Why Are Some Evolutionary Trees in Natural History Museums Prone to Being Misinterpreted?

Amazing Varieties

Visual representations of evolutionary trees can be amazingly varied. Regarding their form, they can differ in shape, design, and orientation. The crucial evolutionary information of a tree is given by the branching order. As long as it remains the same, diagonal, curved, straight, or circular lines can be used (Gregory 2008). Trees may be oriented up or down, to the left or to the right, depending on the position of the root or point of origin. They also can be symmetrical (or balanced) if the number of species at each side of the nodes is the same, or asymmetrical (unbalanced) when not all of the groups represented contain the same number of species. For example, placental mammals are richer in species than monotremes (Crisp and Cook 2005).

Concerning the representation of organisms, evolutionary trees may represent the hypothetical relationships among the entire biodiversity or among members of a kingdom such as the animals. They can be about phyla such as the mollusks or a class such as that of the cartilaginous fishes or chondrichthyes. Orders like primates, families like composite plants, and molecules such as DNA or hemoglobin can also be represented (when using molecular data, scientists use modelling approaches like maximum likelihood and Bayesian methods (Tassy 2011)).

Regarding the method used to build them and the kind of information conveyed, evolutionary trees can be cladograms when composed by species and speciation events and constructed based on cladistic analysis. They can be phylograms when presenting non-equal branch lengths to convey the inferred degree of relatedness among the groups presented. Or they can belong to other types of phylogenies (non-cladistic) if they include different components of the evolutionary history of organisms such as estimates of diversity, enrichment or depletion of species, absolute time, biogeographical relations, and the direction of evolutionary change, in addition to genealogy. These factors provide richer information about organisms than a cladogram sensu stricto. Museums extensively use these representations.

From all this startling variety of representations of the evolution of lineages, we are mostly concerned with “traditional trees of life,” mainly because many of these images are far from Darwinian (moreover, the shape of a real “tree” can unwittingly misrepresent evolutionary relationships) as well as modern images whose design might inadvertently lead to non-scientific notions.

Where Did the Image of the “Tree of Life” Come From, How Did It Become so Widespread, and What Kind of Notions Does It Convey?

One of the most fundamental concepts of Charles Darwin’s argument was that species share a common origin from a single ancestor and have subsequently diverged through time. In the fourth chapter of the Origin, Darwin included a picture that he used as an argument and as an evidence of this notion. The resulting line-diagram is an abstract and irregular design, not a tree in a literal sense and not described by Darwin as such. In his subsequent books, Darwin never included a depiction of this kind. Then, why is it that, for popular accounts of evolution, the chief image in use from late nineteenth century to represent the process is that of a tree?

Darwin came to use the metaphor of a great tree to illustrate his notion of descent with modification as early as 1837 in his Notebook “B”. There, he describes an abstract tree with a main trunk to represent links of ancestry for every species and side branches which ramify further or die out to represent species divergence and extinction. Therefore, to many, the “tree of life” has its roots in one of Charles Darwin’s diagrams and thus represents one of his legacies. “Charles Darwin sketched the branches of a tree in one of his notebooks to illustrate the idea that all life forms branched out from a common ancestry. Since then, the tree of life image has persisted as an aid to understanding evolution”².

Yet history tells us that Darwin’s image was not the one in everybody’s mind until the middle of the twentieth century, as it was only in 1960 that his personal notes began to be printed.

It was Ernst Haeckel, a German naturalist, who published in 1874—in his book Anthropogenie—the image that became iconic of evolutionary theory in the popular realm (Fig. 1). Haeckel’s now famous motif—Systematischer Stammbaum des Menschen—explores the evolutionary history of man. The most fascinating aspect of this image is that it is a real tree—a twisted European Oak with bark and leaves. Popularly, it is assumed to represent an upward journey of life from the most primitive beings at the bottom to the most advanced at the top—man at the summit of all life—even though, to Haeckel, it was a direct chain of human ancestors.

Thus, Haeckel’s Stammbaum des Menschen is the source image for most of the subsequent representations of evolutionary trees in the popular sphere. Not Darwin’s line-diagram of the Origin—in which no single species at the end of the lines can count as a clear culmination—and especially not Darwin’s 1837 ink sketch—which many scholars have described as Darwin’s first evolutionary tree.

In the popular realm, Haeckel’s tree took over Darwin’s 1859 descent diagram. But it was taken out of context. It was assumed to represent a complete evolutionary tree—therefore the popular term “Tree of life.” From this approach, Haeckel’s Stammbaum des Menschen is clearly embedded with non-Darwinian accounts of evolution and also prejudices assumed to be inspired by the prevailing social and philosophical beliefs. The image of a real tree mightily growing upwards offered a view of evolution as a progressive and goal-oriented process that culminates in the appearance of human kind. Moreover, the conflation of religious connotations powerfully embedded in the tree image led to interpretations that modern living beings can be less and more evolved and that the whole process was intended to “produce” human beings.

In the early twentieth century, due to the continuing effort to promote the idea of evolution in layman, evolutionary trees inspired by Haeckel’s started to appear in works aimed at a more popular audience, and their inclusion eventually became standard in biology textbooks (Figs. 2 and 3). Some natural history museums also used them as a new way to explain the organic world through the lens of evolution (Fig. 4). Thus, at least in their beginning, evolutionary trees in the popular sphere were not just embodiments of interpretations of the theory of evolution. They enfolded deeply embedded Western preconceptions of how the world was thought to be according to religious views and other pre-evolutionary views of organic relationships.

By the turn of the century, natural history museums witnessed great changes. For many, a crucial event was the celebration of Darwin’s bicentenary in 2009. Several brand new exhibits of evolution were opened, and outdated ones were overhauled. Today, evolutionary theory is at the heart of most of them. Thus, scientific illustrators and museum displays are making real efforts to change the public perception of evolution. As the metaphor of branching and rebranching to represent the complexity of the natural world proved to be incredibly fecund for practicing biologists and for the general public, in order to distance evolutionary diagrams based on scientific evidence from traditional “tree of life” connotations, the use of cladograms became a common practice from the decade of 1990, and alternative geometries are beginning to appear.

However, even though today it is difficult to find a traditional “tree of life” (inspired by Haeckel’s) in natural history museums, they are still present. Also, the design of many evolutionary trees is still prone to confuse the audience. Below, we revise the most fundamental misunderstandings about trees. Some of them are caused by a lack of phylogenetic literacy, but others are a direct consequence of the design of evolutionary trees (shape, orientation, and organisms presented). Later, we will discuss our findings in five important Western natural history museums.

Problems of Interpretation

Being the direct representation of evolution, evolutionary trees or phylogenies are a central element of modern biology. Tree thinking (the ability to understand evolution as a process of branching and rebranching) among the public in general, however, is not as accepted and widespread as one might expect. “Tree thinking is very much an acquired ability which needs extensive training” (Sandvik 2008), and even graduate students and professors of biology find it difficult to correctly interpret a simple tree drawing (Baum et al. 2005; Bishop and Anderson 1986; Diamond and Scotchmoor 2006; Meir et al. 2007a). There are at least three reasons for this absence of tree thinking: a general misunderstanding of the theory of evolution; a lack of familiarity with phylogenies; and a visual evolutionary culture which sustains an eminently progressive discourse full of prejudices that leads to wrong interpretations of the process (i.e., the widespread presence of traditional “trees of life”).

What Should Be Expected from an Evolutionary Tree in a Museum?

As stated before, tree thinking cannot be taken for granted, and some phylogenetic literacy is needed to derive a proper understanding of evolutionary trees. The role of an evolutionary tree in museums should be (apart from showing the evolutionary relations among sets of species per se) to serve as a tool to help people understand the connections between species that are being proposed, that is, to enable the teaching of the basics in phylogenetic literacy, by taking into consideration that some visual embodiments are prone to reinforce misunderstandings.

Museums should carefully design their evolutionary trees. They should consider the common preconceptions visitors often bring, many of them as a direct result of more than a 100 years of visual culture which has reinforced the notion that evolution is oriented from simple toward complex organisms—communicating the idea of a single ladder of life amidst the extraordinary diversity of organisms—and that humans are at the pinnacle of the evolutionary story. These preconceptions may also be reinforced by the form, shape, orientation, or the group of species shown in modern phylogenies.

In this context, the authors decided to determine by observation if graphic elements within evolutionary trees in some natural history museums are more likely to reinforce rather than amend misinterpretations.

Our Study

There is a previous important study conducted by Teresa E. MacDonald (2010) which explored the use of evolutionary trees in exhibits across 207 informal science institutions and analyzed the form and content of 185 trees. This study was carried out in settings from English-speaking countries (mainly the U.S., UK, Australia, Canada, and New Zealand).

Due to the comprehensive nature of McDonald’s study, the authors decided to focus on five important western natural history museums: Museum für Naturkunde, Berlin; British Natural History Museum, London (Our place in evolution exhibit); American Museum of Natural History, New York (Hall of human origins and Hall of biodiversity); Museum d’histoire naturelle, Paris (Grand Gallery of Evolution and Gallery of Compared Anatomy and Paleontology), and Museo de Historia Natural, México City (Evolution exhibit). Each museum was chosen according to its importance for the establishment of scientific groups, its leadership in museographic techniques and its relevance in education and research locally and globally.

The authors also looked for traditional “Trees of Life.” These are contemporary representations of Haeckel’s tree of 1874. These sorts of trees show evolution as a progression from simple to complex with a distinctively teleological perspective (Bishop and Anderson 1986).

Taking the previously described aspects into consideration, out of the nearly 300 evolutionary trees present in the studied museums (∼270 in the AMNH; ∼10 in the natural history museum of Berlin; five in the natural history museum of Mexico City; six in Our place in evolution Galley at the natural history museum of London, and two in the Great Gallery of Evolution in Paris), we localized 13 evolutionary trees which might be particularly problematic (see Appendix) and three images that are useful to overcome the pitfalls previously described (see Appendix).

An Overview

The British Natural History Museum and the American Museum of Natural History’s evolutionary exhibits rest upon phylogenetic analyses, therefore the main theme for these evolution exhibits is systematics. Cladism is the backbone of New York exhibits, and cladograms are abundant in London in Our place in evolution, which—by means of such trees—explores the characteristics humans share with chimpanzees and gorillas and how humans are related to other apes, living and extinct.

For the Great Gallery of Evolution of Paris, what is important is to show the result of evolution, that is, the biological diversity of our planet, as well as the evolutionary mechanisms behind it. The museum opted to present a universal phylogeny in the form of the three domains proposed by Carl Woese. The main purpose of this tree is to enable visitors to understand that all life is related to a universal ancestor and to give perspective to the relative importance of different organisms. One can see that animals are just a tiny twig recently evolved compared with other eukaryotes.

The Museum of Natural History in Berlin houses one of the most modern evolution galleries of all. Evolution in action opened in July 2007, and it is influenced by current trends in evolutionary science and by contemporary styles of exhibit design. Employing stuffed specimens, interactive and virtual exhibits, fossils, and so forth; it engages visitors with the mechanisms of evolution.

Germany was the birthplace of the famous scientist and illustrator Ernst Haeckel who popularized the image of the tree of life as the iconic image for evolution. Possibly that is the reason behind the numerous evolutionary trees that can be found in this museum, one of which is especially interesting as it employs high technology to present an interactive “complete”³ tree of life with the information made available by the Tree of Life Project Organization.

The Museum of Natural History of México City has a few evolutionary trees, none of which is accurate as they were conceived as merely decorative. Surprisingly, there is a “march of progress” depicting the evolution of horses even though it is an outdated image which promotes several misconceptions such as the notion of progress in evolution and a hierarchy from simple to complex forms in terms of less and more evolved.