Charles Darwin’s work fundamentally reshaped the understanding of life on Earth by proposing that all species share a common ancestry. Before publishing his monumental work, he recorded his developing ideas privately in a series of notebooks. The theory of how species change over time began not with a formal essay, but with a humble, hand-drawn diagram. This simple pencil sketch, titled with the handwritten phrase “I think,” is considered the first visual expression of the concept that all organisms are connected through a branching, family-tree-like structure, known today as the Tree of Life.
Historical Setting and the Notebook’s Context
The sketch is found within Darwin’s “Notebook B,” one of several journals dedicated to his private thoughts on the origin of species. He began filling this notebook around July 1837, less than a year after returning from his five-year voyage aboard the HMS Beagle. Observations made during the journey, particularly concerning the geographical distribution and variations in fauna, had caused him to question traditional views of species fixity. The notebook was labeled “Transmutation of Species,” signaling a departure from earlier creationist-based thinking.
The sketch, drawn on page 36, marks an intense period of theoretical development for Darwin. The phrase “I think” written above the diagram captures a moment of revelation, suggesting a powerful insight into the mechanism of life’s diversity. This private notation indicates the nascent state of his theory, showing he was testing a hypothesis rather than presenting a solidified conclusion.
Interpreting the Core Concepts of the Sketch
The diagram is a rough, abstract drawing that illustrates two core concepts of evolution: common descent and extinction. At the base, a single line represents a common ancestor. The lines branch outward and upward, showing how descendants diverge over time into new species. This structure visually demonstrates common descent, where all life forms are genealogically related through a shared history.
The letters A, B, C, and D are placed along the branches, representing hypothetical species or groups existing at a certain point in time. The lines connecting these letters show the degree of relationship, with closer connections indicating a finer gradation or recent shared ancestry. Darwin’s accompanying text notes that between species A and B there is an “immense gap of relation,” while between C and B there is the “finest gradation,” illustrating varying degrees of evolutionary distance.
Some branching lines terminate abruptly and do not reach the top of the diagram, representing extinction. Darwin understood that for new species to arise, older, less-adapted forms must die out. The model accounts for gaps observed in the natural world, explaining that the absence of intermediate forms is due to the failure and disappearance of those lineages. The sketch, therefore, served as a blueprint for understanding the formation of new genera.
Transitioning to the Tree of Life Diagram
The private sketch in Notebook B served as the conceptual foundation for the single, formal illustration included in the 1859 publication, On the Origin of Species. This published diagram, found in Chapter IV, was a more structured and detailed representation of the same evolutionary principles. The formal diagram replaced the simple branching with a complex, rectangular arrangement, using horizontal lines to represent specific intervals of time, often a thousand generations.
While the notebook sketch used only four letters (A, B, C, D), the published diagram used twelve (A through L) to represent a hypothetical genus of species. This shift was necessary to present the concept of descent with modification as scientific evidence to the broader community. The formal illustration was designed to demonstrate how a common ancestor could produce numerous, distinct species and varieties over vast stretches of geological time.
The published diagram was the only illustration in the entire book, emphasizing its role as a visual argument for the theory of natural selection and the resulting “Tree of Life” metaphor. It formalized the idea that the relationships between all organisms are like the branches of a great tree, where the existing species are the green, budding twigs, and the extinct species are the dead and broken branches. The transition from the simple “I think” sketch to the complex, published diagram represents the journey from a private, exploratory thought to a fully developed, publicly asserted scientific theory.