Writing “I think” above his sketch, Darwin likened the evolutionary relationships between species like that of branches on a tree (common ancestry from a central trunk, continued diversity resulting from many new branches forming, extinction when some branches cease). He wrote in On the Origin of Species (1859, p. 130): “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… As buds give rise by growth to fresh buds, and these, if vigorous, branch out and overtop on all sides many a feebler branch, so by generation I believe it has been with the great Tree of Life, which fills with its dead and broken branches the crust of the earth, and covers the surface with its ever branching and beautiful ramifications.” Darwin also included an evolutionary tree in Origin, and in his transmutation notebook reflected on his studies of coral: “The tree of life should perhaps be called the coral of life, base of branches dead; so that passages cannot be seen. — this again offers contradiction to constant succession of germs in progress no only makes it excessively complicated” (Notebook B, p. 25).
The idea of branching as characteristic of relationships of organisms did not begin – nor end – with Darwin. While it has been argued that Darwin’s was indeed the first to show evolutionary relationships, taxonomic and developmental trees appeared long before.
This is the subject of a new book by Theodore W. Pietsch, Professor of Aquatic & Fishery Sciences at the University of Washington and Curator of Fishes at the Burke Museum of Natural History and Culture (both in Seattle). In Trees of Life: A Visual History of Evolution (John Hopkins, 2012), Pietsch shares and provides context for 230 trees of life and similar diagrams (bracketed tables, maps, webs/networks, and other visual representations of the relationships between organisms). They cover the sixteenth century to the present, and range from depictions of single groups of organisms (types of plants, fish, birds, etc.) to larger categories (kingdoms, phyla, or the whole of life on earth). This book is largely, as he notes in the Preface, “a celebration of the manifest beauty, intrinsic interest, and human ingenuity revealed in trees of life through time” (ix).
The book is organized chronologically, while some chapters deviate from this because of how a particular tree(s) fit into Pietsch’s categories. Among categories defined by botany, the rule of five, time periods, cladistics, molecular biology, and universality, some chapters are devoted to the trees of individual scientists: Darwin, Ernst Haeckel, Alfred S. Romer, and William K. Gregory. The chapter entitled “The First Evolutionary Tree” deals with the work of Jean-Baptiste Lamarck. Pietsch, as does Wheelis (2009), claim that Lamarck’s 1809 tree (or, as he called it, “table, serving to demonstrate the origin of the different animals”) from Philosophie zoologique predates Darwin’s as the first representing an evolutionary framework. Others disagree, such as biologist Mark Pallen (in a blog post listed below), but I guess that’s what makes this book such a useful resource. It brings together many examples of images of interest to researchers, and provides a worthy bibliography and set of notes for those needing to look deeper into the sources of these images. Each chapter also includes a several page commentary by Pietsch about the images representing that chapter.
Trees of Life is a beautiful book, and the diversity of beautiful images within its pages should be of interest to historians of science, biologists, folks working at the intersection of science and art, and, honestly, anyone with a genuine interest in science and the study of the natural world. This is a taxonomy of trees of life, if you will.
Resources (all are from 2009, which while being the Darwin bicentenary was also the 200th anniversary of the publication of Lamarck’s 1809 Philosophie zoologique):
Genetic Future: Lamarck beat Darwin to the tree
The Rough Guide to Evolution (Mark Pallen): Lamarck, Darwin and the Tree of Life
J. David Archibald: “Edward Hitchcock’s Pre-Darwinian (1840) ‘Tree of Life’” (pdf)
KPBS: Perceptions of the Tree of Life Before and After Darwin (with Archibald)
Mark A. Ragan: “Trees and networks before and after Darwin” (pdf)
Mark Wheelis: Darwin: Not the First to Sketch a Tree