Charles Darwin did not discover evolution, a fact known to most modern biologists but not to many others. The idea of “descent with modification,” or what we more colloquially call evolution, was a hotly debated topic by the early nineteenth century. The debate lacked what Darwin provided, along with Alfred Russel Wallace (1823–1913): a mechanism, and the mechanism was natural selection. After the publication of Darwin’s On the Origin of Species in 1859, no one would ever look at a tree of life in the same way. Thus Darwin’s various evolutionary trees, all but one of which went unpublished in his lifetime, deserve a closer look to provide a glimpse of what Darwin was pondering.
Darwin accomplished many things, but he did not draw well. This affected the way he chose to represent his ideas to the public and other scientists. It may explain why in all his publications, and they were numerous, he provided only one evolutionary tree, and that was hypothetical—the single figure in On the Origin of Species. Significantly, Darwin’s trees deal more with his attempts to understand the process of evolution than its pattern. This is not surprising, inasmuch as he was convinced early on that evolution occurred, but he continued to struggle with the process of evolution and how best to represent it.
The other trees (and a few ancillary diagrams) discussed here, none published in Darwin’s lifetime, date from his earliest musings in Notebook B through to his last known tree sketch, a phylogeny of primates that he drew in 1868. Discussion of the tree sketches follows a basically chronological schema, and, as will be seen, a clustering of his thoughts does affect in a general way the time intervals of the various groups of sketches. Exact times of drawing of some of the tree sketches can only be estimated to within a few years, whereas the others are known to the year, month, or even day of drawing.
Darwin’s single most famous tree of life never appeared in print in his lifetime but became synonymous with Darwin because of its appearance in all manner of media leading up to, during, and following the yearlong celebration in 2009 of the bicentennial of his birth and the sesquicentennial of the publication of On the Origin of Species. This branching stick figure appears on page 36 of his Notebook B, written in 1837 and 1838, but is not the first or only such figure in that volume. We must back up to page 26 in Notebook B to see the two earliest surviving diagrams by Darwin (1960) (figure 4.1). We must back up even further to see how Darwin reasoned before he drew trees on page 26. In the pages preceding these diagrams, he mused greatly on many aspects of evolution, but here I concentrate on those comments most specific to tree-like diagrams.
Starting in the middle of page 21 and continuing through page 26 of Notebook B, where the diagrams appear, Darwin (1960) writes:
Changes not result of will of animal, but law of adaptation as much as acid and alkali. Organized beings represent a tree irregularly branched some branches far more branched—hence genera. —As many terminal buds dying as new ones generated [21]. There is nothing stranger in death of species than individuals If we suppose monad definite existence, as we may suppose in this case, their creation being dependent on definite laws, then those which have changed most owing to the accident of positions must in each state of existence have shortest [22] life. Hence shortness of life of Mammalia. Would there not be a triple branching in the tree of life owing to three elements air, land & water, & the endeavour of each one typical class to extend his domain into the other domains, and subdivision three more, double arrangement. —[23] if each main stem of the tree is adapted for these three elements, there will be certainly points of affinity in each branch A species as soon as once formed by separation or change in part of country repugnance to intermarriage increases it settles it [24] We need not think that fish & penguins really pass into each other. —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 [25] contradiction to constant succession of germs in progress no only makes it excessively complicated. [sketch] Is it thus fish can be traced right down to simple organization. —birds —not? [sketch] [26]. (43–46)
FIGURE 4.1 Two phylogenetic trees in Charles Darwin’s Notebook B (1837–1838:26; Cambridge University Library MS.DAR.121.26). The top tree has a three-way split, showing Darwin’s three environments of air, land, and water; the bottom tree bifurcates into a dotted line on the left (likely birds) and solid line on the right (likely fish). (Reproduced by kind permission of the Syndics of Cambridge University Library)
Paraphrasing Darwin’s shorthand, adaptation rather than the will of a species results in changes that can be shown in an irregularly branching tree because branches differentially bud and die. Some species, such as monads, changed little on the tree, whereas others, such as mammals, changed greatly in part because of where by accident they occur on the tree. Life branched into three parts representing the conquest of the three great environments of air, land, and water. Each of these three main branches then invaded the other two environments. Certainly some of the species on the three branches resemble each other, such as fish and penguins, but because of a similar environment, not because of evolutionary relationship. Darwin continues that possibly a better metaphor for evolutionary relationships than a tree would be coral, in which most of the structure is dead and only the tips are alive. Such metaphors complicate our current understanding of how information is passed from one generation to the next. In such a diagram, fish can be easily traced to the bottom of the diagram, but birds cannot.
The first sketch on page 26 then follows (see figure 4.1, top). The meaning of the two smaller bifurcating figures is not clear, although possibly they imply seedlings or new corals. The larger, three-way splitting lines show Darwin’s three environments of air, land, and water, in which the dotted lines indicate the dead parts and the solid lines the live parts of the branching coral of life. The sketch at the bottom of page 26 bifurcates into a dotted line on the left and a solid line on the right (see figure 4.1, bottom). A brush of small branches tops each line. Given what Darwin says immediately above the sketch, the left branch is likely the more poorly known evolutionary history of birds, whereas the right branch represents the better-known (especially fossil) fish.
The very next page in Darwin’s (1960) notes supports this interpretation: “We may fancy according to shortness of life of species that in perfection, the bottom of branches deaden, so that in Mammalia, birds, it would only appear like circles, & insects amongst articulata. —but in lower classes perhaps a more linear arrangement” (44). By circles, Darwin certainly means what we term dotted lines.
On page 36 of Notebook B, we come to the now famous branching stick figure sometimes called the “I think” because these words appear at the top of the page (figure 4.2). Darwin (1960) writes above, to the right of, and below this diagram and continues on page 37:
I think [above] Case must be that one generation then should be as many living as now. To do this & to have many species in same genus (as is) requires extinction [right]. Thus between A & B. immens gap of relation. C & B. the finest gradation, B & D rather greater distinction. Thus genera would be formed. —bearing relation [bottom and top, 37] to ancient types. —with several extinct forms for if each species an ancient (1) is capable of making 13 recent forms, twelve of the contemporarys must have left no offspring at all, so as to keep number of species constant. (45–46)
Darwin grapples with several ideas here. His diagram’s circled 1 is clearly the ancestor in the sketch, the thirteen lines with crossbars are extant forms, and the twelve without crossbars are extinct forms. In today’s usage of branching diagrams, Darwin’s B, C, and D form a polytomy indicating that they are equally related to one another, or, more correctly, that their relationships are not resolved. As Darwin correctly notes, A and B are more distantly related, but A is equidistant from B and C, but in Darwin’s time and even until recently, the relative position on the tree and not just relative branching was intended to show closeness of relationship. He entertains the notion that over geologic time speciations should balance extinctions, something we now know may happen when environmental conditions remain somewhat stable, but at other times, especially times of mass extinctions, this balance skews to extinction and then often back to speciation. If anything, the total count of species has increased in the last 100 million years.
Interestingly, in the tree sketched on page 36, with the exception of one bifurcation (two splits), the remaining ten dividings form polytomies (multiple splits) of three or four branches. In all but a few of Darwin’s trees shown in this chapter, polytomies of three or sometimes more branches far outnumber simple bifurcations. This may simply be habit, but based on the earlier quotes, it may be the case that Darwin viewed the process of speciation as forming more than two new lineages at each such event.
In some instances, Darwin does mention other researchers by name in Notebook B, but not in specific reference to the three diagrams on pages 26 and 36. Julia Voss (2010) suggests that when Darwin (1960) writes in Notebook B that “fish can be traced right down to simple organization,” he may have had Louis Agassiz’s tree-like paleontological chart for fossil fishes in mind (see figure 3.10). Parts of Agassiz’s Recherches sur les poissons fossils appeared from 1833 through 1844. Voss gives the date for publication of the work as 1833, but the diagram did not appear until the final version in 1844 (Brown 1890; Archibald 2009). Thus Darwin could not have had this 1844 figure in mind while writing his 1837 notebook, although he could have been influenced by earlier work of Agassiz and others on the wealth of fish fossils compared with the essentially nonexistent fossil record for birds.
FIGURE 4.2 Darwin’s well-known branching (“I think”) hypothetical stick figure in Notebook B (1837–1838, 36; Cambridge University Library MS.DAR.121.36). (Reproduced by kind permission of the Syndics of Cambridge University Library)
Voss (2010) and Theodore Pietsch (2012) speculate that the thirteen living species shown in figure 4.2 might represent the thirteen species of Galápagos finches that appear in the Birds volume of The Zoology of the Voyage of H.M.S. Beagle (1841). This somewhat lesser-known work was printed between 1838 and 1843 and often appears as a set of five bound volumes but originally was published as nineteen separate, unbound numbers dealing with fossil mammals, living mammals, birds, fish, and reptiles. It should not be confused with the commonly titled Voyage of H.M.S. Beagle, which was Darwin’s first authored book and initially appeared in 1839 simply as Journal and Remarks, 1832–1836, the third volume of the four-volume Narrative of the Surveying Voyages of His Majesty’s Ships Adventure and Beagle, Between the Years 1826 and 1836, Describing Their Examination of the Southern Shores of South America and the Beagle’s Circumnavigation of the Globe. The captain of the Beagle and editor of these volumes, Robert FitzRoy, was none too pleased when Darwin’s contribution became a great success with the public, appearing in many editions, whereas the other volumes languished. FitzRoy’s vehement, ranting denunciation of Darwin’s work on evolution overshadowed the memory of his later careers, checkered in diplomacy but distinguished in meteorology.
In contrast to his Voyage of H.M.S. Beagle, Darwin is usually credited with only editing and superintending the Zoology volumes, although he provided a preface and ecologic, geologic, and geographic information for some of the volumes. As anyone who has edited a book can testify, authors sometimes are a rather prickly and recalcitrant lot. Although John Gould receives credit as the author of the Birds volume of Zoology, Paul Barrett and Richard Freeman (Darwin 1987) note that Gould was in Australia collecting birds and mammals during most of the writing of the volume, much of which fell to Darwin with the assistance of others. Elizabeth Gould, John’s wife, produced all fifty superb plates in the volume. Despite the problems with writing and production, Gould had informed Darwin about the importance of the Galápagos birds in general and the so-called finches in particular. I write “so-called” because in fact the newer evidence based largely on molecular analyses performed in part by my colleague Kevin Burns (Burns et al., 2002) supports the view that these birds are not finches but relate more closely to tanagers found in South America and the Caribbean. More to the point, we know that earlier in 1837, the year that Darwin sketched his now famous Notebook B tree, Gould had told him about the uniqueness of these Galápagos birds, but I am unaware of evidence pointing to thirteen species of finches being identified when Darwin began to write the earlier parts of Notebook B in July 1837.
The 1840s
As amply shown by John van Wyhe (2009), Darwin did not fear publishing his ideas on evolution from the late 1830s through the 1850s, as is often portrayed. Certainly, Darwin was still gathering information and developing his ideas on evolution, but his time during these years was consumed with preparing, editing, writing, and publishing eleven volumes (depending on how one counts) that deal with various aspects of the zoology and geology of the voyage of H.M.S. Beagle and fossil and recent barnacles, as well as over sixty scientific papers (Freeman 1977).
Tree-like diagrams in Darwin’s notes from the 1850s cannot be pinned down as to year, but we do know the month and year of his two drawings from the 1840s. Both are small, obscure figures providing nothing profound, but they offer a glimpse of Darwin’s struggles at the time regarding the process of evolution. The first figure, drawn in July 1843, appears almost as a doodled afterthought, about six years after Darwin drew his tree-like figures in Notebook B. On the front side of a slip of gray paper, he writes:
As all groups by my theory blend into each other, there could be no genera or orders «in same sense that no part or branch of a tree can be said to be distinct» in a «perfect» systema naturæ fossil & recent—but for the existing ones at any one period—these terms useful, implying not separation, but that the species of one genus are more closely related to each other, than to the species of another genus. —not that any barrier exists between these two series of species/over
On the reverse side, he writes, “a tree not good simile—endless piece of sea weed dividing” (Cambridge University Library MS.DAR.205.5.90r–v [angle quotes, or guillemets, are Darwin’s later insertions]).
Darwin’s seaweed sketch appears at the bottom of the reverse side (figure 4.3). Unlike any branching diagram he had done before or would do after, the wispy branches evoke the bending, curving form of a fragile, waterborne plant. One cannot know with certainty Darwin’s thoughts here, but it would seem that he still struggled with the notion of all life through time as continuous and unbroken so that no metaphor—trees, corals, or seaweeds—captured his vision.
FIGURE 4.3 Darwin’s wispy sketch from 1843 (Cambridge University Library MS.DAR.205.5.90v), with branches evoking the bending, curving form of a fragile, waterborne plant, suggesting the notion of life through time as continuous and unbroken seaweed. (Reproduced by kind permission of the Syndics of Cambridge University Library)
FIGURE 4.4 Darwin’s unlabeled tree from 1848 (Cambridge University Library MS.DAR.205.5.127v), preceded by comments on how genera break into groups, with special mention of a genus of pedunculate barnacle. (Reproduced by kind permission of the Syndics of Cambridge University Library)
The second known drawing from the 1840s is on the reverse side of a small, off-white paper (figure 4.4). The front side reads:
Dec. /48/. I have been much struck in Anatifera how the genus, (& I have no doubt universal, as evidenced by sub-genera) breaks up into little groups—hence those who use Dignostic character have generally to refer to only 1 or 2 or 3 species—So again species break up into groups of varieties [reverse side] Genera again in same family are united into little groups—so throughout animal Kingdom—so children even in same Family.—It is universal law. (Cambridge University Library MS.DAR.205.5.127r–v)
Anatifera refers to what we now call Lepas, a genus of pedunculate barnacle that includes the species Lepas anatifera (Pedunculata, Cirrepedia), commonly called a goose or gooseneck barnacle. In letters to various colleagues, Darwin opined as to how he might handle the considerable taxonomic confusion surrounding the names Anatifera and Lepas. He had begun to work on his two monographs on barnacles: two volumes on fossil barnacles published in 1851 and 1854, and two volumes on extant barnacles published in 1851 and 1854. Both volumes from 1851 were titled The Lepadidae, or, pedunculated cirripedes, which includes Lepas anatifera.
The small branching diagram at the bottom of the page is reminiscent of the “I think” diagram on page 36 of Notebook B (see figure 4.2), except that instead of dashes terminating thirteen of the branches, Darwin here uses dots at the termini of some branches and at the nodes of others. Two single branches extend considerably beyond the others. No text or numbers appear on the tree-like sketch, so any suppositions as to what he meant are just that. The comments about Anatifera and other genera breaking into small groups suggest that he was trying to represent the idea of branches dividing into small groups, which in turn break into small groups in a polytomous fashion. The sketch in figure 4.4 suggests this, except that here the number of bifurcations (four) almost matches the number of polytomies (five). Given that Darwin began work on the barnacles in earnest in 1846 and the last of the four monographs appeared in 1854, eight years later, it is rather ironic that all we have preserved from this interval is this one small, rather enigmatic tree-like sketch. It does, however, show that Darwin continued to struggle with how species radiated and how this can be depicted.
The 1850s
Darwin’s six tree sketches done in approximately the last eight years of the 1850s constitute the greatest concentration of his surviving tree sketches, including the only one published in his lifetime, the foldout figure in On the Origin of Species. The dates of production for four of the sketches cannot be established with any certainty, but Darwin probably drew one between 1852 and 1855, whereas the other three likely come from the year 1857 or 1858. Two very important firsts for Darwin appear in several of these sketches. On three of the four sketches, for the first time Darwin writes the names of actual animals, a feature seen in some of his later sketched trees. On the fourth sketch, he identifies geologic time on a tree for the first and only time. I find this somewhat unusual as until the publication of his barnacle monographs between 1851 and 1854, Darwin was known first and foremost as a geologist. Why, then, was geologic time not integral to his visual perceptions of how evolutionary relationships should be presented? Darwin did portray relative time and some ancestor–descendent relationships, but only once did he include geologic time.
The single sketch with geologic time intervals written on it shows five concentric circles, two of which are incomplete (figure 4.5). The inner to outer circles read “Palaeoz” for Paleozoic, “Second” for Secondary, and “Tertiary.” As with Darwin’s small tree sketch from 1848, polytomies—usually trifurcations—with only four bifurcations are most common. Also as with the 1848 sketch, small dots occur at most branchings and the tips of branches. They also appear at intervals along at least three branches. We know what they represent because at the top of the page Darwin writes “Dot means new form,” but unfortunately the text that follows is not discernible with any certainty. Possibly the last word is “Bird.” Darwin clearly had in mind what we today call cladogenesis for branching speciation and anagenesis when it appears that a species evolves into another without splitting.
FIGURE 4.5 Darwin’s radiating tree, likely from 1857 or 1858 (Cambridge University Library MS.DAR.205.5.184r), indicating geologic time intervals written on it as five concentric circles, two of which are incomplete, with the inner to outer circles reading “Palaeoz” for Palaeozoic, “Second” for Secondary, and “Tertiary.” (Reproduced by kind permission of the Syndics of Cambridge University Library)
It is quite likely that Darwin copied the idea of showing geologic time as a circle, possibly from Louis Agassiz and Augustus Gould’s frontispiece for Principles of Zoölogy (1848) (Voss 2010) or from Heinrich Bronn’s similarly shaped frontispiece for Lethaea geognostica (1850–1856) (see figure 7.18). Agassiz and Gould’s frontispiece, “Crust of the Earth as Related to Zoology,” identifies a series of concentric, shaded circles as intervals of Earth history, with the oldest at the center. The circle divides radially into four quadrants identified as the four major animal embranchements first proposed by George Cuvier earlier in the nineteenth century: Articulata, Radiata, Mollusca, and Vertebrata. Recall that Agassiz studied with and was an acolyte of Cuvier. In this diagram, man rests at the top of the circle, embellished with a shining crown.
Each major group within each quadrant emanates as a ray from the center of the circle, showing its origin in the geologic past through to today, with trilobites and ammonites as the only extinct lineages. The relative thicknesses of the rays provide an estimate of their relative species richness at any given time. A dot at the center of the circle, according to Agassiz and Gould, represents “the primitive egg, with its germinative vesicle and germinative dot, indicative of the universal origin of all animals.” Although clearly a nonevolutionary representation, the diagram invokes a comparison between the life history of an individual and the history of life on Earth, a theme strongly repeated in an evolutionary context some twenty years later by Ernst Haeckel (1866) in his coining of the terms “ontogeny” and “phylogeny.” As Julia Voss (2010) argues, Darwin has pulled Agassiz and Gould’s metaphor into an evolutionary realm by literally connecting the dots in his sketch (see figure 4.5). Very likely Darwin knew of this circular diagram, as he possessed a copy of Agassiz and Gould’s book in his library (Cambridge University Library 1961).
Darwin uses the concentric circles for geologic time in one other tree sketch, dating most likely from 1857 or 1858, but without designating geologic times (figure 4.6). This represents one of seven trees by Darwin dealing with mammalian evolution (for six of them, see Archibald 2012). The top of the sketch reads, “Let dots represent Genera ???” To the right of the sketch, Darwin writes “no form intermediate.” A line leads to the largest dot at the base of the tree or to the dotted lines below; thus “no form intermediate” almost certainly refers to the absence of species intermediate between marsupials and placentals (possibly rodents). To the left, the text probably reads, “If these had all given descendants then this wd [would] have been a great series,” and a line leads to an encircled group of five dots near the base of the tree. He thus indicates that this cluster of dots did not radiate into many other species but would have been significant if they had done so. At the very base of the tree, Darwin writes, “Parent of Marsupials & Placentals,” with the word “Rodents” written over the main middle trunk of the tree. Here are two groups that did very successfully radiate, as indicated by many connected dots. To the right, partial circles enclose the tree. Short lines labeled “Rodents” and “Marsupials” point to the middle and lower main trunks, respectively.
FIGURE 4.6 A phylogenetic tree of mammals, drawn by Darwin in 1857 or 1858 (Cambridge University Library MS.DAR.205.5.183r), showing placentals (specifically rodents) and marsupials as sister taxa. No other hypotheses of relationship are given. (Reproduced by kind permission of the Syndics of Cambridge University Library)
FIGURE 4.7 A simple phylogenetic tree, drawn by Darwin in 1857 or 1858 (Cambridge University Library MS.DAR.205.5.184v), indicating the “Parents of Placentals Rodents & Marsupials.” (Reproduced by kind permission of the Syndics of Cambridge University Library)
It at first may seem odd that Darwin should juxtapose rodents and marsupials in such a tree. Based on discussions and correspondence between Darwin and G. R. Waterhouse, there was some idea of a possible relationship between rodents and marsupials. This idea is reinforced in another, simpler diagram, also probably from 1857 or 1858, that appears on the reverse side of the paper bearing the tree shown in figure 4.5. The figure is labeled “Parents of Placentals Rodents & Marsupials” (figure 4.7). Waterhouse espoused the view that rodents in general, and in particular the South American vizcacha (or bizcacha), a relative of the chinchilla, were somehow related to marsupials. This was largely due to the occurrence in both of a dual vaginal opening, which we know is an ancestral trait seen across various mammals and which evolved convergently in marsupials as a dual pseudovagina. Darwin (1859) even expresses this idea in On the Origin of Species:
As the points of affinity of the bizcacha to Marsupials are believed to be real and not merely adaptive, they are due on my theory to inheritance in common. Therefore we must suppose either that all Rodents, including the bizcacha, branched off from some very ancient Marsupial, which will have had a character in some degree intermediate with respect to all existing Marsupials; or that both Rodents and Marsupials branched off from a common progenitor, and that both groups have since undergone much modification in divergent directions. (430)
These tree sketches show Darwin’s toying with the ideas of early mammalian evolution. One of the more astounding of Darwin’s tree sketches done in pencil possibly dates from sometime between 1852 and 1855 and thus predates the three sketches just discussed. It is not a single tree but five trees suffused later with a hodgepodge of brown ink notes overlapping in the upper-right quadrant that all but obscure one of the trees (figure 4.8). The impression of mental doodling is reinforced because Darwin drew this busy sketch and notes on the blank side of an advertisement for the “printing office and stationery warehouse” of Edward Strong, located in Bromley, Kent, about 5 miles (8 km) from Darwin’s home in Downe.
FIGURE 4.8 Five tree sketches, drawn by Darwin sometime between 1852 and 1855 (Cambridge University Library MS.DAR.205.6.51r), with overlapping notes in the upper-right quadrant all but obscuring one of the trees. (Reproduced by kind permission of the Syndics of Cambridge University Library)
For ease of discussion and clarity, the text in the upper-right quadrant was removed to expose the underlying tree, and, as much as possible, legible text replaces Darwin’s writing (figure 4.9). Emerging from this apparent jumble is an exercise in the perceived, and what we now know to be the importance of embryology in the evolutionary process. At the time, scientists, notably Richard Owen, were pressing for changes in embryological development as the foundation for the idea of the archetype as the underpinning for differences among major groups. By this time, Darwin was clearly becoming convinced of the efficacy of his natural selection, but he certainly entertained embryology as an important component of evolution, as this sketch clearly shows. Ironically, embryology did not contribute to the emergence of the Modern Synthesis in the 1930s and 1940s, which melded population genetics and natural selection. Embryology in its incarnation of evodevo (evolution and development) did not join with evolutionary theory until the tools of molecular biology came to the forefront in the last few decades of the twentieth century. Yet here Darwin, like his late-nineteenth-century contemporaries, contemplated its importance.
Darwin likely first drew the large tree in the upper half, possibly writing an abbreviation of “mammal embryo” at its base. He then scratched it out with a series of pencil swirls, to be replaced with the figure just below, showing a tree of mammalian carnivores on the left and what loosely can be called mammalian herbivores on the right. The dual placement of “dog” on the tree was certainly no mistake, as he favored the idea that domestic dogs likely descended from a series of ancestors within the canid family. This tree was probably followed by the simpler version on the left that labels various embryos in ancestral positions. At some point, he added his then emerging information of pigeon breeding at the top left in a tree sketch. Embryos are not mentioned in this tree, but likely he related artificial selection in pigeons to changes in embryological development in the different pigeon breeds. The crossed-out comment on the far right also speaks to early stages of embryological development as an engine for evolutionary change.
Next we must deal with what Darwin penned in brown ink over the scratched-out intricate tree in the upper half (see figure 4.8 [transcriptions mostly from Charles Darwin Papers]). The uppermost and lower-right bubbles read, “When we put horned Bull to Hornless cow & horned calf. the cause was in embryo [?]—but the horns do not appear till nearly adult calf ½ grown. so with bigger horn, so with link, which do not appear till embryo of same size. —Potentially in germ.” The middle bubble reads, “Potentially embryo of fish & Mammal at no [or one?] time[s?] alike.” The lower-left bubble reads, “Horns cd not appear & in embryo, but limbs cd be appear longer, supposing that we had means of measurement but there is no reason to suppose they do.”
FIGURE 4.9 The same five trees shown in figure 4.8, with the overwritten text removed and transcriptions of Darwin’s written comments added. (The transcriptions are mostly from Charles Darwin Papers.)
These penned comments overlying the tree sketches demonstrate Darwin’s struggle with understanding how traits are passed from one generation to the next. It is tempting to suggest that Darwin comes close to understanding the underlying germ-line inheritance of traits. This was not to be, for in The Variation of Animals and Plants Under Domestication (1868), he published his theory of “pangenesis,” which argued that “gemmules” passed from the soma, or body, of the organism to the gonads and thus to the next generation. We now know this not to be the case, as it necessitates the ideas of “use and disuse” and “inheritance of acquired characteristics,” which Lamarckian evolutionary theory included. His ideas of inheritance never caught hold, but he never abandoned them, even mockingly calling them “my much despised child” in a letter to E. Ray Lankester in 1870 (Darwin 2010:69).
Although any specifics from the stream-of-consciousness jottings on the tree sketches in figure 4.8 may not be very important, Darwin does provide his general impressions about the importance of embryology to evolutionary change and how traits pass from one generation to the next. The tree sketches merely served as a vehicle for these musings.
Darwin’s “Big Book”
By 1858, Darwin had completed some ten and a half chapters for his planned large work, or as he sometimes called it, his “Big Book” dealing with evolution by means of natural selection. Except for the first two chapters, which grew into The Variation of Animals and Plants Under Domestication (1868), the remainder of the manuscript was not published until 1975, when it appeared as an edited volume, Charles Darwin’s Natural Selection: Being the Second Part of His Big Species Book Written from 1856 to 1858. This work includes two diagrams dealing with biological relationships: Darwin called one a table, and the other forms a multipage precursor to the tree that appears in On the Origin of Species. This precursor tree (actually trees) did not see wider circulation until its publication in Natural Selection (Darwin 1975) and still remains rather obscure.
Darwin’s table in the same work, as well as two other related figures showing biological relationships and hybridizations, warrant brief comment. These three sketchy diagrams attempt to show the importance for evolution that Darwin placed on what he most often referred to as “hybridism.” Darwin felt strongly that hybridization must play a role in evolution, such as in the formation of new species, but not until the advent of more molecularly based tools in the mid-twentieth century could this be demonstrated (see chapter 7). Darwin (1975) describes his table as providing for “all the well authenticated crosses which I have heard of in one order of Birds, the Rasores [no longer recognized]; in order that those who have not attended to the subject, may see how numerous the crosses have been, & between what different forms” (434). Figure 4.10A presents Darwin’s original sketch (a more readable version is in Darwin 1975:434). Various birds referred to as the then recognized Rasores, such as pheasants and peacocks, are listed down the middle of the table, with brackets on either side linking species known to hybridize. Darwin indicates, “The Brackets, imply that hybrid offspring has been produced by the two forms so connected” (436). Small numbers barely discernible next to the species names refer to several pages of footnotes documenting the degree of fertility in the hybrids. This diagrammatic table is not tree-like; two other of Darwin’s unpublished diagrams explore hybridization in birds, one of which at first appears tree-like, whereas the other uses some form of unrooted networks.
Stylistically, unrooted networks still exist, mostly for the comparison of populations within a species, usually by means of molecular techniques. Unrooted networks for systematic research reached their heyday in the 1960s and 1970s under the auspices of phenetics (see chapter 7). Unrooted networks have an even longer run, both before and after Darwin. For example, the English naturalist Hugh Edwin Strickland (1811–1853) published what he termed a map on which connecting lines of various lengths indicate relationships of birds (Strickland 1841; Voss 2010). In 1856, Alfred Russel Wallace used Strickland’s technique to create an unrooted network to show the relationships within the now rejected avian order Scansores (birds with two toes facing forward and two facing backward). While it is tempting to claim that Strickland’s and Wallace’s unrooted networks represent pre-Darwinian evolutionary trees, nothing of their form or in the text indicates evolution as the basis for the arrangements. Rather, these visual schemes cleverly present a rather older heritage of unrooted networks useful in representing relationships.
Of the two additional Darwinian diagrams noted earlier, the first one appears to be some sort of tree and the other appears to be an unrooted network, but both more likely represent Darwin’s attempts at creating a bracketing scheme in order to join hybridizing pairs of birds. Figure 4.10B, done sometime between the end of September and the end of December 1857, shows crosses between various species of mostly birds in the pheasant family listed down the middle of the figure. What appear to be branching figures or trees on either side more likely show nested brackets pointing to hybridizing pairs. Darwin drew lines through the text on this page as well as through the figure, indicating his rejection of this attempt. A probably later effort, shown in figure 4.10C, reads across the figure in Darwin’s hand, “Make Table of Pheasants & Fowls Crossing,” likely a note to himself. It presents Darwin’s experimentation possibly with an unrooted network diagram. Although the intent of his artistry may not be clear, its content attempts to show some aspects of hybridization. If Darwin’s comment about a table refers to the table that eventually appeared in Natural Selection in 1975, Darwin possibly had abandoned an unrooted network for a simpler bracketing version.
The relationship of each of these three hybridization diagrams to tree-like figures must remain tangential, but not so Darwin’s large, carefully prepared foldout tree (actually trees) finally published in Natural Selection in 1975. Its publication was, of course, beaten by well over one hundred years with the appearance of the single, foldout figure in On the Origin of Species, published in 1859. The two sets of diagrams are similar, with the Natural Selection version more complex. The fact that it opens downward rather than upward, as in On the Origin of Species, seems inconsequential. Darwin identified the Natural Selection figures as comprising four diagrams, labeled I through IV (figure 4.11). The text in the upper-right quadrant details how the compositor was to arrange lettering on the diagram. Darwin expended some 3,500 words explaining these four diagrams, whereas for the single diagram in On the Origin of Species (figure 4.12), he used some 2,700 words in explanation. These texts allow us to understand more clearly what Darwin attempted to tell us about his ideas compared with earlier tree sketches. None of the trees in these two books tells us the specific patterns of relationship among plants and animals, but they attempt to show how Darwin perceived the process of evolution to unfold. About the diagrams in Natural Selection, Darwin (1975) writes, “The complex action of these several principles, namely, natural selection, divergence & extinction, may be best, yet very imperfectly, illustrated by the following Diagram, printed on a folded sheet for convenience of reference. This diagram will show the manner, in which I believe species descend from each other & therefore shall be explained in detail: it will, also, clearly show several points of doubt & difficulty” (238–39).
FIGURE 4.10 Three diagrams drawn by Darwin showing his views and evidence for hybridization in various groups of birds: (A) a self-described table from 1858 indicating well-authenticated crosses for birds within the then recognized order Rasores (Cambridge University Library MS.DAR.12.88); (B) an eventually rejected figure (note diagonal pencil lines) from 1857 showing crosses between various species of birds mostly in the pheasant family (MS.DAR.205.7[1].33v); and (C) a figure likely from 1857 or 1858 showing various crosses of birds over which Darwin has written “Make Table of Pheasants & Fowls Crossing,” possibly referring to the table in (A) (MS.DAR.205.7[1].86r). (Reproduced by kind permission of the Syndics of Cambridge University Library)
FIGURE 4.11 Darwin’s figures, drawn in 1858, for Natural Selection (1975) (Cambridge University Library MS.DAR.10.2.26R–S), comprising four diagrams labeled I through IV. (Reproduced by kind permission of the Syndics of Cambridge University Library)
FIGURE 4.12 Darwin’s only phylogeny published in his lifetime appears in On the Origin of Species (1859) as a foldout inserted between pages 116 and 117.
Darwin’s explanation, although rather long-winded, summarizes his several points. In diagram I, A through M are species of plants within a single genus with the species A and M: “the two most distinct forms in all respects.…From our principle of divergence, the extreme varieties of any of the species, & more especially of those species which are now extreme in some characters, will have the best chance, after a vast lapse of time, of surviving; for they will tend to occupy new places in the economy of our imaginary country” (239). Based on modern studies, the argument that extreme forms tend to survive is highly questionable, but Darwin does show how species may diverge through time.
For the sake of argument, Darwin (1975) makes “A the most moisture loving & M the least moisture-loving species.” In his thought experiment, he then evolves each of the two species in what he sees as the most likely scenarios:
We will first take the simplest case. Let M inhabit a continuous area, not separated by barriers, & let it be a very common & widely diffused & varying plant. From the fact of M. being very common & widely diffused, it clearly has some advantages in comparison with most of the other inhabitants of the same country…it could endure still more drought.…As m1 tends to inherit all the advantages of its parent M, with the additional advantage of enduring somewhat more drought, it will have an advantage over it, & will probably first be a thriving local variety, which will spread & become extremely common & ultimately, supplant its own parent. We may now repeat the process…perhaps many thousands of generations may pass before m1 will produce another variety m2, still more drought-enduring & yet inheriting the common advantages of m1 & M.…
In each stage of descent, there will be a tendency in the new forms to supplant its parent, though probably, as we shall see, very slowly, & so ultimately cause its extinction. (239)
The successive replacement within the M lineage in Darwin’s diagram refers to what is now called anagenesis. The process that Darwin envisions for species A is now called cladogenesis for obvious reasons—it results in the increase of the number of clades and species. Here Darwin shows mostly cladogenesis with some anagenesis but with the result of three species at the end where there had been one at the beginning. Unlike for M, Darwin notes that A varies greatly, giving rise to many varieties and eventually species. The printed capital letters and ink sketching appear clearly in figure 4.11. Less discernible faint solid and dotted pencil lines radiate from many of the species emanating from A, likely added by Darwin to emphasize the splitting nature of speciation in A as compared with M.
The other ten species, B through L, which at first glance appear to have become extinct, in fact survive, but Darwin has placed at each terminus only “&” or “&c,” indicating the continuation of each of these lines, for he notes, “The other species of the genus, B to L, are supposed to have transmitted unaltered descendants” (244). This essentially defines what in the twentieth century Niles Eldredge and Stephen Jay Gould (1972) included as the equilibrium part of their theory of punctuated equilibrium.
The only difference that Darwin (1975) wishes to impart between diagram I and diagram II is that in diagram II “everything is the same as in diagram I…except that it is left to mere chance in each stage of descent, whether the more or less moisture loving varieties are preserved” (244), with the result that he shows the descendants in diagram II as less separated from each other on the horizontal scale compared with those in diagram I. He has argued, then, that natural selection will cause more divergence than chance alone, an idea that finds support in modern population studies. Notably, Darwin did not pencil in any additional radiations of species emanating from A in diagram II, as he had done in diagram I.
Although diagrams III and IV are not trees, comments are germane. As figure 4.11 shows, M is farther away from L than L is from K in all four diagrams. These differences have been lost in printed reproductions of this figure, such as in Natural Selection (Darwin 1975), that show equal spacing; thus Darwin’s intent is lost. Darwin states, “So again m10 having constantly diverged from the characters of M will now stand more distant from L, than M originally stood. This is represented in the Diagram III” (245). He says that the descendants will go on to produce “more & more new specific forms & thus more & more modified or divergent.” In diagram IV, Darwin attempts to show, not very successfully, the principle of what he terms analogy and what we now call convergence. He further notes that if all the intermediates became extinct, the remaining forms in diagram IV might be placed in separate genera.
A final interesting point Darwin (1975) makes about his diagrams in Natural Selection deals with the irksome question of the origin of higher taxa, such as families, orders, and classes:
Now for a moment let us go back many stages in descent: on our theory the original twelve species A to M are supposed to have descended & diverged from some one species, which may be called Z, of a former genus.…Z will have become the ancestor of two or three very distinct groups of new species; & such groups, naturalists call genera. By continuing the same process, namely the natural selection of generally the most divergent forms, with the extinction of those which have been less modified & are intermediate, Z may become the ancestor of two very distinct groups of genera; & such groups of genera, naturalists call Families or even Orders. (246)
Rush to Publish
Understandably, the publication of On the Origin of Species in 1859 heralded the acceptance of trees of life into the scientific as well as the public psyche. Never mind that the sole figure in Darwin’s work was not a tree of life that showed the relationships of any specific species to one another but a tree that hypothesized Darwin’s views of how evolution may have unfolded. The book provided the archetypical watershed moment when people acceptingly visualized nature as a great tree with its numerous branchings and rebranchings. A tree now was the metaphor for the history of life.
Even though Darwin does not mention human evolution in On the Origin of Species, the publication engendered a truly world-changing perception of nature and our place in it, for which reason one might conclude that his foldout tree became an icon of biology. But such was not the case. Indeed, today the tree is not even universally recognizable among biologists. Darwin’s (1960) small figure from Notebook B is far more recognized and reproduced, especially since the celebration of the bicentenary of his birth in 2009 (see figure 4.2). This tree, not the one in On the Origin of Species, has come to symbolize Darwin’s evolutionary views.
The foldout diagram in On the Origin of Species (see figure 4.12) both resembles and differs from those in Natural Selection (1975). In Natural Selection, as in On the Origin of Species, the “species are supposed to resemble each other in unequal degrees, as is so generally the case in nature, and as is represented in the diagram by the letters standing at unequal distances” (Darwin 1859:116). In the latter, Darwin has added fifteen horizontal lines with the explanation, “The intervals between the horizontal lines in the diagram, may represent each a thousand generations; but it would have been better if each had represented ten thousand generations. After a thousand generations, species (A) is supposed to have produced two fairly well-marked varieties, namely a1 and m1. These two varieties will generally continue to be exposed to the same conditions which made their parents variable” (117). He continues discussion a few pages later:
If we suppose the amount of change between each horizontal line in our diagram to be excessively small, these three forms may still be only well-marked varieties; or they may have arrived at the doubtful category of sub-species; but we have only to suppose the steps in the process of modification to be more numerous or greater in amount, to convert these three forms into well-defined species: thus the diagram illustrates the steps by which the small differences distinguishing varieties are increased into the larger differences distinguishing species. (120)
For Darwin, then, the same processes—notably, natural selection—act over all time frames and eventually over all taxonomic levels to produce varieties and species to orders and classes. Only the length of time provides the variable.
In On the Origin of Species, Darwin does not introduce a hypothetical case, such as the moisture-loving through drought-resistant plants he introduced in Natural Selection, but instead remains abstract in his discussion. He still argues that “the more diversified in structure the descendants from any one species can be rendered, the more places they will be enabled to seize on, and the more their modified progeny will be increased,” but here he also now admits, “I am far from thinking that the most divergent varieties will invariably prevail and multiply: a medium form may often long endure, and may or may not produce more than one modified descendant” (119).
The dashed lines at the top of the foldout in On the Origin of Species represent a more abstracted continuation of divergence and speciations shown lower in the diagram. In this tree, species I diverges but to a lesser degree than species A, but Darwin does not offer any statements as to differences or significances for species I. He simply calls it a second species following “analogous steps.” After referring to species I, he does, as in Natural Selection, speak to the lack of divergence in some species: “The other nine species (marked by capital letters) of our original genus, may for a long period continue transmitting unaltered descendants; and this is shown in the diagram by the dotted lines not prolonged far upwards from want of space” (121). Following this on the same page, he notes that extinction is an important principle: “Hence all the intermediate forms between the earlier and later states, that is between the less and more improved state of a species, as well as the original parent-species itself, will generally tend to become extinct.” The remainder of Darwin’s discussion of the foldout covers in detail a point made in his discussion in Natural Selection—that the divergence of lower taxa such as species eventually leads to higher taxa such as orders and classes.
Darwin (1859) concludes the chapter on natural selection and his description of his foldout diagram by paying homage to the metaphor of a tree representing the history of life. It bears repeating in its entirety:
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. The green and budding twigs may represent existing species; and those produced during each former year may represent the long succession of extinct species. At each period of growth all the growing twigs have tried to branch out on all sides, and to overtop and kill the surrounding twigs and branches, in the same manner as species and groups of species have tried to overmaster other species in the great battle for life. The limbs divided into great branches, and these into lesser and lesser branches, were themselves once, when the tree was small, budding twigs; and this connexion of the former and present buds by ramifying branches may well represent the classification of all extinct and living species in groups subordinate to groups. Of the many twigs which flourished when the tree was a mere bush, only two or three, now grown into great branches, yet survive and bear all the other branches; so with the species which lived during long-past geological periods, very few now have living and modified descendants. From the first growth of the tree, many a limb and branch has decayed and dropped off; and these lost branches of various sizes may represent those whole orders, families, and genera which have now no living representatives, and which are known to us only from having been found in a fossil state. As we here and there see a thin straggling branch springing from a fork low down in a tree, and which by some chance has been favoured and is still alive on its summit, so we occasionally see an animal like the Ornithorhynchus or Lepidosiren, which in some small degree connects by its affinities two large branches of life, and which has apparently been saved from fatal competition by having inhabited a protected station. 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. (129–30)
Marsupials: Sister Taxa or Ancestors?
Perhaps the most intriguing of Darwin’s trees, or actually pair of trees, comes from a ten-page letter that Darwin wrote to the geologist Charles Lyell on September 23, 1860, during Darwin’s vacation with his family at a seaside hotel in Eastbourne. The letter was written just ten months after the publication of On the Origin of Species. Its interest lies first in the fact that unlike all but a few of Darwin’s trees, it details the relationships of an actual group—mammals. Second, it is a pair of trees with competing views on the origin of placental and marsupial mammals, something Darwin never did before or after, as far as we know. Third, unlike most of his other trees, except those in On the Origin of Species and Natural Selection, it comes with a commentary of his views, sometimes in rather frank terms, because it was meant for only Lyell’s eyes (for details of Darwin’s exchange of letters with Lyell, see Archibald 2012).
The two trees in question lay out two schemes for the origin of the two great clades of living therian mammals: marsupials and placentals (figures 4.13 and 4.14). Here the concentration concerns the two phylogenies and what Darwin said about them. At the top of the first tree, labeled “Diagram I,” Darwin writes, “A. Unknown form probably intermediate between reptiles mammals, Reptiles & Birds as intermediate as Lepidosiren now is between Fish and Beatractians [Batrachians],—probably more This unknown form probably more closely related to Ornithorhynchus than to any other known form” (see figure 4.13). On the opposite side, labeled “Diagram II,” Darwin writes “A (as on other side).” On both diagrams, an A was positioned at the origin of each tree; thus the description was intended to be the ancestor in both trees.
The two main branches of diagram I (see figure 4.13) are more symmetrical than those in diagram II (see figure 4.14), and the relative lengths of the branches vary slightly. The topology or relative positions of the branchings in the two diagrams appears almost identical, except that the first node in the marsupial side is reversed. The differences appear inconsequential in the positioning and orientation of terminal taxa names. Names listed in each tree at the tips of terminal branches are identical, except for minor differences such as capitalizations.
FIGURE 4.13 A phylogenetic tree, drawn by Darwin in 1860, showing a nonmarsupial, nonplacental mammal as the common ancestor, A, of both marsupials and placentals. (Reproduced with permission of the American Philosophical Society)
In diagram I, on the “True Marsupial” branch, “Kangaroo fam.” and “Didelphys fam.” refer to what we today essentially recognize as the families Macropodidae and Didelphidae, respectively, although the content of each group in Darwin’s time was somewhat broader. On the “True Placental” side, the first branch is “Rodents” with “Insectivores” as the second branch. The orders Rodentia and Carnivora are recognizable. Insectivora tended to include any small placental mammals with sharp teeth that could not be aligned with any other placentals. Consequently, Insectivora constitutes what biologists term a wastebasket taxon, and current classification utilizes the name Lipotyphla or Eulipotyphla, including shrews, moles, hedgehogs, and a few lesser-known groups. There is then a three-way split between one branch with “Ruminants” and “Pachyderms” (thick-skinned mammals), the second for “Carnivores,” and the third for “Quadrumana.” Johann Friedrich Blumenbach (1779) proposed “Quadrumana” (referring to four hands) for all primates except humans in contradistinction to his “Bimana” (two hands) for humans. Darwin here simply used this older terminology. Thomas Henry Huxley (1863) argued convincingly that the so-called higher apes were reasonably put in Carl Linnaeus’s (1758) Primates along with humans. The names Quadrumana and Bimana were out of favor by the end of the nineteenth century in favor of Primates, including humans.
FIGURE 4.14 A phylogenetic tree, drawn by Darwin in 1860, showing a marsupial as the common ancestor, A, of both marsupials and placentals. (Reproduced with permission of the American Philosophical Society)
The most interesting aspects of the two trees are not Darwin’s views of the relationships within marsupials and within placentals but the alternative hypotheses he presented regarding the origin of marsupials and placentals (Archibald 2012). On diagram I above the split between “True Placental” and “True Marsupial,” Darwin writes “Mammals not true marsupials not true placentals” (see figure 4.13). On diagram II, the earliest ancestor is labeled “true Marsupials lowly developed” followed by “True Marsupials higher developed?” with the split leading to “Placentals” and “Present Marsupials” (see figure 4.14).
In the letter to Lyell accompanying these diagrams, Darwin indicated that as a general rule he preferred the tree in diagram I, but if the embryological brain of marsupials closely resembled that of placentals, he should strongly prefer the tree in diagram II, as this agreed with the antiquity of Microlestes. We now know that the marsupial embryologic brain does not indicate its being a precursor to the placental brain and that the Cretaceous fossil Microlestes (now Thomasisa) is not a marsupial.
Although it would be tempting to suggest that Darwin favored one of the trees over the other, especially if it were the correct version shown in diagram I, the evidence in his letter and diagrams is not definitive on this matter. The best we can argue from current evidence is that Darwin did weigh alternative views for the monophyletic versus the successive origin of living mammals, which no one else was doing with the same acumen 150 years ago (for further discussion, see Archibald 2012).
Man’s Place in Nature
Darwin drew two phylogenetic trees dealing with primate evolution in general and human evolution in particular, neither published in his lifetime. The simpler and probably earlier of the two trees appears as a pencil sketch on the reverse of an undated, nicely handwritten ink text with scrawled pencil comments at the bottom (Cambridge University Library MS.DAR.80.B118r). The portion of the text in ink almost certainly belongs to Charles Darwin’s daughter Henrietta Emma “Etty” Darwin, who helped her father in his work on The Descent of Man (1871) (Browne 2002).
Her clear, easily readable text describes aspects of W. C. L. Martin’s views on primate classification. She notes that Martin (1841:361) groups Simiadae into three subfamilies, one including the chimpanzee, the orangutan, and Hylobates; the second including Semnopithecus and probably Colobus; and the third including Cercopithecus, Macaca, and Cynocephalus. In Martin’s classification, Simiadae corresponds to what we now call Catarrhini, the group including Old World monkeys and apes, except that Martin, as did others at the time, placed humans in their own order: Bimana. In The Descent of Man, Darwin (1871) used Simiadae to include not only Catarrhini but also Platyrrhini (New World monkeys): “The Simiadae then branched off into two great stems, the New World and Old World monkeys [including apes]; and from the latter, at a remote period, Man, the wonder and glory of the Universe, proceeded” (213).
Below Henrietta Darwin’s clearly discernible text, her father notes in almost illegible pencil something concerning Ludwig Rütimeyer’s ideas on New and Old World monkeys, probably referring to his paper on Eocene Swiss mammals that also included primates (Rütimeyer 1862). Darwin also notes Jean Albert Gaudry’s association of Semnopithecus and Macaca, most likely from the paper in which he describes fossils of the extinct monkey Mesopithecus from Greece (Gaudry 1862). There is a final comment at the very bottom about the fossil ape Dryopithecus from France, which was named and described by Édouard Lartet in 1856. Based on these dates, the document was written in at least 1862 but possibly before 1865, because the latter is the year in which the English biologist St. George Jackson Mivart published one of the earliest primate phylogenetic trees (see chapter 5).
Darwin’s simple primate evolutionary tree on the reverse side incorporates elements from all these authors, except for Mivart’s publications, which appeared later (figure 4.15). “Man,” “Gibbon Man,” and “Orang Gibb” form a four-way split with another bifurcating branch that bears no name. This, in turn, joins a branch that bifurcates into one branch leading to “Semnopith” and the other to “Macaca.” Below where these two branches join is written “Dryopithecus.” Why Darwin placed Dryopithecus below Old World monkeys rather than below apes as described by Lartet is unclear, given that he wrote of it as being an ape about the size of a human in The Descent of Man (1871; Begun 2009). Below this branch Darwin wrote “Old World,” with the right branch labeled “New World,” both of which probably refer to the two main lineages: Old World monkeys, apes, and humans, and New World monkeys.
FIGURE 4.15 A simple phylogenetic tree, drawn by Darwin after 1862 and possibly before 1865 but not published (Cambridge University Library MS.DAR.80.B118v), showing general relationships among primates. (Reproduced by kind permission of the Syndics of Cambridge University Library)
The second, more elaborate, and better-known primate evolutionary tree bears the date April 21, 1868 (figure 4.16). It represents the last known phylogenetic tree that Darwin produced and shows the evolutionary relationships of major groups of living primates. This is rightfully termed a rough sketch, but the roughness comes mostly from its execution and less from Darwin’s uncertainty about relationships (Pietsch 2012). At first glance, the overall messiness of the sketch suggests indecisiveness on Darwin’s part as to primate relationships. Deconstructing the tree shows this is not the case. With one possible exception, Darwin knew quite well the relationships he wished to show; he simply struggled with how best to show them, especially given his general lack of artistic expertise.
Although speculative, a likely inspiration for this tree came from two trees of primate evolution published by Mivart in 1865 and 1867 (see figure 5.1). Mivart was an early convert to Darwin’s arguments that natural selection was the driving force of evolution, but he soon reversed himself in large part because of his devout Catholic faith, though still believing that evolution had occurred. Because of both his antiselectionist views and his continued support of evolution, he was shunned not only by Darwinists but by the Catholic Church as well. Mivart’s tree published in 1865 was based on the axial skeleton (vertebral column), and his tree published in 1867 was based on the appendicular skeleton (fore- and hind limbs). Darwin’s primate tree was sketched in 1868.
In a letter dated December 9, 1867, Darwin thanked Mivart for his “Memoir on the Append. skeleton of the Primates,” which he says he is glad to receive at present “as I am now attending to some point in the natural history of man.” No copy of this reprint is known in Darwin’s preserved collections, but a partially uncut and lightly annotated copy of the journal is known, and Darwin (2005:469) cites Mivart in The Descent of Man (1871). Additionally, Darwin (2008) wrote to Mivart on April 6, 1868, and apparently saw him during his stay in London from March 3 to April 1, 1868. It is conceivable that they discussed primate relationships, for Darwin produced his primate tree less than a month later. Although the relationships of the primates are quite different in the three trees (compare figures 4.16 and 5.1), and incidentally Darwin’s tree reflects more closely our current understanding of primate relationships, Darwin’s tree and Mivart’s two are strikingly similar in their stick-like form. Further, Mivart specifically used two names for groups of Old World monkeys, Semnopithecinae and Cynopithecinae (now considered obsolete names), that also appear on Darwin’s primate tree.
Darwin drew his primate tree in brown ink, likely starting from the top of the tree and working down or at least from the middle, rather than from the bottom up, because the first layer of the sketch is clearly composed of dashed marks joining three main branches, which for convenience I will call the human branch on the left, the three ape branches in the middle, and the OWm (Old World monkey) branch on the right (figure 4.17A). Despite the untidy juncture of the three main branches, the human and ape branches join just above the OWm branch. A line appears to stray from the base of human branch across to the OWm branch, but given its closeness to the base is likely a simple slip of the pen. Dashes join the three central ape branches as a three-way split in two places, farther up the tree and also farther down, but the same tripartite ape occurs in both versions. On the right for the OWm branch, the dashes form a single line. At this early stage, Darwin had likely written “Man.” horizontally above the leftmost dashed branch. Possibly, he next inked over all three of the main branches with solid lines (see figure 4.17B). On the far left “Man.” was crossed out and “Homo” was written vertically in lighter ink above it, possibly at the same time that he wrote “Hylobates” vertically on the leftmost of the central branches, followed in the middle by a lightly written vertical “Orangutan” with a vertical “Gorilla & Chimpanzee” on the rightmost central branch. It is likely at this time that he added the two versions of bifurcating solid lines to the rightmost OWm branch and “Semnopithecus” above the leftmost of these OWm branches. Because the dashed lines do not appear to go farther down the tree, it seems that the solid ink lines in the lower part of the tree were added at this time. Darwin appears next to have written vertically “Old World Monkeys” along the main branch and probably “New World Monkey” horizontally along a right diagonal line. Below this, a right solid branch was marked “Lemuridae” in light ink, finally with “Primates” written vertically near the base. Why Darwin added the left branch labeled “Old World Monkeys” provides the one mystery about this tree. Was he simply sloppy in placing them in two places, did he see these as a group of monkeys separate from the genera of Old World monkeys he listed, or did he perceive of Old World monkeys as a larger group also giving rise to apes and humans? In the probably earlier tree in figure 4.15, he used only the terms “Old World” and “New World.”
FIGURE 4.16 A phylogenetic tree, drawn by Darwin in 1868 but not published (Cambridge University Library MS.DAR.80.B91r), showing specific hypotheses of relationships among various groups of primates. (Reproduced by kind permission of the Syndics of Cambridge University Library)
FIGURE 4.17 A deconstruction and reassembly of the tree shown in figure 4.16. The letters A, B, and C refer to the three discernible stages of construction of the tree based on personal observation.
Darwin then had a change of heart about the arrangements of some names. From left to right on the top branches, he crossed out “Homo” and wrote “Man” twice; “Gorilla & Chimp” replaced a crossed-out “Hylobates”; “Orang-utan” was written over “Orangutan”; “Hylobates” replaced a scratched-out “Gorilla & Chimpanzee”; “Cercopithecus, Macacas, Baboons” is written over “Semnopithecus”; and “Semnopithecus” is written on the rightmost branch (see figure 4.17C). Below this, solid lines nearly obliterate “Old World Monkeys,” to be replaced by “Old World Monkeys” with a line pointed at a left side branch. Four additional unlabeled branches obliterate “New World Monkey,” which is replaced by the same phrase below with a line pointed at this branch. Finally, “Lemuridae” is written over by the same word on the lowest right branch, and the vertical “Primates” is crossed out and replaced with a horizontal “Primates.” Various lines were further darkened. By rewriting the sequence from left to right as “Man Gorilla & Chimp Orang-utan Hylobates Cercopithecus, Macacas, Baboons Semnopithecus,” Darwin almost certainly thought that he showed the degree of relatedness to humans going from colobine, or leaf-eating, monkeys (Semnopithecus) on the right through to apes and humans on the left. Not readily apparent to Darwin’s contemporaries was that the ordering of names in this manner does not indicate relationship; rather, the relative branching sequence indicates relationship. Unfortunately this error remains quite common. Voss (2010) repeats this mistake when referring to Darwin’s primate phylogeny: “The evolutionary diagram thus makes two statements about human beings: that gorillas and chimpanzees are our closest living relatives; and second, that humans represent just one part of the primate tree” (182). Whereas the first statement is not true, I agree with her second conclusion (also Pietsch 2012), but this was not, as she states, peculiar to Darwin. In 1866, for example, Ernst Haeckel published a family tree of mammals in which humans are shown as simply another branch of primates, just as Darwin did two years later in his sketch tree of primates. Very likely, Darwin knew of this tree, as he possessed a copy of Haeckel’s book in his library (Cambridge University Library 1961). It is true, as Voss notes, that Haeckel (1874) also produced trees in which humans clearly sat astride the tree of life. Indeed, on the reverse side of Darwin’s tree sketch in figure 4.16, he scribbled, “Arrangement as far as I can make out by comparing the work views of Huxley various naturalists as in whose judgment much reliance can be placed—For myself I have no clues whatever to form an opinion” (Cambridge University Library MS.DAR.80.B91v).
In no manner does Darwin’s tree sketch elevate humans above other apes. In his diagram, Darwin correctly did group apes and humans together to the exclusion of the two main branches of cercopithecids, or Old World monkeys. Today. the hypothesis of the evolutionary branching sequence for humans and apes can be written as (gibbon(orangutan(gorilla(human + chimp)))). The parentheses circumscribe ever smaller and more terminal branches, as if one has sliced through the tree. Darwin did not know, as we do today, that chimps and humans share an ancestor, to the exclusion of other apes. He did provide the correct relative branching positions of New World monkeys and lemurs, as had other authors. As noted, the only truly incongruous branch and label is for Old World monkeys; what he meant in this instance, if he knew, remains unclear.
This evolutionary tree, which at first looks rather helter-skelter, is in fact a quite coherent exercise of how Darwin saw man’s place in nature, at least as interpreted from others’ work. Darwin (1871) never published any version of this tree, but as is well known, he correctly predicted the birthplace of the human lineage: “In each great region of the world the living mammals are closely related to the extinct species of the same region. It is therefore probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man’s nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere” (199).
Process Rather Than Pattern
What general conclusions might we draw about Darwin’s various trees, all but one unpublished in his lifetime? None of the trees, except arguably the two competing about mammal origins and primate relationships, intended to show patterns of evolution for specific taxa. Rather, for Darwin the process, not the pattern, of evolution took precedence in his tree sketches. Although interested in relationships of major groups, Darwin instead used his sketches to help him work out how he perceived evolution to operate. It was not until the next generation of biologists, notably Darwin’s acolyte Haeckel, that we see a clear attempt to show the pattern of evolution. These are, of course, what we most often visualize as “trees of life”—the true order of nature. As Darwin writes in On the Origin of Species (1859), “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” (129). Darwin was correct, and much more was to come, even in his lifetime.
