Trees of Life: A Visual History of Evolution by Theodore W. Pietsch (review)

The late Stephen Jay Gould warned us that visual representations of evolution almost always have a hidden agenda, although his point is most relevant for images depicting human origins. Curiously, there are few of these in Pietch’s cornucopia of evolutionary trees, drawn from over three centuries of naturalists’ efforts to represent the relationships between species. His collection is good on the pre-Darwinian period, showing the variety of alternative schemes—maps, circles, and the like—tried out before Darwin consolidated the idea of a tree to represent common descent (related species branch out from a common ancestor). We see that you do not have to be an evolutionist to draw a branching tree, although Pietsch does not notice that most scholars think J.-B. Lamarck’s branching diagram of 1809 was not meant to represent the course of evolution.

Pietsch is a biologist himself and most of his trees are technical representations of how particular groups of living things have evolved. His main interest is in how trees branch, and he concludes with modern cladograms in which the branches represent only degrees of relationship, not evolutionary pathways. From the late nineteenth century he includes Ernst Haeckel’s trees, noting those that run through the whole animal kingdom up to humanity, making us seem the goal of creation. But he says little about the techniques Haeckel used to imply that all the branches leading off the main stem were dead ends, in effect reintroducing a linear chain of being with the “lower” races intermediate between apes and Europeans.

Of the few trees depicting recent human origins, there are two here, by Arthur Keith and Henry Fairfield Osborn, which show the Neanderthals as products of a line quite separate from the one leading to modern humans. This is the once-popular idea of parallel evolution: independent lines driven in the same direction by an inbuilt trend (seen also in Osborn’s trees of mammal evolution, which are more like candelabra with parallel branches). The most extreme version of this theory had the living human races evolving from separate ape species, but there were few visual depictions of this version and none reproduced here. The Darwinian model of common descent is incompatible with this polytypic view, but a study of trees is not necessarily the best way of understanding why. I fear that Pietch’s fascination with branching has blinded him to some of the other issues involved—a case of not seeing the trees for the branches, perhaps.