swap these gene clusters between individuals of completely different orders. Unlike mammals, who can only spread their genes into the next generation and only by mating with mammals of the same species, bacteria live in a massive orgy of DNA-mixing that makes the 1970s look like a church fundraiser (unless, that is, your local church is a pentecostalist megachurch). Some biologists even question whether we should use the term “species” when describing bacteria.
As a result, the Tree of Life for micro-organisms such as bacteria and archeans is a swirl of interlocking pathways. In the more extreme cases, not only do organisms swap packets of DNA, but one organism colonises another and comes to live permanently in a state of symbiosis with its host. The early microbes that swalled chloroplasts and mitochondria became the true bacteria. Those that became nucleated and swallowed mitochondria became the eukaryotes — that’s us, by the way, along with plants and fungi and protists.
One of the most vocal advocates of this view if life is W. Ford Doolittle. His map of evolution looks more like a mesh than a tree. He calls it the web of life.
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There is little doubt that Doolittle’s web is a much better representation of single-celled evolution than Haeckel’s oak. It is also very up-to-date. It really does represent the best current knowledge. I think he has overplayed his hand by assuming the deep roots of the web are a “Common Ancestral Community of Primitive Cells.” It’s not that I disagree with Doolittle. I think a community of primitive cells is a vastly more likely progenitor of life than a single species, but we know so little about that phase of life that I think it is presumptious to map it as factual.
Resurrecting the Tree
Given that Darwin’s Tree is an excellent metaphor for large multicellular life forms and Doolittle’s Web is an excellent metaphor for single-celled organisms, how can we choose between the two? Well actually, we don’t have to. There are many types of tree besides imposing oaks. Here are two common trees that can act as metaphors for both the webbiness and the branchiness of life’s history.
The banyan tree, as illustrated in Nature over 200 years ago.
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The base of a strangler fig (personal photograph). The upper reaches of mature strangler figs branch just like other trees.
Darwin’s Tree of Life is not entirely outmoded; it still has utility provided one adapts it to meet new evidence and one recalls Darwin’s original intent. The Tree of Life is a metaphor.
