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Aristotle's Ladder, Darwin's TreeThe Evolution of Visual Metaphors for Biological Order$

J. Archibald

Print publication date: 2014

Print ISBN-13: 9780231164122

Published to Columbia Scholarship Online: November 2015

DOI: 10.7312/columbia/9780231164122.001.0001

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The Paragon of Animals

The Paragon of Animals

(p.200) Chapter Eight The Paragon of Animals
Aristotle's Ladder, Darwin's Tree

J. David Archibald

Columbia University Press

Abstract and Keywords

This chapter examines how perceptions of our place in nature evolved over time as we discovered and attempted to understand the vast living and nonliving world that surrounds and defines us. How the vast majority of people view humans' place in nature, even when accepting that evolution occurred, can be summed up by a quotation in William Shakespeare's Hamlet: “What a piece of work is a man! How noble in reason, how infinite in faculty! In form and moving how express and admirable! In action how like an Angel! In apprehension how like a god! The beauty of the world! The paragon of animals!” This chapter looks back at the earliest tree-like attempts to visualize supposed racial differences as well as the ancestry of all living humans and proceeds by discussing Charles Darwin's evolutionary trees and the question of when humans departed Africa as well as its implication for the meaning of the human race. It concludes that evolution has not produced a clearcut phylogeny of humans or any group of plants or animals that would result in a neatly branching family tree.

Keywords:   nature, humans, evolution, animals, ancestry, Charles Darwin, evolutionary trees, Africa, human race, phylogeny

Where do we or, more broadly, where does our species, Homo sapiens, belong in this nexus of relationships? Are we the pinnacle of creation espoused by the ancients, and not so ancients, or do we fit as one small twig on the vast tree (or web) of evolutionary relationships? In large measure, one’s worldview dictates the answer; but in spite of what we as individuals believe about this vexing issue, our understanding of our place in nature has expanded in the past two thousand years almost beyond comprehension, to the point that we now realize that we constitute but a minuscule part of life on Earth. Some deny this, some revel in it, but most do not even think about it. To a great extent, the perceptions of our place in nature evolved over time as we discovered and attempted to understand the vast living and nonliving world that surrounds and defines us.

What metaphor best describes our place in the profusion of animate nature? It depends. Are we these unique creatures cast in God’s image, just below the angels? We recall Hamlet’s musing in act 2, scene 2, of Shakespeare’s Hamlet (or similarly in the rock-opera Hair): “What a piece of work is a man! How noble in reason, how infinite in faculty! In form and moving how express and admirable! In action how like an Angel! In apprehension how like a god! The beauty of the world! The paragon of animals!” This is certainly how the vast majority of people view humans’ place in nature, even when accepting that evolution occurred. Not much has changed in our overall perceptions of ourselves stretching back to the Aristotelian scala naturae, although as figured, this concept of humans in a biological context dates from only the eighteenth century with Charles Bonnet’s (1745, 1781) representations of humans at the top of the ladder (see figure 1.4) or the stairway (see figure 1.5). Even the first attempts at placing humans in a tree figure were of a nonevolutionary sort, such as those by Carl Edward von Eichwald in 1829 (see figure 3.3) and Edward Hitchcock in 1840 (see figure 3.11).

(p.201) Our philosopher Hamlet’s stirring words may treat humanity as the “paragon of animals,” but it does not end there. In fact, this praise is a sham: “And yet to me, what is this quintessence of dust? Man delights not me, no, nor woman either.” Although Shakespeare’s intent lies elsewhere for poor mad Hamlet, it does serve the warning that biologically humans really are not the pinnacle of evolution whether at the top of the ladder or perched in the tree, except possibly in our minds and in our definitions. We are but a twig on the bush of human evolutionary history that becomes less and less certain with the discovery of each new fossil hominid and the completion of each new genetic sequence. For many this is troubling, but for many others it is liberating.

Grades, Clades, and Races

Humans, as do most other species, attempt to differentiate “us from them.” Some birds use song to distinguish between neighbors, dogs may sniff their canine encounters, but humans as visually oriented mammals largely use sight to tell differences among humans. No wonder, then, that humans from their very beginnings visually recognized both physical and cultural distinctions among one another. One of the earliest tree-like attempts at visualizing supposed racial differences as well as the ancestry of all living humans dates from at least the early seventeenth century, often as an engraving in early copies of the King James Bible. This representation is biblical, not scientific, showing in an unmistakable tree form the sons of Noah—Japheth, Shem, and Ham (Iaphet, Sem, and Cham in figure 8.1A)—giving rise to peoples of Europe, Asia, and Africa, respectively. In this scene, the ark rests on Mount Ararat, Armenia (present-day Turkey), with the tree springing from Noah and his ark. The “Hebrewes” at the top are likely shown as the Chosen People of the Hebrew Bible. Although a proverbial kernel of truth might be found in such a tree, it purports to show the ancestry of all humankind coming from Noah.

Such a view of human history within the Western tradition lasted well into the nineteenth century, but among many educated people it began to crumble by time of the publication of On the Origin of Species. Critics quickly lambasted Darwin for toppling humans from the pinnacle of creation. The only problem was that Darwin (1859) never mentioned human origins in this book, other than his cryptic sentence on the third to last page: “Light will be thrown on the origin of man and his history” (488). Not until The Descent of Man, and Selection in Relation to Sex (1871) did he broach the topic when he presciently predicted, “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).

The only known evolutionary trees including humans that Darwin produced were his unpublished sketchs (see figures 4.15 and 4.16), the version in 1868 possibly from the influence of St. George Mivart’s (1865, 1867) published primate trees (see figure 5.1). Nevertheless, Darwin’s (1859) volume opened the floodgates. Only six (p.202)

The Paragon of Animals

Figure 8.1 (A) Noah and his descendants (ca. 1611); (B) Ernst Haeckel’s “Family Tree of Monkeys with Humans,” from Natürliche Schöpfungsgeschichte (1868).

months later, Thomas Henry Huxley and Samuel Wilberforce proved to be the most vociferous participants in a heated discussion in Oxford when Wilberforce purportedly asked whether Huxley’s ancestry from apes came through his mother’s or his father’s side, and Huxley retorted something to the effect that he would rather claim an ape as an ancestor than someone such as Wilberforce who abused his considerable debating powers on such important matters. Whatever the truth of the actual exchange, a number of Huxley’s published essays, especially in 1861 and 1862, spelled out his views on human ancestry.

In 1863, Huxley’s slim, 159-page Evidence as to Man’s Place in Nature appeared. It included thirty-two figures, possibly the best known being the frontispiece (see figure 1.6A). Not calling it a progression, he later writes of species lying below humans on a scale of evolution. He does not present any phylogenetic trees. Although a considerable amount of the book’s content appeared in earlier essays, Evidence reached an even wider audience. In the book, Huxley clearly aligns humans with apes. He explains that through embryological development, humans, (p.203) as with all animals, start life as a fertilized egg that proceeds through various stages of vertebrate development and ends in a form far closer to apes than to any other animal. Also in 1863, Darwin’s geological mentor and friend Charles Lyell published Geological Evidences of the Antiquity of Man, which examined early humans. Unlike Huxley’s book, which discussed humans within a biological context, Lyell’s dealt with early humans in Europe in more of an archaeological context. Although Lyell had come around to accepting some aspects of Darwin’s descent with modification by means of natural selection, he remained, as Janet Browne (2002) notes, a “reluctant evolutionist.”

The younger Darwinian adherents St. George Mivart and Ernst Haeckel first attempted to place humans in the context of evolutionary trees. The earliest such human-inhabited trees belong to Mivart (1865, 1867), who includes only living primates based on the axial and appendicular skeletons, respectively (see figure 5.1), and to Haeckel (1866), whose mammal tree includes humans and the fossil ape Dryopithecus (see figure 5.4). Haeckel unmistakably places Dryopithecus on the human lineage in his tree, yet in his classification it belongs to the same family as living great apes. This ambiguity becomes somewhat clearer in his text when he notes that although it can be classified with other living apes, “the recently discovered Dryopithecus fossil represents a very important form between the gorilla and man” (2:155). Haeckel hedges his bets here. He continues this theme on the next page, again noting that whereas Dryopithecus may be classified as an ape, it cannot be excluded from human ancestry.

Dryopitheus presents an interesting history. Éduoard Lartet (1801–1871) named and described it in 1856 as a fossil ape, but not a human relative, based on a partial limb bone, some teeth, and a lower jaw from a site in southwestern France. Today, various species of Dryopithecus occur at sites in Europe, Africa, and Asia, possibly dating from 12 to 9 mya, and although placed among apes it begs the question of if or how it may relate to human ancestry, given that chimps and humans share an ancestor perhaps between 13 and 7 mya based on current genomic and fossil data (Begun 2009).

Haeckel (1866) includes in his classification of the human family (Erecta) the genus Homo along with Pithecanthropus, a then undiscovered, hypothetical ancestor. To my knowledge, this represents Haeckel’s first mention of this genus, at least in a book. In Natürliche Schöpfungsgeschichte (The History of Creation, 1868:514), Haeckel hypothesized species names such as Pithecanthropus primi-genus, or Homo primigenus, and the better-known Pithecanthropus alalus, which he includes in his very stylized “Family Tree of Monkeys with Humans” (see figure 8.1B). He further hypothesized that fossils of these forms might be discovered in what is now Indonesia. Amazingly, following Haeckel’s predictions, in 1891 the Dutchman Eugène Dubois (1858–1940) recovered fossil remains in Java that he named Pithecanthropus erectus, or Java Man, now known as Homo erectus. In addition to placing Pithecanthropus as ancestral to Homo, Haeckel divides “speaking man” (Homo) into “wooly-haired” (Ulotriches) and “straight-haired” (Lissotriches) modern humans. Late in the same book, Haeckel further divides wooly-haired and straight-haired humans into ten species and twenty races. In the volume’s final figure, Haeckel presents a family tree of his human species and races. He does not stop here, for in the second edition of Natürliche Schöpfungsgeschichte (p.204) (1870) he further divides humans into twelve species and thirty-six races. The tree shown in figure 8.2A comes from E. Ray Lankester’s translation (Haeckel 1876). Recall that Haeckel (1870) presented his map of human migration using a modified tree-like form originating in his hypothetical Lemuria in the Indian Ocean (see figure 5.7).

Cast in today’s sensibilities and understanding of species concepts, Haeckel’s ideas might be offensive as well as absurd. Within his era, however, Haeckel’s views on human races varied only slightly from the societal norms for both evolutionists and nonevolutionists. In addition, Haeckel has been portrayed as a harbinger or even an enabler of Nazi racial propaganda, a charge that Robert Richards (2008) ably rebuts in his biography of Haeckel. As to the notion of multiple species of humans, even in Haeckel’s day this was not a widely held view. Part of the problem, which still plagues us, concerns the more general issue of what constitutes a species. For example, throughout On the Origin of Species, Darwin struggled with differentiating between varieties and species. Given this, the importance of Haeckel derives from his being one of the first scientists to attempt to trace human evolution, even

The Paragon of Animals

Figure 8.2 Haeckel’s (A) “Pedigree of the Twelve Species of Men,” from The History of Creation (1876), and (B) figural tree showing an orangutan, a chimpanzee, a gorilla, and a human, from Anthropogenie; oder, Entwickelungsgeschichte des Menschen (1874).

(p.205) if quite fancifully and often tinged with racism. This culminates in Anthropogenie; oder, Entwickelungsgeschichte des Menschen (Anthropogeny; or, Evolutionary History of Man, 1874). It is in this volume that we find Haeckel’s famous “Stammbaum des Menschen” (Genealogical Tree of Humans), his “great oak” (see figure 5.9).

In this book as well as his earlier works, Haeckel argues in the text and shows on his trees the origin of humans as having occurred as a singular event, or what we today call monophyly. Yet in the text, he confounds what we know as grades versus clades of evolution, the problem that later scientists such as (Alfred Sherwood Romer 1933, 1971) struggled with in deciding between horizontal (grades) or vertical (clades) classification (see figure 6.8C). For Haeckel (1874), the best-known and disturbing example comes in a figural tree showing an orangutan, a chimpanzee, a gorilla, and a human labeled “Neger,” a German word now often regarded as an ethnic slur (see figure 8.2B). In all of Haeckel’s trees and elsewhere in his text, he claims monophyly for humans, yet an English translation of the text explaining the diagram reads, “Both the African Manlike Apes [referring in his text to gorillas and chimpanzees] are black in colour, and like their countrymen, the Negroes, have the head long from back to front (dolichocephalic). The Asiatic Man-like Apes are, on the contrary mostly of a brown, or yellowish brown colour, and have the head short from back to front (brachycephalic), like their countrymen, the Malays and Mongols” (1876:180–81).

In its repulsive absurdity, this particular example shows us that Haeckel and some of his contemporaries too often did not realize the inconsistency in confusing grades and clades in evolution. It had a special grip on those biologists such as Haeckel who saw no problems couching racist views under the aegis of science. Even seemingly progressive ideas of improving humankind quickly turned ugly because of value judgments that people necessarily placed on various “desirable” attributes. Such was the case for the concept of eugenics, a term coined by Francis Galton (1822–1911) in Inquiries into Human Faculty and Its Development (1883): “We greatly want a brief word to express the science of improving stock, which is by no means confined to questions of judicious mating, but which, especially in the case of man, takes cognisance of all influences that tend in however remote a degree to give to the more suitable races or strains of blood a better chance of prevailing speedily over the less suitable than they otherwise would have had. The word eugenics would sufficiently express the idea” (25). For what was to come in the twentieth century, this was chillingly prescient.

Galton was a cousin of Charles Darwin, who for a number of years entertained the misguided idea that he could apply principles of animal breeding to humans. Although at first seemingly well meaning, it fostered some of the most heinous acts against humanity. Accordingly, I find a tree-like symbol used as the logo for the Second International Eugenics Congress, held at the American Museum of Natural History in New York in 1921, especially apropos in its portent (figure 8.3A). Henry Fairfield Osborn, president of the American Museum of Natural History, presided at this congress. Haeckel died in 1919, before the founding of the National Socialist Party, and thus we do not know how he might have viewed the ascendance of the Nazis. We do know, however, that Osborn supported both Mussolini’s and Hitler’s “racial hygiene” programs (Rainger 1991).

(p.206) The idea of northern European racial superiority permeated not only Osborn’s social views but his science as well, in this way not unlike Haeckel. Osborn’s book Men of the Old Stone Age: Their Environment, Life and Art (1915) fairly represented our knowledge at the time, as well as fairly showing racial attitudes. Osborn’s views on the origin of humans, however, found little traction. As discussed in chapter 5, he was no fan of Darwin, instead invoking his neo-Lamarckian aristogenesis, including the inevitability that this unseen force would result in humans. In keeping with these views, Osborn argued that although humans shared ancestry with the apes, the human lineage had a long and separate history stretching back into the Oligocene epoch. In support of this idea, he lauded the importance of the Late Pliocene Eanthropus dawsoni, known to posterity as “Piltdown Man.” As were many scientists at the time, Osborn was taken in by the Piltdown hoax and even included it on “Ancestral Tree of the Anthropoid Apes and of Man,” showing the human lineage dating back to the Oligocene (see figure 8.3B). A number of new fossil hominid species as well as new material of known species since Haeckel’s work some forty years earlier adorn Osborn’s aristogenetic upward march to Homo sapiens. These fossil species also occur in the figure that he describes as a “tree showing the main theoretic lines of descent of the chief Pre-Neolithic races discovered in Western Europe” (see figure 8.3C). Note the major racial categories “Narrow heads” and “Broad heads” that further subdivide the human lineages. Osborn indicates that these branches of Homo sapiens known from the Upper Paleolithic likely separated one from another in the Lower Paleolithic of Asia, likely based on the aforementioned finds that had been made there only a few decade earlier.

Our Split with Chimps: Possibly Older Than We Thought

As discussed in chapter 7, fully twelve years earlier, in 1904, George Nuttall had produced a monographic analysis of blood sera from a variety of animals. His repeated phylogeny by Dubois from 1896 (see figure 7.6A) shows a split between apes and human ancestors in the mid-Miocene. Although the timing of the split was not at issue for him, Nuttall indicated that his blood serum work exhibited a similar pattern of splitting among monkeys, apes, and humans. Both the timing of these splits (unlike Osborn’s estimates) and the pattern of splitting held fairly constant until about the 1960s. When dramatic ideas of human evolution did arrive, they occurred because of fieldwork, first in southern and then in eastern Africa starting in the 1920s. In 1925, Raymond Dart (1893–1988) described and named Australopithecus africanus based on the partial skull of a young individual. It was largely dismissed as an ape by the scientific establishment because it occurred in Africa rather than Europe or Asia, where other hominids were known; its brain size fell (p.207)

The Paragon of Animals

Figure 8.3 (A) Tree-like symbol used as the logo for the Second International Eugenics Congress, held at the American Museum of Natural History in New York in 1921; Henry Osborn’s (B) “Ancestral Tree of the Anthropoid Apes and of Man” and (C) “tree showing the main theoretic lines of descent of the chief Pre-Neolithic races discovered in Western Europe,” from Men of the Old Stone Age (1915).

(p.208) in the range of apes, not early humans; and someone outside the scientific inner circle had described it. This all began to change with the recovery of a number of other specimens of australopithecines from South Africa and then in the Rift Valley of East Africa beginning the 1950s, largely through the tenacity of Louis Leakey (1903–1972) and his second wife, Mary (1913–1996). Many paleoanthropologists have followed in the footsteps of Dart in South Africa and the Leakeys in East Africa. Although consensus building among paleoanthropologists may be like herding cats, something like seven to nine species of Australopithecus are now generally recognized, ranging in age from about 4 to 2 mya. How they relate to one another; to even earlier possible hominids, dating to perhaps 7 mya; and to the origin of Homo remains in dispute. Of interest here is how they have been portrayed in evolutionary trees to answer particularly the questions of when the human and chimpanzee lineages separated from each other and when and how humans migrated from Africa.

When did the chimpanzee and human lineages split? Recall that until 1963, the commonly accepted wisdom held that chimpanzees and gorillas formed a clade with humans as their sister group, such as shown by George Gaylord Simpson (1963) based on fossils, and that this split could be as old as 12 mya (see figure 7.7B and C). In the same year (and publication), however, Morris Goodman used molecular techniques to argue that these three lineages formed an undifferentiated tritomy (see figure 7.7A). Further, in 1967 Vincent Sarich and Allan Wilson, also using molecular techniques, argued that this three-way split dated to about 5 mya (see figure 7.6B). In 1984 and again in 1987, Charles Sibley and Jon Ahlquist resolved this tritomy, once again with molecular techniques, to show that chimpanzees and humans were each other’s nearest relatives and that the split was between 7.7 and 6.3 mya. In the space of about a hundred years, then, evolutionary trees with humans as the pinnacle of creation changed to trees with humans as the sister clade of chimpanzees. Further, the staggering array of pre-australopithecines, australopithecines, and early Homo coming from Africa, even if their relationships could not and still cannot be sorted out, proved Darwin correct: humans’ origins were to be found in Africa among their nearest ape relatives.

The newest twist for the split of the chimpanzee and human lineages comes from fossil material as well as molecular data. The often provocative weekly magazine New Scientist ran a cover story in November 2012 by one of its editors, Catherine Brahic, with the teasing title “Our True Dawn” and the intriguing argument that humans have been human for a lot longer than we thought. The two accompanying comparative tree figures present the dilemma (figure 8.4A). (p.209)

The Paragon of Animals

Figure 8.4 (A) Old and new views of the split between humans and chimpanzees, with the earlier dates for the fork in the evolutionary road having significant consequences for our understanding of the human family tree, redrawn after Catherine Brahic’s “Our True Dawn” (2012); (B) Rebecca Cann, Mark Stoneking, and Allan Wilson’s U-shaped mitochondrial phylogeny, based on 147 people drawn from five geographic populations, supporting the ancestry from one African woman 200,000 years ago, from “Mitochondrial DNA and Human Evolution” (1987); (C) Stoneking’s four models for the origin and spread of modern humans, from “Human Origins” (2008).

([B] and [C] reproduced with permission of the Nature Publishing Group)

(p.210) The old view dates the human–chimp split between 6 and 4 mya, based mostly on the molecular data that argued for around seventy-five mutations in genomes between each generation. The species Australopithecus afarensis of the fossil Lucy fame, which almost all accept as being a literally upright early representative of the human lineage at almost 4 mya, bangs up against the youngest molecularly based dates for the human–chimp split. But the problem with the molecular data, according to the paleoanthropologists, is that they bracket quite possible to very likely pre-australopithecine members of the human lineage from Central and East Africa back as far as 7 mya (see “possible human ancestors” in figure 8.4A). Enter new direct measurements of generational mutation rates of change that may be as low as thirty-six per generation, or half the original seventy-five mutations for older estimates. For both molecular data and fossils, the split now can be pushed back to between 8 and 7 mya, although a minority of paleoanthropologists argue for as early as 13 mya. In the teaser for the article, Brahic writes that the argument for the timing of when humans and chimps split appears to be on the verge of being settled. It certainly makes for good press, but I would not hold my breath, given the perceived importance of the issue.

Leaving Home: A Recent Departure from Africa

The next and more widely known revelation does not entail our deepest roots but our much more recent heritage: When did we depart Africa? And what does this imply about the meaning of human races? A paper published in 1987 by Rebecca Cann, Mark Stoneking, and Allan Wilson proclaimed, “Mitochondrial DNAs from 147 people, drawn from five geographic populations have been analysed by restriction mapping. All these mitochondrial DNAs stem from one woman who is postulated to have lived about 200,000 years ago, probably in Africa. All the populations examined except the African population have multiple origins, implying that each area was colonised repeatedly” (31). So there was an Eve after all, and she hailed from Africa 200,000 years ago. Although the paper did not say it specifically, the implication emerged that all living humans could be traced to this one woman, or at least to the population to which she belonged. Being based on mitochondrial DNA, which is inherited only through the maternal lineage, the study could not say from where the father hailed. This first attempt is shown in figure 8.4B in a nice upside-down U-shaped but still discernible branching tree. One of the more curious aspects of this tree is how the different geographic groups cluster, or rather do not cluster in many instances. None of the geographic groups forms a cluster totally separate from any other. Cann and her colleagues found that the tree first breaks into one branch that includes only Africans, whereas the other branch includes Africans as well as non-Africans (see figure 8.4B), which leads the authors to conclude that the common mitochondrial ancestor was African. What does this say about the usually perceived racial groupings based on aspects of skin color, hair texture, skull shape, and facial features? This study and others the authors cite indicate that various molecularly based differences can be greater within usually identified races than between them. Some African maternal (p.211) lineages share a more recent ancestry with non-Africans than with Africans, as the authors’ tree suggests. Thus much of what we attribute to race might be inferred more as holdovers of some wider ancestral traits and less from those with whom we share more recent ancestry.

The results of this and similar studies did not escape critics, most notably those who supported what came to be called the Multiregional Hypothesis (for example, Wolpoff et al. 1988) for the origin of modern humans versus what became known as the Out of Africa or African Eve Hypothesis. Before the latter hypothesis appeared, the unchallenged idea was that Homo erectus originated in Africa some 2 mya, subsequently spreading throughout much of the Old World. Following this, these archaic forms remained an interbreeding population, and through gene flow and local adaptation this Old World population evolved from Homo erectus to archaic Homo sapiens and then to modern humans. In the late 1960s and into the early 1970s, some even thought that the australopithecines constituted only one or a few species (for example, Wolpoff 1968) that fit into a neat straight-line evolutionary portrayal, such as Rudolph Zallinger presented in F. Clark Howell’s Early Man (1965) (see figure 1.8A). This soon disabused idea melted away as more and more different australopithecines turned up in the fossil record. Even as evidence mounted, we could not rid ourselves of the idea that we represent this singular evolutionary progression rather than the reality that we are but one twig on the tree, even though most of the rest had been hacked away by extinction—possibly through some of our agency.

Out of the numerous hypotheses for the origin and spread of modern humans, Stoneking (2008) presents succinct tree models for the four most clearly articulated hypotheses. He identifies these as the candelabra, multiregional evolution, replacement, and assimilation models (see figure 8.4C). Stoneking regards the candelabra as perhaps the oldest model, which prevailed for decades. According to this model, the common human ancestor dates back to some 2 mya, with modern humans arising separately and probably in considerable isolation in Africa, Europe, Asia, and Australasia. As Stoneking notes, this view helped foster racist ideas. He identifies specifically Carleton Coon (1904–1981), but the even earlier ideas of Osborn fall within this hypothetical framework. Within this racist scenario, Europeans had a longer time in which to evolve, whereas Africans were the last to do so.

Stoneking’s three other models share the now strong evidence that until between 2 and 1.5 mya, human evolution took place in Africa. Following this, ancestral humans began a series of migrations throughout the Old World, but when and how they occurred is in dispute. In the multiregional evolution model, the ancestral humans left Africa and spread out in the Old World, but as Stone-king shows by the small horizontal arrows, considerable migration and gene flow occurred during this long interval so that throughout the Old World humans evolved in concert, although local conditions certainly caused differing adaptational modifications (see figure 8.4C, upper right). Stoneking’s replacement model, by contrast, argues that modern humans arose some 300,000 to 200,000 years ago, spreading out of Africa between 100,000 and 50,000 years ago and replacing the local populations as they migrated (see figure 8.4C, lower left). This represents one form of the Out of Africa model. The fourth and final model, assimilation, differs in that it argues that some evidence suggests that the process did not (p.212) involve solely replacement, but that as modern humans spread from Africa, they on occasion interbred with indigenous archaic humans (see figure 8.4C, lower right). A study showing that non-Africans share about 2 to 3 percent of their genome with Neanderthals (Currat and Excoffier 2011) bolsters the idea that as modern humans migrated out of Africa, they may have mated with Neanderthals. Other explanations suggest that this small percentage may be a genomic holdover that was shared by the last Neanderthal and human ancestor before that common ancestor departed from Africa, even before the more recent exodus of modern humans. Whichever, if any, of these hypotheses proves correct, the evidence that we can hang on the family tree shows that, in a very real sense, we are all Africans.

As these four hypotheses show, evolution has not produced a nice neat phylogeny of humans or, for that matter, any group of plants or animals resulting in a neatly branching family tree. Evolution, as with life in general, is complex and inconveniently messy. When viewed backward through a telescope of geologic time, evolutionary patterns might seem discrete episodes of splitting and resplitting with the twain never meeting, but even our best, albeit myopic, visual metaphors are just that—metaphors.