December 3, 2010 | 32
It never fails. Whenever scientists announce the discovery of a hitherto unknown fossil species intermediate between two already known forms there is always one newspaper or magazine that calls it a "missing link". Score another point for evolutionary science—another gap in the fossil record has been filled in.
I hate the phrase "missing link". It immediately sends up a red flag in my mind, and is almost always a sure indicator that the person employing it has only a very superficial understanding of the way evolution works. To understand why this is, however, we have to inspect the intellectual baggage that the phrase carries with it.
Long before the explosion of evolutionary ideas during the mid-19th century, scholars of various stripes arranged all of nature according to an intricately graded scale called the Great Chain of Being. (Arthur Lovejoy’s book by the same name remains one of the most comprehensive studies of this concept through history.) Rooted in the ruminations of Plato and Aristotle, but most famously adopted by Medieval theologians and Renaissance thinkers, this concept ranked the natural world into a static hierarchy which elucidated the character of the Almighty. The chain was not a ramshackle assemblage of minerals, plants, and animals, but instead a carefully ordered sequence of increasing complexity and closeness to God in which our species occupied a crucial hinge point – animal in body but infused with a divinely-prescribed soul.
There could be no gap in this linear sequence. Given God’s wisdom and grace, every link in the chain had to be filled in. This caused some rather hairy dilemmas. Prior to the discovery of the great apes by Western naturalists during the 18th century, there was nothing between the vulgar monkeys and us. Strange "wild men" captured in western Africa and seen in the jungles of Indonesia eventually filled this gap—what we now call orangutans, gorillas, chimpanzees, and bonobos—and were comfortably slotted in between us and "lower" primates. They were exceptionally humanlike in body, but could not speak or reason, and hence were cast as near-humans which lacked souls.
By the dawn of the 19th century, however, the Great Chain of Being was no longer useful as an organizing concept. Naturalists were cataloguing such a wide diversity of species that they could not all be ranked one above the other. The hierarchy only worked if there were relatively few creatures which were disparate in character. The circulation of evolutionary hypotheses and speculation during the early 19th century also broke down the idea that nature was static, and of course we all know that in 1859 Charles Darwin took the controversial idea of evolution by natural selection and presented it as an idea which merited careful, thorough investigation. Darwin did not shoehorn life into constrained, straight-line hierarchies, but perceived innumerable, branching lineages which were constantly changing according to the exigencies of local ecology.
Fig. 1 – The single image in On the Origin of Species, depicting the branching nature of evolutionary change. From Darwin, 1859.
But the old imagery still held fast. By the late 19th century the order of the Great Chain had become impressed onto the geologic timescale and the appearance of life on Earth. (Just as fish were inferior to amphibians, for example, fish appeared before amphibians in Earth history.) Given the capricious nature of the fossil record, however, paleontologists did not have a complete record to investigate. There were significant geologic and evolutionary gaps in the history of life. Darwin knew this well and accounted for it in On the Origin of Species, but other naturalists took the lack of finely graded transitional series as a sign that evolution had occurred in rapid jumps or by some mechanism very different from natural selection.
It wasn’t all that long before potential transitional series began to be identified. During the 1870’s T. H. Huxley—the English anatomist who was evolution’s staunchest public advocate—presented plausible scenarios for the evolution of whales from land-dwelling carnivorous mammals, of horses from tiny, multi-toed ancestors, and of birds from dinosaurlike reptiles. Of these only the horse sequence was represented by relatively complete string of species which could be considered as ancestors and descendants, but Huxley underscored each case as a relatively straightforward, stepwise process of change along a linear evolutionary pathway.
This is the scientific context in which the phrase "missing link" became popular, especially in reference to human evolution. Darwin’s evolutionary theory predicted long strings of finely-graded species in the fossil record, and now that they knew what to look for paleontologists were beginning to find them. As was the case when the intellectual grip of the Great Chain remained firm, what existed in the shadowy place between monkey and human was of extreme interest. Were our ancestors really like the apes kept in the zoological gardens, or were they more of a "pre-sapiens" type—like us but rougher around the edges? The Neandertals, first discovered in Germany right around the time that evolutionary debate was ramping up in the 1860’s, were too close to us to fill that space. No one could agree whether they were a distinct species, a throwback, or pathological individuals of our own species. Instead the first fossil to be widely heralded as a missing link was "Java Man"—known to us today as Homo erectus—discovered by the Dutch physician Eugene Dubois in Indonesia. There was debate about this fossil just as there was about the Neandertals, but it was the first fossil with the right mix of "human" and "ape" characteristics to fit snugly in a straight-line progression of human evolution from primitive, brutish ancestors to modern humanity.
Stripped of its theological underpinnings, the image of minutely-graded, linear evolutionary transitions proliferated. There is perhaps no more direct way to illustrate macroevolutionary change than to lay out a series of transitional forms which document the change from the archaic into the modern. This kind of imagery is quick, easy, and powerful, but it also obscures the grander patterns of the fossil record which paleontologists have recognized for well over a century.
Fig. 2 – The traditional image of horse evolution from the archaic Eohippus [bottom] through the modern Equus [top]. From Matthew, 1926.
Let’s take the evolution of horses as an example. The evolution of the modern Equus from the diminutive Eohippus is one of the most iconic of all evolutionary transformations, and typically a series of multi-toed horses are slotted between the two following the pattern which Huxley and other naturalists laid out. But, as paleontologists have recognized since the beginning of the 20th century, there were too many fossil horses to fit into this geologic chain. Horse evolution was a branching process in which multiple species and genera lived alongside one another, some leaving descendants and others sinking into extinction. The technical literature on horses from the past century is full of branching evolutionary trees, yet the chosen depiction for books and museums—perhaps in response to the threat of creationism and Christian fundamentalism which reared its ugly head in 1920’s America—was of a linear series in which both the geologic and anatomical gaps had all been filled in. If a horse did not fit on the neat march towards modern Equus, it was simply left out as an aberrant side branch.
The dichotomy between what scientists know and what the public is presented with still hangs on now, even as research conducted within the past 30 years has shown horse evolution to be a tangled story which cannot be corralled into a straight-line narrative. Not only were there multiple radiations of horses over time, but there were even some reversals in which large lineages became dwarfed. There was no onward-and-upward march towards Equus. Instead, in what Stephen Jay Gould called "Life’s Little Joke," the only reason we can hold horses up as an evolutionary icon is that there is one genus left, making it easy to condense all those fossils into a linear pathway.
Fig. 3 – A modern (2005) vision of horse evolution. Any straight-line march of fossil horses would have to leave out numerous collateral relatives. From MacFadden, 2005.
Horses have not been the only groups of vertebrates to fall victim to this kind of imagery. Since it is the standard mode of evolutionary storytelling, the origins of the first tetrapods from fish, the earliest birds from dinosaurs, of whales, of elephants, and—of course—humans have all been cast this way. The cases of feathered dinosaurs and early humans, especially, underscore the struggle between what paleontologists have found and the traditional narrative of linear change.
An influx of new fossils and the development of new techniques have revolutionized our understanding of bird origins during the past decade. The origin of birds is much more complicated than the straightforward transformation of a single lineage of small, feathered, predatory dinosaurs into flying avians as has often been portrayed. Many traits once thought to be unique to birds and their direct ancestors—such as feathers and complicated sets of air sacs which leave telltale pockets on skeletons—have been found to be widespread among dinosaurs. Just last year, in fact, paleontologists found that Tyrannosaurus rex—a distant cousin of the dinosaurs which were ancestral to the first birds—suffered infestations of a microorganism which bored holes into the jawbones of their hosts and are still commonly seen in living hawks and pigeons. Rather than dinosaurs being birdlike, it is perhaps more proper to say that living birds are dinosaurlike, and we cannot hope to achieve a comprehensive understanding of avian evolution without more fully understanding dinosaurian evolution as well.
Fig. 4 – The lower jaws of a hawk [left] and a Tyrannosaurus [right] showing lesions created by the microorganism Trichomonas gallinae (or a similar species in the case of Tyrannosaurus). From Wolff et al., 2009.
Human evolution has also been traditionally depicted in an upward push towards modernity, as if we had been driving our own evolution all along. The most famous depiction of this—adapted from an illustration in the 1965 Time–Life book Early Man—is called the "March of Progress". It is exactly what it sounds like—a parade of human evolution from an archaic ape to a tool-wielding modern human.
Yet we know human evolution never took such a direct path. As with horses, during the past six million years there have been multiple species of human living on the planet at any one time. It is truly unusual that there is only one species today, and the rapid discovery of new species has shown the human family tree to be rather bushy. Even our own direct ancestry was marked by branching events. In 2007 a team of paleontologists lead by Fred Spoor announced that fossil evidence from Ileret, Kenya showed that Homo habilis and Homo erectus—long regarded as two successive stages in our own ancestry—had overlapped with each other for a few hundred thousand years.
News reports crowed this discovery rewrote our evolutionary history—a bit of boilerplate which is trotted out almost anytime anything new is discovered about human evolution—but it actually was perfectly consistent with what we know from other fossil transitions. As outlined by Gould and Niles Eldredge in their theory of punctuated equilibrium, descendant species can rapidly branch off from an ancestral stock which is going through a period of little change (stasis). The pattern this creates is of ancestral and descendant species overlapping in time. In this case, Homo erectus branched off from a population of Homo habilis and both coexisted for a time before the ancestral species became extinct, and depicting their relationship as simply a straight line blinds us to this branching trend. (Of course, the alternative scenario of both Homo species having branched off independently from an earlier common ancestor is also possible, but there is no hard evidence to support this hypothesis and it would still be best understood within a more tangled evolutionary scenario.)
Given all that we have learned about evolutionary patterns in the fossil record, why do we keep returning to the same outdated imagery?
At the beginning of the 20th century, American fundamentalism was gaining momentum and the public circus that was the Scopes trial turned the teaching of evolution into a controversial public issue. Now, at the beginning of the 21st century, anti-scientific opposition to evolution remains a prominent cultural force. Be it straight-up young-Earth creationism or its insidious sibling intelligent design, fundamentalism-fueled views of science and nature abound. Groups such as the National Center for Science Education are continually tracking the spread of anti-evolution agendas which would further erode the quality of scientific understanding. Perhaps this is why we keep returning to the March of Progress. When the fossils and stratigraphy are laid out so plainly, how can any reasonable person deny that evolution is a reality? Yet, by preferring this antiquated mode of imagery, we may have hamstrung ourselves. Given all that we have gleaned about evolution from the fossil record—especially the major pattern of contingent radiations cut back by extinction before bursting into numerous splendid forms all over again—why not bring this wonderful "tangled bank" imagery to the public?
Of course the problem is not just one of imagery or the way in which science education has suffered. Anti-science attitudes are influenced by religion, politics, and other sociocultural factors which make it impossible to offer up any single antidote. Nevertheless, we have to do more than simply say "We have the fossils; we win." If we become so focused on rhetoric and winning an argument that we strip evolutionary changes from their full context, then we are only setting ourselves up for failure as public presentations of evolution continue to drift from what we truly understand.
This is a boom time for paleontology. New species are being discovered and described at an astonishing rate and techniques from molecular biology, genetics, embryology, geochemistry, and other disciplines are allowing us insights into prehistoric life which have never been possible before. (See David Sepkoski and Michael Ruse’s recently-published The Paleobiological Revolution for an excellent overview of these changes to the discipline.) The age when museums would send out their best bone hunters for top museum-quality specimens alone is over. We are now in a time when paleobiology is more vibrant than it has ever been. This reinvigorated science is providing the detailed context by which to understand life as it is today, and it is essential in understanding what G. G. Simpson once called the "tempo and mode of evolution." We should take full advantage of this, and step one involves casting out types of imagery which constrain rather than enlighten. I, for one, would not miss the phrase "missing links" at all.
Darwin, C. 1859. On the Origin of Species by Means of Natural Selection. London: John Murray.
Eldredge, N., and Gould, S.J. 1972. Punctuated equilibria: An alternative to phyletic gradualism. Models in Paleobiology. San Francisco: Freeman, Cooper and Company. pp 82–115
Gould, S. J. 1991. Bully for Brontosaurus. New York: W.W. Norton and Company. pp. 168–181
Howell, F. C. 1965. Early Man. New York: Time Life Books. pp. 41–45
Huxley, T. H. 1870. The anniversary address of the president. Quarterly Journal of the Geological Society, London, XXVI: xxix–lxiv
Lovejoy, A. 1964. The Great Chain of Being. Cambridge: Harvard University Press.
MacFadden, Bruce. 2005. Fossil horses—Evidence for evolution. Science 307: 1728–1730
Matthew, W. D. 1926. The Evolution of the Horse: A Record and Its Interpretation. The Quarterly Review of Biology, 1: 139–185.
Sepkoski, D., and Ruse, M. 2009. The Paleobiological Revolution. Chicago: University of Chicago Press.
Simpson, G. G. 1944. Tempo and Mode in Evolution. New York: Columbia University Press
Spoor, F.; Leakey, M. G.; Gathogo, P. N; Brown, F. H.; Anton, S. C.; McDougall, I.; Kiarie, C.; Manthi, F. K.; Leakey, L. N. 2007. Implications of new early Homo fossils from Ileret, east of Lake Turkana, Kenya. Nature 448: 688–691
Wolff, E.; Salisbury, S.; Horner, J.; Varricchio, D. 2009. Common avian infection plagued the tyrant dinosaurs. PLoS One 4: e7288 http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0007288
About the author: Brian Switek is a freelance science writer and a paleontology research associate at the New Jersey State Museum. He has written articles on paleontology for a variety of popular and academic publications—from the London Times to Evolution: Education and Outreach—and he presently blogs at WIRED Science’s Laelaps and Smithsonian magazine’s Dinosaur Tracking. His first book—Written in Stone: Evolution, the Fossil Record, and Our Place in Nature—has just been published by Bellevue Literary Press. To keep up with Brian’s latest work, follow him on Twitter @Laelaps or friend him on Facebook.
The views expressed are those of the author and are not necessarily those of Scientific American.
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