Darwin's Evolutionary Trees

The Museum's library is part of the Darwin Manuscripts Project, which aims to digitize the papers, manuscripts, and correspondence of Charles Darwin. Among these papers are some of Darwin's trees of life—visual representations of how organisms are related— that represent the scientist's brainstorming about his theories.

Charles Darwin published only one tree of life in his lifetime, a foldout that appears in On The Origin of Species. But Darwin drew many trees over his career, including this one for primates, the last one he was known to produce, dated April 21, 1868. Scholars suspect that Darwin, not a skilled draftsman, was likely using his trees as a tool for working out relationships in his own mind—a form of thinking out loud. 

Trees of life like this one helped Darwin refine his ideas visually.
Reproduced with the permission of the Syndics of Cambridge University Library and William Huxley Darwin

“I think it was one step beyond doodling,” says J. David Archibald, author of Aristotle’s Ladder, Darwin’s Tree: The Evolution of Visual Metaphors for Biological Order. “Darwin was a terrible artist, but a wonderful writer. His work led to an explosion of evolutionary trees. Evolution took over the iconography of trees and the non-evolutionists stopped using them.”

Scientists and philosophers have long striven to organize life—and where humans rank within it—into some coherent pattern, from the scala naturae of ancient Greece and Rome to the great chains of being in the Middle Ages to intricate cladograms of today. So what was Darwin getting at with this tree full of scratched over labels and apparent second thoughts?

Evidence has borne out Darwin's early idea that primates like chimpanzees and bonobos were the closest living ancestors to humans.
Courtesy of Wikimedia Commons

Dr. Archibald, a paleomammalogist, speculates that Darwin was sure of the relationships, basing them, as he acknowledges, on the work of other naturalists. He was just not sure how best to illustrate them. In the end, his tree is close to the modern understanding, correctly branching the relative positions of lemurs and New World monkeys, linking humans and apes with Old World monkeys, and identifying humans as a branch of apes. Also, although Darwin did not know, as we do now, that chimps and bonobos are our closest relatives, he did argue that humans’ closest living relatives were gorillas and chimpanzee, and posited that humans probably originated in Africa. Abundant fossil evidence has proven his prediction right.

Much has changed in evolutionary biology since Darwin sketched out this tree three years before publishing The Descent of Man in 1871. Debates have waxed and waned on how best to group organisms, and on what basis. The most commonly used system today is cladistics, which uses shared derived traits to discover how close—or distant—the relationship is between groups of organisms. And the relatively recent revolutions of genome sequencing and supercomputing have both further refined and complicated the process.

Trees of life, or phylogenetic trees, continue to be useful in science today, though they're more complicated than Darwin may have envisioned.
Courtesy of Wikimedia Commons

And yet, the key to building trees of life still lies in the foundation Darwin laid for the fact of evolution itself: “descent with modification by means of natural selection.” As Archibald concludes in his book, “Before Darwin’s On The Origin of Species was published in 1859, evolutionary trees of life were a novelty; after Darwin, they were a necessity.”

Learn about Darwin’s family life in Darwin at Home. To try some of the scientist's experiments yourself, read How to Experiment Like Darwin. For more about the origin of the Darwin Manuscripts Project, read Digitizing Darwin’s Work.

A version of this story originally appeared in the Fall issue of the Member magazine Rotunda.