This “Tree of Life” sketch is seen in Darwin’s “B” notebook, at a press preview of the new “Darwin” exhibition at the American Museum of Natural History November 15, 2005 in New York City (Image: Mario Tama / Getty)

Read our related editorial: Uprooting Darwin’s tree

IN JULY 1837, Charles Darwin had a flash of inspiration. In his study at his house in London, he turned to a new page in his red leather notebook and wrote, “I think”. Then he drew a spindly sketch of a tree.

As far as we know, this was the first time Darwin toyed with the concept of a “tree of life” to explain the evolutionary relationships between different species. It was to prove a fruitful idea: by the time he published On The Origin of Species 22 years later, Darwin’s spindly tree had grown into a mighty oak. The book contains numerous references to the tree and its only diagram is of a branching structure showing how one species can evolve into many.

“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 tree-of-life concept was absolutely central to Darwin’s thinking, equal in importance to natural selection, according to biologist W. Ford Doolittle of Dalhousie University in Halifax, Nova Scotia, Canada. Without it the theory of evolution would never have happened. The tree also helped carry the day for evolution. Darwin argued successfully that the tree of life was a fact of nature, plain for all to see though in need of explanation. The explanation he came up with was evolution by …