The tree of life: with Darwin from Genesis to genomics

Last week a team of scientists published a new “tree of life” in the journal Nature Microbiology. The evolutionary diagram, representing the interconnected family history of all life forms, deviates from earlier ones by granting significantly more space to bacteria, while eukaryotes – including fungi, plants, and animals – are relegated to one slender branch. But why is a multi-coloured, sprawling diagram like this one referred to as a tree in the first place? And why do scientists invoke the biblical image of immortality when they trace the course of evolution?

Trees of life are often misread as chronologies of evolutionary history. However, because scientists cannot travel back in time, they cannot know with certainty how one species evolved into another. Instead they compare extant life forms and fossils to infer how closely related they are. In the past, trees of life were constructed by comparing visible physical characteristics. Following recent advancements in molecular biology, scientists can now calculate organisms’ degrees of affinity by comparing their genomes. The methods have been refined, yet even the most advanced representations remain approximations of life’s history. Trees are not records but models.

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Before the theory of evolution was accepted, family trees were only one of several such models used by naturalists to conceptualize the order of nature, or the way species and other taxa are related to one another. Trees were pitted against maps, networks, and other models. Naturalists typically defended their own choices as “natural”, while branding those of their rivals as “artificial”.

Charles Darwin’s theory of evolution was one of many rival propositions put forward in this debate. Darwin had come to believe that the order of nature was not fixed, but rather a complex web of family relations. In On the Origin of Species, he explained that:

the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical ... the natural system is genealogical in its arrangement, like a pedigree.

Darwin imagined this pedigree as a tree. When he first set out to formulate his evolutionary ideas, in his private “B” notebook of 1837, he famously sketched three tree-like, ascending and branching genealogical diagrams to visualize various aspects of what he was already naming “the tree of life”. Several naturalists before him, including Buffon, Antoine Duchesne, Peter Simon Pallas, and Augustin Augier, had already resorted to the vocabulary of trees and genealogy for classification. But Darwin deviated from earlier naturalists because he imagined one tree for all of life, and because he meant the metaphor to be read literally. Also, no naturalist before him had named his tree after the tree of life in the Bible.

Readers of the Guardian may not know their Bible as well as Darwin’s contemporaries did, yet in Victorian Britain it would have escaped no one that he was naming his tree after the tree in the Garden of Eden. The biblical tree of life is a symbol of immortality and regeneration. It is prominently placed in the Bible, appearing both in Genesis, the first book of the Old Testament, and in Revelation, the last book of the New.

Darwin’s appropriation of the tree of life was no mistake. He had grown up in a devout Unitarian family and knew his Bible well; before he had decided for a career in science, he had planned to become a priest. He also had a more personal relationship to the Edenic tree. When he travelled around the world on board the HMS Beagle, he brought a copy of John Milton’s Paradise Lost wherever he landed. Young and curious, Darwin devoured Milton’s retelling of the creation story, in which the tree of life was granted an even more prominent place than in the Bible. It was only shortly after his return to England, that he first recorded his idea of a family tree of all life, “the tree of life”.

Darwin, who loved poetry and who knew his religious symbolism, apparently enjoyed the pun enough to repeat it twenty-two years later, in On the Origin of Species. By then he had also added a second layer of religious symbolism. In March 1841 Darwin read On Heroes, Hero-Worship, and the Heroic in History, by Thomas Carlyle, a family acquaintance. The Scotsman’s plea for grandeur included a description of the world tree Yggdrasil, the “old Norse view of Nature”. “All Life is figured by them as a Tree”, wrote Carlyle in what could perhaps be seen as a prophetic vision of modern biology and the diagram published last week.

The similarities between Carlyle’s world tree and Darwin’s evolutionary tree were striking. Darwin not only drew on Carlyle’s vision but also on his words: “I find no similitude so true as this of a Tree,” wrote Carlyle in 1841. “I believe this simile largely speaks the truth”, echoed Darwin in 1859.

Darwin had made his genealogical tree more compelling by associating it to religious themes of unity and continuity. Secularization may have undermined our hopes of eternal life in heaven, yet evolutionary theory has enabled us to see a genetic continuation in our descendants, whether humans or not. The tree of life is dead; long live the tree of life.

Petter Hellström is a PhD candidate at the Department of History of Science and Ideas, Uppsala University. For more detail, see Petter Hellström, “Darwin and the tree of life: The roots of the evolutionary tree”.

[This article was edited at 16:06 to clarify the reference to Darwin’s notebook and to update two links to provide better additional source information.]