The past 150 years have seen a gradual reduction in the anthropocentricism in biological systematics. Darwin (1859) removed the basis of beliefs that evolution is a directed process, and that Homo sapiens was the goal of evolution (or creation). The Darwinian mode of evolution is also incompatible with the idea that extant species can stand in ancestor–descendant relations to each other. Instead, extant species share common ancestors, from which they all descend. At this stage, the two first narrative devices lost their justification. Of course, this did not prevent them from turning up in a variety of scientific accounts through the decades (e.g., Romer 1959). By now, however, such distortions of evolutionary history are largely restricted to the popular domain.
The second milestone was the “cladistic revolution” (Halstead 1978), started by Hennig (1950). His “phylogenetic systematics” identified monophyly as a desirable property of systematic entities. By doing so, Hennig removed human opinion and authoritative decisions from evolutionary biology. The phylogenetic (or cladistic) way of systemising threw out the arbitrariness of classification and replaced it by the scientific rigor of phylogenetics. It might not be intuitively clear why this should be relevant for anthropocentricism. The relevance lies in the fact that there is not a single paraphyletic taxon that incorporates the human species. On the other hand, quite many paraphyletic groupings denote taxa that would have been monophyletic if they had not been erected to the exclusion of a taxon containing man. Obviously, one unwritten rule for the erection of paraphyla was that they must not contain Homo sapiens. Examples of such paraphyla are “Pongidae” (= Hominidae sensu lato less Homo), “Prosimii” (= Primates less Anthropoidea), “Marsupionta” (= Mammalia less Placentalia), “Reptilia” (= Amniota less Aves and [crown-]Mammalia), “Anamnia” (Vertebrata less Amniota), “Osteichthyes” (= Osteognathostomata less Tetrapoda), “Invertebrata” (Metazoa less Vertebrata), “Coelenterata” (Eumetazoa less Bilateria), “Protoctista” (Eukaryota less multicellular organisms), “Prokaryota” (Vita less Eukaryota). Inventing paraphyletic taxa that include our species may be a useful and funny exercise. Such taxa, like O’Hara’s (1992, p. 154) “Anarthropoda”, are so obviously absurd that no evolutionary taxonomist would ever have adopted them, no matter how large the phenetic gap between two sister taxa. When the phenetic gap that isolates tetrapods from lungfishes is large enough to justify the erection of a paraphyletic “Osteichthyes”—why doesn’t the same apply to other taxa with many and very distinct autapomorphies, like the echinoderms and cnidarians? However, I suspect it is unthinkable even for evolutionary taxonomists to subdivide the Bilateria into the two superphyla “Anechinodermata” (= Chordata + Enteropneusta + Pterobranchia + Protostomia + Acoelomorpha) and Echinodermata, or to subdivide the Metazoa into the two subkingdoms “Acnidaria” (= Bilateria + Ctenophora + Porifera + Trichoplax adhaerens) and Cnidaria.
One might have thought that with the acceptance of Darwinian evolution and cladistic methodology, anthropocentricism was finally banned from evolutionary narratives, at least in scientific publications. This was the background for limiting my case study to textbooks that explicitly adopted a cladistic perspective. As one aim of phylogenetic systematists is to present phylogenetic history in a way that is independent of the opinion of authorities, one might expect that they are more conscious about avoiding anthropocentricisms than the average author of evolutionary textbooks. However, as my results show, this is merely wishful thinking. Two distorting devices can still occur even in state-of-the-art phylogenetic cladograms, and at least one of them is prevalent in textbooks: the taxon containing Homo sapiens is quite consistently placed at the rightmost position of each cladogram. There was also a trend that the resolution of cladogram branches is biased in favour of our own species (p < 0.10).
These two narrative devices—i.e., O’Hara’s fourth one and the one proposed by me—can be illustrated with Fig. 2. All trees in this figure are cladograms of the same taxon, viz. jawed vertebrates (Gnathostomata). Figure 2a is perfectly balanced, thus having eliminated device no. 4, but the taxon containing man (Choanata) is placed at the right, thus using device no. 5. The balance of the tree gives as much emphasis to “side branches” as to the branch leading to Homo sapiens and other Choanata. On the other hand, this branch is emphasized more than the others by placing it in the upper right corner, thus implying that one “has to read the graph” towards Choanata.
Figure 2b is attempting to distribute taxa along the left–right axis in an objective way, thus eliminating device no. 5, but it is extremely unbalanced, thus using device no. 4. Every taxon’s relative place was determined according to the number of described extant species within the taxon. This places the taxon containing man, Theria, somewhere in the middle of the tree. On the other hand, all taxa not including man are collapsed into only one single terminal taxon, resulting in a perfectly comb-like tree topology, again at the disadvantage of non-human taxa. Of course, it is possible to combine the advantages of both cladograms in one figure, just turning some branches of Fig. 2a around their own axes, the result of which is shown in Fig. 2c.
Figure 2d shows a fourth out of the over 36,000 possibilities to depict gnathostome phylogeny using 8 tips. In this last illustration, the cladogram is maximally unbalanced, but this time to the advantage of the beluga or European sturgeon (Huso huso) and to the disadvantage of Sarcopterygii (including Homo sapiens) and five other side branches. Such presentations are quite uncommon, even though ichthyocentric cladograms are as justified as anthropocentric cladograms (but see Scott (1986) for a papiliocentric tree of life).
Finally, Fig. 3 shows an attempt of displaying the phylogeny of Gnathostomata as balanced as possible, resolving all branches to the same degree as the therian branch in Fig. 2a. Figure 3 also illustrates that there are natural constraints to perfectly balanced trees: the tree of life is not itself perfectly balanced (Guyer and Slowinski 1991; Mooers 1995; Mooers and Heard 2002), and several of the branches in Fig. 3 cannot be shown with higher resolution because they represent single species. This is why this tree does not have 27 = 128 tips but merely 76.
One might argue that the use of these narrative devices has certain advantages, i.e., that anthropocentric cladograms are not only negative. As regards device no. 4, using balanced trees such as Fig. 3 needs far more space for representing the same bunch of the tree than comb-like trees such as Fig. 2b. This depends of course on the aim of the cladogram. When one wants to tell the history of man (or any other taxon), and nothing else, comb-like trees are, of course, perfectly suited. However, in zoology textbooks this is normally not the intention, instead the reader is expected to get an impression of zoological diversity. This may be better achieved by using balanced trees. What is otherwise suggested is that there is no “relevant” or “interesting” diversity in “side branches”.
It might also be argued that, because the tree of life itself is not balanced (cf. Fig. 3), cladogram balance is not a desirable attribute in the first place. In most cases, however, cladogram resolution is not constrained by the topology of the tree of life. Moreover, even in the cases where it is, it may be very useful heuristically to visualise that some tips in fact are species or species-poor taxa. This makes readers of the cladogram understand that biodiversity is indeed not distributed uniformly across the tree of life (as implicitly suggested by the straitjacket of Linnean categories, see below).
A further problem of balanced trees, also pointed out by O’Hara (1992, p. 150f), is that there are certain well-known taxon names and that these are not equally distributed among taxa with the same rank.Footnote 2 As illustrated by Fig. 2a, very few of the names appearing at the tips of the cladogram will be known to a reader who is not already familiar with vertebrate phylogeny. That Choanata includes lungfishes (Dipnoi) and terrestrial vertebrates (Tetrapoda) is only known to specialists (many of whom would not even use the scientific name Choanata, but prefer other names such as Rhipidistia sensu lato). Likewise, that Callorhinchus and Chimaeroidea together constitute the chimaeras (Holocephalii), or that Galea and Squalea together represent the rays and sharks (Elasmobranchii), will not be visible from the cladogram, unless names of higher taxa are displayed above the tips (as is shown in Fig. 2a, but not Fig. 2b–d). In such cases, perfect tree balance does not necessarily seem to outweigh the loss of information. A compromise would be to display one more branching in the rightmost branch of Fig. 2a, and to add names of higher taxa above the names of the tips.
As regards device no. 5, also the anthropocentric left–right orientation has certain advantages, not the least a better orientation of untrained biologists in the tree. However, this argument seems to be mostly a question of habit (and practice), as it also could be applied to device no. 3. (In fact it has been applied, by claiming that evolutionary systematics is more in accord with commonsense than phylogenetic systematics; Halstead 1978; Mayr and Bock 2002.) After all, taxa have to be sorted in one way or another, and as all sorting criteria are equally correct, it does not really seem to matter whether the criterion chosen is the phylogenetic distance to Homo sapiens.
What, then, are the disadvantages of anthropocentric cladograms, and do they weigh more than the advantages? Evidence that anthropocentric cladograms are indeed problematic comes from student questionnaires (O’Hara 1997; H. Sandvik, unpublished manuscript). If students are asked to draw the tree of life, they generally produce drawings that place Homo sapiens in a prominent position (either on top of the drawing or on its right-hand side). This in itself does not tell us whether the choice to draw a tree in this way is based on misconceptions about evolutionary processes, or just a matter of habit. However, there is also overwhelming evidence that many students are unable to read cladograms: even many graduate students of biology pay more attention to the left–right ordering of taxon names on the cladograms tips, than to the topology of the cladogram displayed beneath the taxon names.
A further problem is that students often regard Linnean categories (i.e., labels such as “family”, “subphylum”, “order” etc.) as scientific statements. They assume that the different levels in the Linnean hierarchy have distinct meanings and that the assignment of a given category to any one taxon is either right or wrong. Of course, Linnean categories are entirely arbitrary and do not carry any information whatsoever (de Queiroz and Gauthier 1992; Ereshefsky 1994, 2001, 2002; Donoghue 2001). This misconception is further reinforced by the fact that many textbook authors tend to adjust the resolution of cladograms to Linnean categories (for a refreshing exception, see Ax 1995–2001). In other words, taxa that got Linnean categories attached to them are overrepresented among the tips of cladograms. In some textbooks, this is even aggravated by the pruning of less-known (and supposedly less important) branches (i.e., O’Hara’s second narrative device). Cladograms therefore often only display the taxon names that the students are supposed to memorise, which is a somewhat biased sample of the taxa that in fact exist in nature.
These problems re-enforce each other in misleading at least untrained readers of cladograms. To be sure, none of those problems is solved by avoiding anthropocentricism in cladograms alone. It is at least as important to teach students how to read cladograms (O’Hara 1997; Sandvik, unpublished manuscript). O’Hara (1992, p. 156) emphasized that it “should always be made clear in a tree diagram that it is the topological relations of the branches that carries meaning, and not their left-to-right positioning”. However, stating this point somewhere in the vicinity of a tree diagram does not prevent the readers of the diagram from unconsciously noticing the implicit sequencing of taxa. As graphs appeal to the optical memory, the right taxon will almost necessarily be remembered as the “focal” one, even if the accompanying text tells the reader otherwise. In addition to increased awareness about the fact that tree thinking is an ability that has to be acquired, authors and teachers should become more aware of how they present cladograms. It they are not, it turns out that more or less unconsciously the result becomes anthropocentrically biased.
This seems to suggest that other modes to determine taxa positions along the left–right axis should be considered by authors and teachers. Several such alternatives to the anthropocentric mode have been proposed. de Queiroz and Gauthier (1992) suggested to sort taxa according to the number of species contained in them: at each node, the taxon encompassing most species—either described or estimated, excluding or including fossil ones—is placed to the right of the smaller taxon (cf. Figs. 2b, c and 3). Secondly, several authors seem to use a kind of implicit complexity measure: at each node, the taxon having acquired more, or more complex, autapomorphies is placed to the right of the taxon that has undergone less evolutionary change. This is evidently how e.g., Ax (1995–2001) has ordered taxa in cladograms that do not contain Homo sapiens—even though this criterion is repeatedly violated in cladograms that do contain us. A third alternative, which has been carried through in a Norwegian textbook of mine (Sandvik 2001), is alphabetic order: at each node the two taxa are ordered simply by their scientific names. There are advantages and disadvantages to all those methods. The latter one is deliberately arbitrary. It is perhaps best suited to illustrate that left–right order does indeed not carry any meaning whatsoever. The apomorphy-based method is somewhat more in accordance with intuition. However, it accepts the horizontal orientation of a cladogram as a narrative axis, at which most change occurs. This, and the fact that there is no objective measure of the complexity of autapomorphous (or any other) traits (Johnson 1968; Ghiselin 1969; Griffiths 1973), suggests that this method has its weaknesses. The method based on species numbers might be the best in order to express the importance of taxa in terms of biodiversity. As such, this method might be recommended for most uses.
All three methods are probably better than leaving the left–right ordering to “chance”, since the result is not random at all, but anthropocentrically biased. This is most likely explainable by an innate psychological bias to think anthropocentrically. If this disposition is not kept in check by conscious decisions, it obviously introduces systematic biases in how we choose to represent evolutionary history. These findings do of course not relate to the validity of the scientific results displayed by the cladograms. To the contrary, it is exactly because the graphical representation of the results is irrelevant to their correctness that scientists are entirely free to choose whatever resolution and ordering of taxa they like. Only because of this freedom, unconscious biases can affect the graphical result in the way they do and may thereby reveal some underlying expectations or worldviews of the scientists. Given that textbook authors—particularly cladistic ones—should be expected to be especially aware of issues that can confuse or mislead readers, this subconscious bias seems to be strong indeed.
In conclusion, the cladistic revolution has indeed largely removed three of the narrative devices that distort accounts of evolutionary history. However, two devices can also be found in cladistic depictions of phylogenetic relationships, viz. differential resolution of branches and left–right ordering. The latter narrative device has been found to be present even in textbooks that explicitly adopt a cladistic perspective. O’Hara (1992, pp. 154–156) has provided a very useful guide on how to “tell the tree”, and authors of textbooks and of original papers presenting phylogenetic research would profit from following those suggestions. The current paper illustrates that having the right intentions does not suffice to produce unbiased evolutionary narratives. Obviously, the subconsciousness even of trained cladists is more anthropocentric than we would like to acknowledge.