The evolutionary version of von Baer’s law suggests that embryos may give us better clues about ancestry than adults—but not because they represent ancestral adults in miniature, as Haeckel and the recapitulationists believed. Rather, embryos indicate ancestry because generalized features of large groups offer better clues than do specialized traits of more-restricted lineages. In a standard example, some parasites become so anatomically degenerate as adults that they retain no distinctive traits of their larger affiliation—as, for example, in the parasitic barnacle Sacculina that, as an adult, becomes little more than an amorphous bag of feeding and reproductive tissue within the body of its crab host. But the larval stages that must seek and penetrate a crab can hardly be distinguished from the early stages of ordinary barnacles. Darwin made the key point succinctly when he stated in The Origin of Species that “community in embryonic structure reveals community of descent.”
Von Baer’s law makes good sense—but nothing in Darwinian theory implies or requires its validity, while evolution itself clearly permits embryology to proceed in either direction (or in no linearized manner at all)—either from embryonic similarity to adult discordance (as in groups that follow von Baer’s principle), or from larval discordance to adult likeness (as in several invertebrate groups, notably some closely related sea urchin species, where larvae have adapted to highly different lifestyles of planktonic floating versus development from yolk-filled eggs that remain on the sea floor, while the highly similar adults of both species continue to live and function like ordinary sea urchins).
The “bottom line”—to use a popular phrase from another walk of human life—may now be simply stated: the validity and relative frequency of von Baer’s law remains an open, empirical question within evolutionary theory, an issue that can only be resolved by observational evidence from a wide variety of organisms. Moreover, this issue has become quite important in the light of current excitement over recent advances in genetics that have finally allowed us to identify and trace the genes regulating early development. In this crucial and valid context, Richardson wisely chose to reevaluate our complacency about the probable validity of von Baer’s law.
Richardson realized that the continuing republication of Haeckel’s fraudulent figures might be tipping our beliefs in von Baer’s favor for indefensible reasons of inherited and unquestioned tradition (based on falsified drawings, to boot), rather than by good observational evidence. He therefore called attention to this likely source of unrecognized bias as he marshaled several colleagues to make the basic observations that could resolve a truly open question, falsely regarded by many colleagues as an issue decided long ago, partly on the basis of Haeckel’s doctored evidence.
The jury will be out for some time as they debate, and actively research, this important issue, too long neglected, in the sciences of natural history. But the 1997 paper of Richardson and six colleagues has already poked some important holes in the old and (as we now learn) poorly documented belief in early embryonic similarity among related lineages, followed by increasing disparity toward adulthood. The early embryonic stages of vertebrates are not nearly so similar as Haeckel’s phony drawings had led us to believe. For example, at the stage that Haeckel chose for maximal similarity, the somite count (number of vertebral segments) of actual vertebrate embryos ranges from eleven for a Puerto Rican tree frog to sixty for a “blind worm” (the common name for an unfamiliar group of limbless amphibians with a basically snakelike adult form). Moreover, although Haeckel drew his embryos as identical in both size and form, actual vertebrate embryos at their stage of maximal anatomical similarity span a tenfold range in body size.
In short, the work of Richardson and colleagues goes by a simple and treasured name in my trade: “good science.” The flap over Haeckel’s doctored drawings should leave us feeling ashamed about the partial basis of a widely shared bias now properly exposed and already subjected to exciting new research. But Haeckel’s high Victorian (or should I say Bismarckian) misdeeds provide no fodder to foes of Darwin, or of evolution—although we should feel sheepish (and well instructed) about our belated obedience to a grand old motto: Physician, heal thyself.
In other words, to give von Baer and Agassiz a final due, we need not fear the first and second stages of a scientific revolution because we will fight like hell (perhaps unwisely and too well, but at least with gusto) so long as we regard a new idea as either ridiculous or opposed to “religion” (that is, to conventional belief). But we must beware the dreaded third stage—for when we capitulate and then smugly state that we knew it all along, we easily fall into the greatest danger of all—arrogant complacency—because we have ceased to question and observe. And no situation in science could possibly be more abscheulich—atrocious!
23
Tales of a Feathered Tail
ONE FINE DAY, OR so THE LEGEND PROCLAIMS, JOSEPH Stalin received a telegram from his exiled archrival, Leon Trotsky. Overjoyed by the apparent content, Stalin rounded up the citizenry of Moscow for an impromptu rally in Red Square. He then addressed the crowd below: “I have received the following message of contrition from Comrade Trotsky, who has obviously been using his Mexican retreat for beneficial reflection: ‘Comrade Stalin: You are right! I was wrong! You are the leader of the Russian people!’”
But as waves of involuntary applause rolled through the square, a Jewish tailor in the front row—Trotsky’s old school chum from yeshiva days—bravely mounted the platform, tapped Stalin on the shoulder, and took the microphone to address the crowd. “Excuse me, Comrade Stalin,” he said. “The words, you got them right; but the meaning, I’m not so sure.” Then the tailor read the telegram again, this time with the intended intonation of disgust and the rising inflection of inquiry: ‘“Comrade Stalin: You are right?? I was wrong?? You are the leader of the Russian people??’”
Velociraptor, a ground-running, meat-eating dinosaur from the Gobi, may well have been covered in feathers.
I have never been able to regard this joke with equanimity, because I can’t help wondering what happened to the poor tailor, who undoubtedly suffered far more than Trotsky, albeit anonymously. But I value the tale as a lesson about the importance of context. We may get every word right but, in a pungent military acronym (the superlative degree of SNAFU, I have been assured by army grammarians), still get the meaning FUBAR (defined by the dictionary, in genteel terms, as “fouled [euphemism] up beyond all recognition”). As an unfortunate, yet eminently understandable, consequence of its central status in biology and its challenging implications for our view of human origins and history, evolution probably exceeds all other scientific subjects in featuring straightforward facts enshrouded in difficult or ambiguous meanings.
The popular understanding of evolution includes at least two false assumptions, so widely shared and so deeply (if unconsciously) embedded in the context of conventional explanations that many plain facts, easily grasped at a superficial level of overt recitation, almost always enter the public discourse of newspapers, films, and magazines in a highly confused form that “science writers” either mistake for the actual opinions of scientists or, more cynically, choose to present as the literary equivalent of “easy listening” for succor in drive-time traffic jams.
In the picture conveyed by these two related fallacies, evolution becomes, first of all, the transformation of one kind of entity into another, body and soul. So fish evolve into amphibians in a “conquest” of the land, and apes leave the safety of trees, eventually to become human by facing the dangers of terra firma with a weapon in their liberated hand and a fresh twinkle of insight emanating from an enlarged organ behind their eye. In the second component of this transformational view, descendants win victory from the heart of their valor in the face of natural selection—for “later” must mean “better,” as the land yields to explorational metaphors of conquest or colonization while the African savannas, for the first time in planetary history, ring with sounds of progress now expressed in t
he voice of real language.
But evolution proceeds by the branching of bushes, not by the morphing of one form into another, with the old disappearing into the triumph of the new. Novelties begin as little branches on old trees, not as butterflies of Michael Jordan refashioned from the caterpillar components of Joe Airball. Moreover, most novelties, at least at their origin, grow as tiny twigs of addition to persisting and vigorous bushes, not as higher realizations of ancestors that literally gave their all to a transcendence of their former grubby selves.
Amphibians and all their descendants have done well enough on land, but fins beat feet on the vertebrate bush, where the majority of twigs (species) sprout among fishes. I do not deny the transient success, and interesting novelties, of humans. But Homo sapiens occupies only one twig on a modest primate bush of some two hundred species, and even our most distantly related sub-groups, in both evolutionary and geographic terms (say, the San of southern Africa and the Sami of northern Finland), show very little genetic divergence, whereas two populations of the same species of chimpanzee, separated by only a few hundred miles of African real estate, have evolved many more genetic differences, one from the other. (This initially surprising fact makes evident sense once we recast our conceptions in properly bushy terms. All living humans descended from common ancestors who lived in Africa less than 200,000 years ago—despite our subsequent spread throughout the world. The two chimpanzee populations may have remained in geographical proximity, but they split from a common ancestor far longer ago, thus providing much more time for the evolution of genetic differences in the separated groups.)
Finally, and at the broadest scale, we will grasp the principle that novelty arises by branching and not by the wholesale transformation of all ancestors into better descendants only when we recognize that bacteria still constitute most of life’s tree—including the entire basal trunk that they built by themselves at life’s cellular origin—and that all multicellular kingdoms occupy just a few, if admittedly quite healthy, branches at the terminus of a single bough.
Many of my essays stress this theme of mentally liberating bushes versus constraining ladders because I believe that no other misconception so skews public understanding of evolution. I have treated a variety of topics under this rubric: why the air bladders of fishes evolved from lungs and not vice versa, as nearly everyone assumes (including Darwin himself in this case); why the cramming of primates into a halfway corner, and not at a triumphant terminus, of a linear walk through the hall of fossil mammals in New York’s Museum of Natural History makes such revolutionary sense; and why the “out of Africa” theory (on the origin of all modern humans from a recent population of African ancestors) and not the multiregional theory (of our threefold parallel origin from ancestral Homo erectus populations in Europe, Africa, and Asia) represents conventional evolutionary thought based on origin by branching and not the iconoclastic shock featured in most press reports, which have also misconstrued the truly peculiar and theoretically unlikely multiregional theory as transformational orthodoxy.
But I have heretofore desisted in applying this favorite theme to serious public misunderstandings of the apparently accurate claim that birds descended from dinosaurs—probably because I don’t like to attack generalities head-on but prefer the path of insinuation by small but fascinating tidbits and also because dinosaurs really are just a tad overexposed and scarcely need more publicity from this student of snails. But my tidbit just arrived in the professional literature, thus permitting a tale about the bushy reform of avian origins at two levels: first the dreaded generality and then the tidbit.
1. The basic relationship of birds and dinosaurs. I don’t mean to toss any cold water upon the almost surely valid claim, and one of the most interesting conclusions of late-twentieth-century paleontology, that birds descended from a lineage of small-bodied, bipedal dinosaurs. But the conventional interpretive “take” on this accurately stated fact could benefit from a flat-out dousing, if only because the gain in general understanding of evolution might more than compensate the loss of a charming but truly misleading characterization of a fact.
I should point out, first of all, that the basic claim does not justify the feelings of surprise or weirdness conveyed by most popular accounts. Birds did not evolve from massive sauropods or antediluvian, tanklike ankylosaurs or even from the large tyrannosaurs (which do, in fact, lie fairly close to birds on the dinosaur bush). Rather, birds branched off from a lineage of small, two-legged, meat-eating, running dinosaurs—full members of the group by proper criteria of descent (hence the validity of the one-liner that “birds evolved from dinosaurs”), but scarcely a version calculated to evoke either the fear or the power associated with our usual icon of dinosaurian immensity.
Moreover, the assertion of this evolutionary linkage during the past twenty years does not mark a stunningly new and utterly surprising discovery, but rather reaffirms an old idea that seemed patently obvious to many paleontologists in Darwin’s day (notably to T. H. Huxley, who defended the argument in several publications), but then fell from popularity for a good reason based on an honest error.
The detailed anatomical correspondence between Archaeopteryx, the earliest toothed bird of Late Jurassic times, and the small running dinosaurs now known as maniraptors (and including such popularized forms as Deinonychus and Troodon) can hardly fail to suggest a close genealogical relationship. But following Huxley’s initial assertion of an evolutionary link, paleontologists reached the false conclusion that all dinosaurs had lost their clavicles, or collar-bones—a prominent component of bird skeletons, where the clavicles enlarge and fuse to form the furcula, or wishbone. Since complex anatomical structures, coded by numerous genes working through intricate pathways of development, cannot reevolve after a complete loss, the apparent absence of clavicles in dinosaurs seemed to preclude a directly ancestral status to birds, though most paleontologists continued to assert a relationship of close cousinhood.
The recent discovery of clavicles in several dinosaurs—including the forms closest to birds—immediately reinstated Huxley’s old hypothesis of direct evolutionary descent. I don’t mean to downplay the significance of a firm resolution for the evolutionary relationship between birds and dinosaurs, but for sheer psychological punch, the revivification of an old and eminently sensible idea cannot match the impact of discovering a truly pristine and almost shockingly unexpected item in nature’s factual arsenal.
But I throw my iciest pitcher of water (a device to open eyes wide shut, not an intended punishment) into the face of a foolish claim almost invariably featured—usually as a headline—in any popular article on the origin of birds: “Dinosaurs didn’t die out after all; they remain among us still, more numerous than ever, but now twittering in trees rather than eating lawyers off John seats.”
This knee-jerk formulation sounds right in the superficial sense that buttresses most misunderstandings in science, for the majority of our errors reflect false conventionalities of hidebound thinking (conceptual locks) rather than failures to find the information (factual lacks) that could resolve an issue in purely observational terms. After all, if birds evolved from dinosaurs (as they did) and if all remaining dinosaurs perished in a mass extinction triggered by an impacting comet or asteroid 65 million years ago—well, then, we must have been wrong about dinosaurian death and incompetence, for our latter-day tyrannosaurs in the trees continually chirp the New Age message of Jurassic Partk, life finds a way. (In fact, as I write this paragraph, a mourning dove mocks my mammalian pretensions in a minor key, from a nest beneath my air conditioner. Sic non transit gloria mundi!)
Only our largely unconscious bias for conceiving evolution as a total transformation of one entity into a new and improved model could buttress the common belief that canonical dinosaurs—the really big guys, in all their brontosaurian bulk or tyrannosaurian terror—live on as hawks and hummingbirds. For we do understand that most species of dinosaurs just died, plain and simple, withou
t leaving any direct descendants. Under a transformational model, however, any ancestral bird carries the legacy of all dinosaurs at the heart of its courageous persistence, just as the baton in a relay race embodies all the efforts of those who ran before.
However, under a corrected branching model of evolution, birds didn’t descend from a mystical totality but only from the particular little bough that generated an actual avian branch. The dinosaurian ancestors of birds lie among the smallest bipedal carnivores (think of little Compsognathus, tragically mistaken for a cute pet in the sequel to Jurassic Party—creatures that may be “all dinosaur” by genealogy but that do not seem so functionally incongruous as progenitors of birds. Ducks as direct descendants of Diplodocus (a sauropod dinosaur of maximal length) would strain my credulity, but ostriches as later offshoots from a dinosaurian line that began with little Oviraptor (a small, lithe carnivore of less than human, and much less than ostrich, height) hardly strains my limited imagination.
As a second clarification offered by the branching model of evolution, we must distinguish similarity of form from continuity in descent: two important concepts of very different meaning and far too frequent confusion. The fact of avian descent from dinosaurs (continuity) does not imply the persistence of the functional and anatomical lifestyle of our culture’s canonical dinosaurs. Evolution does mean change, after all, and our linguistic conventions honor the results of sufficiently extensive changes with new names. I don’t call my dainty poodle a wolf or my car a horse-drawn carriage, despite the undoubted ties of genealogical continuity.
To draw a more complex but precise analogy in evolutionary terms: Mammals evolved from pelycosaurs, the “popular” group of sail-backed reptiles often mistaken for dinosaurs in series of stamps or sets of plastic monsters from the past. But I would never make the mistake of claiming that Dimetrodon (the most familiar and carnivorous of pelycosaurian reptiles) still exists because I am now typing its name, while whales swim in the sea and mice munch in my kitchen. In their descent from pelycosaurs, mammals evolved into such different creatures that the ancestral name, defined for a particular set of anatomical forms and functions, no longer describes the altered descendants. Moreover, and to reemphasize the theme of branching, pelycosaurs included three major subgroups, only two bearing sails on their backs. Mammals probably evolved as a branch of the third, sailless group. So even if we erroneously stated that pelycosaurs still lived because mammals now exist, we could not grant this status to a canonical sail-backed form, any more than we could argue for brontosaurian persistence because birds descended from a very different lineage of dinosaurs.