Trees are used as imagery for many purposes. A Tree of Life shows organisms and their evolutionary relationships.

In July 1837, Charles Darwin sketched the first tree to show evolutionary relationships based on his thoughts about natural selection. This page of his notebook predates publication of the most important book in the history of biology, Origin of Species By Means of Natural Selection by 22 years.

Darwin annotated the tree (letters A-D) with his thoughts about the mechanisms of evolution. At the top of the diagram, he wrote “I think.”

We have been making tree of life diagrams ever since as our understanding of evolutionary relationships among organisms changed. The figure on the right is a recent tree of life showing the current 3 Domains, Bacteria, Archaea, and Eukarya. It appears that the Eukarya, all modern multicellular organisms, arose from the Archaea, and some biologists now use 2 Domains, with the Eukarya nested in the Archeae.

Note: Our Trees 2026 edition returns this afternoon.

Figures: Left, Darwin’s first sketch of a tree of life; Right, a modern tree of life showing three domains, Bacteria, Archaea, Eukarya