By Stephen Jay Gould

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the evolution of life on earth The history of life is not necessarily progressive; it is certainly not predictable. The earth’s creatures have evolved through a series of contingent and fortuitous events By Stephen Jay Gould

ome creators announce their inventions with grand imally conspicuous place at the very end of his introduction: “I éclat. God proclaimed, “Fiat lux,” and then flooded am convinced that Natural Selection has been the most impor- his new universe with brightness. Others bring forth tant, but not the exclusive, means of modification.” great discoveries in a modest guise, as did Charles Darwin in defining his new mechanism of evolu- Reality versus Conceit Stionary causality in 1859: “I have called this principle, by which NATURALSELECTION is not fully sufficient to explain evo- each slight variation, if useful, is preserved, by the term Natur- lutionary change for two major reasons. First, many other caus- al Selection.” es are powerful, particularly at levels of biological organization Natural selection is an immensely powerful yet beautifully both above and below the traditional Darwinian focus on or- simple theory that has held up remarkably well, under intense ganisms and their struggles for reproductive success. At the low- and unrelenting scrutiny and testing, for 135 years. In essence, est level of substitution in individual base pairs of DNA, change natural selection locates the mechanism of evolutionary change is often effectively neutral and therefore random. At higher lev- in a “struggle” among organisms for reproductive success, lead- els, involving entire species or faunas, punctuated equilibrium ing to improved fit of populations to changing environments. can produce evolutionary trends by selection of species based (Struggle is often a metaphorical description and need not be on their rates of origin and extirpation, whereas mass extinc- viewed as overt combat, guns blazing. Tactics for reproductive tions wipe out substantial parts of biotas for reasons unrelat- success include a variety of nonmartial activities such as earlier ed to adaptive struggles of constituent species in “normal” and more frequent mating or better cooperation with partners times between such events. in raising offspring.) Natural selection is therefore a principle of Second, and the focus of this article, no matter how ade- local adaptation, not of general advance or progress. quate our general theory of evolutionary change, we also yearn Yet powerful though the principle may be, natural selection to document and understand the actual pathway of life’s his- is not the only cause of evolutionary change (and may, in many tory. Theory, of course, is relevant to explaining the pathway cases, be overshadowed by other forces). This point needs em- (nothing about the pathway can be inconsistent with good the- phasis because the standard misapplication of evolutionary theory, and theory can predict certain general aspects of life’s geo- ory assumes that biological explanation may be equated with logic pattern). But the actual pathway is strongly underdeter- devising accounts, often speculative and conjectural in practice, mined by our general theory of life’s evolution. This point needs about the adaptive value of any given feature in its original en- some belaboring as a central yet widely misunderstood aspect vironment (human aggression as good for hunting, music and of the world’s complexity. Webs and chains of historical events religion as good for tribal cohesion, for example). Darwin him- are so intricate, so imbued with random and chaotic elements, self strongly emphasized the multifactorial nature of evolu- so unrepeatable in encompassing such a multitude of unique tionary change and warned against too exclusive a reliance on (and uniquely interacting) objects, that standard models of sim- natural selection, by placing the following statement in a max- ple prediction and replication do not apply.

92 SCIENTIFIC AMERICAN Reprinted from the October 1994 issue COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. MARK MC MENAMIN Mount Holyoke College sensitive dependence on minute and unmeasurablechimerical differences dream. in History includes tooany muchmoment, chaos,but theor natureextremely of universal knowcomplexity the position and motionshatters of all particlesthis in the cosmos at century, once said that he could echoingspecify the growingall andfuture confident statesdeterminism ifof thehe precisionbeforehand.anyPierre-Simonwith dictedlate Laplace, could 18th adequate, after a sequence of events unfolds, but it cannot be pre- us by possessing a notochord, or dorsal stiffening rod,small andis simpleamong swimming creature, showing its allegiance to mediate determination by timeless laws of nature. by long chains of unpredictable antecedenttory includesstates, toorather much thancontingency, im- or on tinyshaping and unknowable of disparitiespresent in startingresults points. And his- initial conditions, leading to massively divergent outcomes based animals died out before the Cambrian explosion of modern life. These thin, These life. modern of explosion Cambrian the before out died animals Ediacaran The ago. years million 600 some appeared which Ediacaran, called fauna, multicellular first earth’s the from organism prominent www.sciam.com SPECIMENS CONTAINING SLAB genus the phylum,chordate our of member one (Only all. at earth million years ago, then no vertebrates would have inhabitedtion of themulticellular animal life in the Cambrian explosion 530 eage had not been among the few survivors of the initial radia- four among a multitude: (1) If our inconspicuouspathway andthat wouldfragile not have lin-led to consciousness. alternative Toan on citehistory sentjust and differently occurred have outcome of thousands of linked events, any one of which plexity.could Humans arose, rather, as a fortuitous and contingent come based on themes of progress and increasing neural com- second ago, because evolutionary theory predicts such an out- Homo sapiens History can be explained, with satisfying rigor if evidence be Pikaia, has been found among these earliest fossils. This did not appear on the earth, just a geologic of Pteridinium from Namibia shows a shows Namibia from COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. AMERICAN, SCIENTIFIC 2004 COPYRIGHT life’s history on our planet to a paleontological examination of the contingent pattern of pathway, we must go beyond principles of evolutionary theory remarkable behavioral complexity. ically marginal and probably doomed to extinction the despitechimpanzee andtheir gorilla today, would have become ecolog- then our ancestry might have ended in a line of apes that, like the drying African savannas just two to four million years ago, a small lineage of primates had not evolved upright posture on tion that had prevailed for 100 million years previously). If (4) would still be dominant and mammals insignificant (the situa- had not struck the earth 65 million years ago, then dinosaurs er haveIf becomea largeterrestrial. extraterrestrial (3) body capable of bearing weight on land, then vertebrates might nev- finned fishes had not evolved finlobe- of group unpromising andbones small a If with(2) fauna.) brian a strong central axis the rarest fossils of the Burgess Shale, our best preserved Cam- gin of Species, objectivity. Darwin himself, in the closing lines of vailing “certainties,” however hard they may be all science is socially aimingembedded, and all scientists record pre- for pure and intended planetary steward. for a history culminating in humans as life’s highest expression est social traditions and psychological hopes of Western tionalculture deterministic models of Western science and to the deep- Such a view of life’s history is highly contrary both to conven- millions of plausible alternatives that happened not to occur. episodes, rather than by gradual improvement. improvement. gradual by than rather episodes, quirky and quick in move to tends life of history The life. multicellular in experiment failed ultimately and a separate represent also may but forms modern some to ancestral be may organisms sheetlike quilted, Therefore, to understand the events and generalities of life’s Science can, and does, strive to grasp nature’s factuality, but expressed Victorian social preference more than — the single actualized version among SCIENTIFIC AMERICAN SCIENTIFIC On the Ori- 93 Left wall of minimal complexity Precambrian

Bacteria because the earth’s surface became molten early in its history, a result of bombard- ment by large amounts of cosmic debris during the solar system’s coalescence and of heat generated by radioactive decay of short-lived isotopes. These oldest rocks Bacteria Present are too metamorphosed by subsequent heat and pressure to preserve fossils (although some scientists interpret the pro-

Frequency of Occurrence portions of carbon isotopes in these rocks as signs of organic production). The oldest rocks sufficiently unaltered to retain cellular fossils—African and Aus- tralian sediments dated to 3.5 billion Complexity years old—do preserve prokaryotic cells PROGRESS DOES NOT RULE (and is not even a primary thrust of) the evolutionary process. For reasons (bacteria and cyanophytes) and stroma- of chemistry and physics, life arises next to the “left wall” of its simplest conceivable and preservable tolites (mats of sediment trapped and complexity. This style of life (bacterial) has remained most common and most successful. A few bound by these cells in shallow marine creatures occasionally move to the right, thus extending the right tail in the distribution of complexity. Many always move to the left, but they are absorbed within space already occupied. waters). Thus, life on the earth evolved Note that the bacterial mode has never changed in position, but just grown higher. quickly and is as old as it could be. This fact alone seems to indicate an inevit- nature’s record in writing: “As natural prey through time). But laws of nature do ability, or at least a predictability, for selection works solely by and for the not tell us why we have crabs and snails life’s origin from the original chemical good of each being, all corporeal and at all, why insects rule the multicellular constituents of atmosphere and ocean. mental endowments will tend to progress world and why vertebrates rather than No one can doubt that more com- towards perfection.” persistent algal mats exist as the most plex creatures arose sequentially after Life’s pathway certainly includes complex forms of life on the earth. this prokaryotic beginning—first eu- many features predictable from laws of Relative to the conventional view of karyotic cells, perhaps about two billion nature, but these aspects are too broad life’s history as an at least broadly pre- years ago, then multicellular animals and general to provide the “rightness” dictable process of gradually advancing about 600 million years ago, with a relay that we seek for validating evolution’s complexity through time, three features of highest complexity among animals particular results—roses, mushrooms, of the paleontological record stand out in passing from invertebrates, to marine people and so forth. Organisms adapt to, opposition and shall therefore serve as vertebrates and, finally (if we wish, albeit and are constrained by, physical princi- organizing themes for the rest of this ar- parochially, to honor neural architecture ples. It is, for example, scarcely surpris- ticle: the constancy of modal complexity as a primary criterion), to reptiles, mam- ing, given laws of gravity, that the largest throughout life’s history; the concentra- mals and humans. This is the conven- vertebrates in the sea (whales) exceed the tion of major events in short bursts inter- tional sequence represented in the old heaviest animals on land (elephants to- spersed with long periods of relative sta- charts and texts as an “age of inverte- day, dinosaurs in the past), which, in bility; and the role of external impositions, brates,” followed by an “age of fishes,” turn, are far bulkier than the largest ver- primarily mass extinctions, in disrupting “age of reptiles,” “age of mammals,” tebrate that ever flew (extinct pterosaurs patterns of “normal” times. These three and “age of man” (to add the old gender of the Mesozoic era). features, combined with more general bias to all the other prejudices implied by Predictable ecological rules govern the themes of chaos and contingency, require this sequence). structuring of communities by principles a new framework for conceptualizing I do not deny the facts of the preced- of energy flow and thermodynamics and drawing life’s history, and this article ing paragraph but wish to argue that our (more biomass in prey than in predators, therefore closes with suggestions for a dif- conventional desire to view history as for example). Evolutionary trends, once ferent iconography of evolution. progressive, and to see humans as pre- started, may have local predictability dictably dominant, has grossly distorted (“arms races,” in which both predators The Lie of “Progress” our interpretation of life’s pathway by and prey hone their defenses and weap- THEPRIMARY paleontological fact falsely placing in the center of things a ons, for example—a pattern that Geerat about life’s beginnings points to pre- relatively minor phenomenon that arises J. Vermeij of the University of California dictability for the onset and very little for only as a side consequence of a physical- at Davis has called “escalation” and doc- the particular pathways thereafter. The ly constrained starting point. The most umented in increasing strength of both earth is 4.6 billion years old, but the old- salient feature of life has been the stabil-

crab claws and shells of their gastropod est rocks date to about 3.9 billion years ity of its bacterial mode from the begin- DAVID STARWOOD

94 SCIENTIFIC AMERICAN DINOSAURS AND OTHER MONSTERS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. ning of the fossil record until today and, with little doubt, into all future time so long as the earth endures. This is truly the “age of bacteria”—as it was in the beginning, is now and ever shall be. For reasons related to the chemistry of life’s origin and the physics of self- organization, the first living things arose Time at the lower limit of life’s conceivable, preservable complexity. Call this lower limit the “left wall” for an architecture of complexity. Because so little space exists between the left wall and life’s initial bacterial mode in the fossil record, only one Anatomical Diversity direction for future increment exists—toward greater complexity at the right. NEW ICONOGRAPHY OF LIFE’S TREE shows that maximal diversity in anatomical forms (not in number Thus, every once in a while, a more com- of species) is reached very early in life’s multicellular history. Later times feature extinction of most of these initial experiments and enormous success within surviving lines. This success is measured plex creature evolves and extends the in the proliferation of species but not in the development of new anatomies. Today we have more range of life’s diversity in the only avail- species than ever before, although they are restricted to fewer basic anatomies. able direction. In technical terms, the distribution of complexity becomes more left wall but still hold that evolution with- nore just as many lineages adapting strongly right skewed through these oc- in particular groups differentially favors equally well by becoming simpler in casional additions. complexity when the founding lineage form. The morphologically degenerate But the additions are rare and epi- begins far enough from the left wall to parasite, safe within its host, has just as sodic. They do not even constitute an evo- permit movement in both directions. Em- much prospect for evolutionary success lutionary series but form a motley se- pirical tests of this interesting hypothesis as its gorgeously elaborate relative cop- quence of distantly related taxa, usually are just beginning (as concern for the sub- ing with the slings and arrows of outra- depicted as eukaryotic cell, jellyfish, trilo- ject mounts among paleontologists), and geous fortune in a tough external world. bite, nautiloid, eurypterid (a large relative we do not yet have enough cases to ad- of horseshoe crabs), fish, an amphibian vance a generality. But the first two stud- Steps, Not Inclines such as Eryops, a dinosaur, a mammal ies—by Daniel W. McShea of the Uni- EVENIFCOMPLEXITY is only a drift and a human being. This sequence can- versity of Michigan on mammalian ver- away from a constraining left wall, we not be construed as the major thrust or tebrae and by George F. Boyajian of the might view trends in this direction as trend of life’s history. Think rather of an University of Pennsylvania on ammonite more predictable and characteristic of occasional creature tumbling into the suture lines—show no evolutionary ten- life’s pathway as a whole if increments of empty right region of complexity’s space. dencies to favor increased complexity. complexity accrued in a persistent and Throughout this entire time, the bacteri- Moreover, when we consider that for gradually accumulating manner through al mode has grown in height and re- each mode of life involving greater com- time. But nothing about life’s history is mained constant in position. Bacteria rep- plexity, there probably exists an equally more peculiar with respect to this com- resent the great success story of life’s path- advantageous style based on greater sim- mon (and false) expectation than the ac- way. They occupy a wider domain of plicity of form (as often found in para- tual pattern of extended stability and environments and span a broader range sites, for example), then preferential evo- rapid episodic movement, as revealed by of biochemistries than any other group. lution toward complexity seems unlikely the fossil record. They are adaptable, indestructible and a priori. Our impression that life evolves Life remained almost exclusively uni- astoundingly diverse. We cannot even toward greater complexity is probably cellular for the first five sixths of its his- imagine how anthropogenic intervention only a bias inspired by parochial focus on tory—from the first recorded fossils at might threaten their extinction, although ourselves, and consequent overattention 3.5 billion years to the first well-doc- we worry about our impact on nearly to complexifying creatures, while we ig- umented multicellular animals less than every other form of life. The number of Escherichia coli cells in the gut of each hu- STEPHEN JAY GOULD taught biology, geology and the history of science at Harvard Uni- man being exceeds the number of hu- versity from 1967 until his death in 2002 at age 60. The influential and provocative evo- mans that has ever lived on this planet. lutionary biologist had a Ph.D. in paleontology from Columbia University. Well known for One might grant that complexifica- his popular writings, in particular a monthly column in Natural History magazine, he was tion for life as a whole represents a the author of more than a dozen books, including Full House: The Spread of Excellence from T H E A U T H O R

DAVID STARWOOD pseudotrend based on constraint at the Plato to Darwin and The Mismeasure of Man.

600 million years ago. (Some simple brian time or by failure of unicellular life 2 multicellular algae evolved more than a to achieve some structural threshold act- billion years ago, but these organisms being as a prerequisite to multicellularity. long to the plant kingdom and have no More curiously, all major stages in or- genealogical connection with animals.) ganizing animal life’s multicellular archi- This long period of unicellular life does tecture then occurred in a short period be- 3 include, to be sure, the vitally important ginning less than 600 million years ago transition from simple prokaryotic cells and ending by about 530 million years without organelles to eukaryotic cells ago—and the steps within this sequence 5 with nuclei, mitochondria and other com- are also discontinuous and episodic, not plexities of intracellular architecture— gradually accumulative. The first fauna, but no recorded attainment of multicel- called Ediacaran to honor the Australian 6 lular animal organization for a full three locality of its initial discovery but now billion years. If complexity is such a good known from rocks on all continents, con- 4 thing, and multicellularity represents its sists of highly flattened fronds, sheets and initial phase in our usual view, then life circlets composed of numerous slender 9 certainly took its time in making this cru- segments quilted together. The nature of cial step. Such delays speak strongly the Ediacaran fauna is now a subject of against general progress as the major intense discussion. These creatures do not 7 theme of life’s history, even if they can be seem to be simple precursors of later 13 8 plausibly explained by lack of sufficient forms. They may constitute a separate

atmospheric oxygen for most of Precam- and failed experiment in animal life, or PATRICIA J. WYNNE 1. Vauxia (gracile) 11. Micromitra 22. Emeraldella 34. Sidneyia 10 2. Branchiocaris 12. Echmatocrinus 23. Burgessia 35. Odaraia 12 3. Opabinia 13. Chancelloria 24. Leanchoilia 36. Eiffelia 11 4. Amiskwia 14. Pirania 25. Sanctacaris 37. Mackenzia 5. Vauxia (robust) 15. Choia 26. Ottoia 38. Odontogriphus 6. Molaria 16. Leptomitus 27. Louisella 39. Hallucigenia 17 7. Aysheaia 17. Dinomischus 28. Actaeus 40. Elrathia 8. Sarotrocercus 18. Wiwaxia 29. Yohoia 41. Anomalocaris 9. Nectocaris 19. Naraoia 30. Peronochaeta 42. Lingulella 10. Pikaia 20. Hyolithes 31. Selkirkia 43. Scenella 21. Habelia 32. Ancalagon 44. Canadaspis 16 20 19 33. Burgessochaeta 45. Marrella 24 46. Olenoides 14 15

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COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. they may represent a full range of di- than variations on anatomical themes es- posed to this view allow that Cambrian ploblastic (two-layered) organization, of tablished during the Cambrian explosion diversity at least equaled the modern which the modern phylum Cnidaria within five million years. Three billion range—so even the most cautious opin- (corals, jellyfishes and their allies) remains years of unicellularity, followed by five ion holds that 500 million subsequent as a small and much altered remnant. million years of intense creativity and then years of opportunity have not expanded In any case, they apparently died out capped by more than 500 million years the Cambrian range, achieved in just five well before the Cambrian biota evolved. of variation on set anatomical themes million years. The Cambrian explosion The Cambrian then began with an as- can scarcely be read as a predictable, in- was the most remarkable and puzzling semblage of bits and pieces, frustrating- exorable or continuous trend toward event in the history of life. ly difficult to interpret, called the “small progress or increasing complexity. shelly fauna.” The subsequent main We do not know why the Cambrian Dumb Luck pulse, starting about 530 million years explosion could establish all major ana- MOREOVER, WEDONOT know why ago, constitutes the famous Cambrian ex- tomical designs so quickly. An “external” most of the early experiments died, while plosion, during which all but one modern explanation based on ecology seems at- a few survived to become our modern phylum of animal life made a first ap- tractive: the Cambrian explosion repre- phyla. It is tempting to say that the vic- pearance in the fossil record. (Geologists sents an initial filling of the “ecological tors won by virtue of greater anatomical had previously allowed up to 40 million barrel” of niches for multicellular organ- complexity, better ecological fit or some years for this event, but an elegant study, isms, and any experiment found a space. other predictable feature of convention- published in 1993, clearly restricts this pe- The barrel has never emptied since; even al Darwinian struggle. But no recognized riod of phyletic flowering to a mere five the great mass extinctions left a few spe- traits unite the victors, and the radical al- million years.) The Bryozoa, a group of cies in each principal role, and their oc- ternative must be entertained that each sessile and colonial marine organisms, do cupation of ecological space forecloses early experiment received little more not arise until the beginning of the sub- opportunity for fundamental novelties. than the equivalent of a ticket in the sequent, Ordovician period, but this ap- But an “internal” explanation based on largest lottery ever played out on our parent delay may be an artifact of failure genetics and development also seems nec- planet—and that each surviving lineage, to discover Cambrian representatives. essary as a complement: the earliest mul- including our own phylum of verte- Although interesting and portentous ticellular animals may have maintained a brates, inhabits the earth today more by events have occurred since, from the flow- flexibility for genetic change and embry- the luck of the draw than by any pre- ering of dinosaurs to the origin of human ological transformation that became dictable struggle for existence. The his- consciousness, we do not exaggerate greatly reduced as organisms “locked in” tory of multicellular animal life may be greatly in stating that the subsequent his- to a set of stable and successful designs. more a story of great reduction in initial tory of animal life amounts to little more Either way, this initial period of both possibilities, with stabilization of lucky internal and external flexibility yielded a survivors, than a conventional tale of GREAT DIVERSITY quickly evolved at the dawn of multicellular animal life during the Cambrian range of invertebrate anatomies that may steady ecological expansion and mor- period (530 million years ago). The creatures have exceeded (in just a few million years phological progress in complexity. shown here are all found in the Middle Cambrian of production) the full scope of animal Finally, this pattern of long stasis, Burgess Shale fauna of Canada. They include form in all the earth’s environments to- with change concentrated in rapid epi- some familiar forms (sponges, brachiopods) day (after more than 500 million years of sodes that establish new equilibria, may that have survived. But many creatures (such as the giant Anomalocaris, at the lower right, additional time for further expansion). be quite general at several scales of time largest of all the Cambrian animals) did not live Scientists are divided on this question. and magnitude, forming a kind of fractal for long and were so anatomically peculiar Some claim that the anatomical range of pattern in self-similarity. According to (relative to survivors) that we cannot classify this initial explosion exceeded that of the punctuated equilibrium model of spe- them among known phyla. modern life, as many early experiments ciation, trends within lineages occur by died out and no new phyla have ever accumulated episodes of geologically in- arisen. But scientists most strongly op- stantaneous speciation, rather than by gradual change within continuous pop- 41 ulations (like climbing a staircase rather than rolling a ball up an inclined plane). Even if evolutionary theory implied a potential internal direction for life’s pathway (although previous facts and argu- 43 44 45 46

SCIENTIFIC AMERICAN 97 COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. CLASSICAL REPRESENTATIONS OF LIFE’S HISTORY reveal the severe biases of another invertebrate, although they did not go away or stop evolving. viewing evolution as embodying a central principle of progress and When land vertebrates arise (panel 2), we never see another fish, even complexification. In these paintings by Charles R. Knight from a 1942 issue though return of land vertebrate lineages to the sea may be depicted of National Geographic, the first panel shows invertebrates of the Burgess (panel 3). The sequence always ends with mammals—even though fishes, Shale. But as soon as fishes evolve, no subsequent scene ever shows invertebrates and reptiles are still thriving—and, of course, humans. ments in this article cast doubt on such radically different in effect began with tologists had previously realized. These a claim), the occasional imposition of a the presentation of data by Luis and four properties encompass the radical rapid and substantial, perhaps even tru- Walter Alvarez in 1979, indicating that implications of mass extinction for un- ly catastrophic, change in environment the impact of a large extraterrestrial ob- derstanding life’s pathway as more con- would have intervened to stymie the pat- ject (they suggested an asteroid seven to tingent and chancy than predictable and tern. These environmental changes trigger 10 kilometers in diameter) set off the last directional. mass extinction of a high percentage of great extinction at the Cretaceous-Ter- Mass extinctions are not random in the earth’s species and may so derail any tiary boundary 65 million years ago. Al- their impact on life. Some lineages suc- internal direction and so reset the path- though the Alvarez hypothesis initially cumb and others survive as sensible out- way that the net pattern of life’s history received very skeptical treatment from comes based on presence or absence of looks more capricious and concentrated scientists (a proper approach to highly evolved features. But especially if the trig- in episodes than steady and directional. unconventional explanations), the case gering cause of extinction be sudden and Mass extinctions have been recog- now seems virtually proved by discovery catastrophic, the reasons for life or death nized since the dawn of paleontology; the of the “smoking gun,” a crater of appro- may be random with respect to the orig- major divisions of the geologic time scale priate size and age located off the Yu- inal value of key features when first were established at boundaries marked catán peninsula in Mexico. evolved in Darwinian struggles of nor- by such events. But until the revival of in- This reawakening of interest also in- mal times. This “different rules” model terest that began in the late 1970s, most spired paleontologists to tabulate the of mass extinction imparts a quirky and paleontologists treated mass extinctions data of mass extinction more rigorously. unpredictable character to life’s pathway only as intensifications of ordinary Work by David M. Raup, J. J. Sepkoski, based on the evident claim that lineages events, leading (at most) to a speeding up Jr., and David Jablonski of the Universi- cannot anticipate future contingencies of of tendencies that pervaded normal ty of Chicago has established that multi- such magnitude and different operation. times. In this gradualistic theory of mass cellular animal life experienced five ma- To cite two examples from the im- extinction, these events really took a few jor (end of Ordovician, late Devonian, pact-triggered Cretaceous-Tertiary ex- million years to unfold (with the appear- end of Permian, end of Triassic and end tinction 65 million years ago: First, an ance of suddenness interpreted as an ar- of Cretaceous) and many minor mass ex- important study published in 1986 not- NATIONAL GEOGRAPHIC tifact of an imperfect fossil record), and tinctions during its 530-million-year his- ed that diatoms survived the extinction they only made the ordinary occur faster tory. We have no clear evidence that any far better than other single-celled plank- (more intense Darwinian competition in but the last of these events was triggered ton (primarily coccoliths and radiolaria). tough times, for example, leading to even by catastrophic impact, but such careful This study found that many diatoms had more efficient replacement of less adapt- study leads to the general conclusion that evolved a strategy of dormancy by en- ed by superior forms). mass extinctions were more frequent, cystment, perhaps to survive through The reinterpretation of mass extinc- more rapid, more extensive in magnitude seasonal periods of unfavorable condi-

tions as central to life’s pathway and and more different in effect than paleon- tions (months of darkness in polar spe- CHARLES R. KNIGHT, COURTESY OF

98 SCIENTIFIC AMERICAN DINOSAURS AND OTHER MONSTERS COPYRIGHT 2004 SCIENTIFIC AMERICAN, INC. cies as otherwise fatal to these photosyn- tance. In Freud’s three examples, Coper- (later oceanic tableaux depict only such thesizing cells; sporadic availability of sil- nicus moved our home from center to pe- returning reptiles as ichthyosaurs and ica needed to construct their skeletons). riphery; Darwin then relegated us to “de- plesiosaurs). But fishes did not stop Other planktonic cells had not evolved scent from an animal world”; and, final- evolving after one small lineage managed any mechanisms for dormancy. If the ter- ly (in one of the least modest statements to invade the land. In fact, the major minal Cretaceous impact produced a of intellectual history), Freud himself dis- event in the evolution of fishes, the origin dust cloud that blocked light for several covered the unconscious and exploded and rise to dominance of the teleosts, or months or longer (one popular idea for a the myth of a fully rational mind. modern bony fishes, occurred during the “killing scenario” in the extinction), then In this wise and crucial sense, the Dar- time of the dinosaurs and is therefore diatoms may have survived as a fortu- winian revolution remains woefully in- never shown at all in any of these se- itous result of dormancy mechanisms complete because, even though thinking quences—even though teleosts include evolved for the entirely different function humanity accepts the fact of evolution, more than half of all species of verte- of weathering seasonal droughts in ordi- most of us are still unwilling to abandon brates. Why should humans appear at nary times. Diatoms are not superior to the comforting view that evolution means the end of all sequences? Our order of radiolaria or other plankton that suc- (or at least embodies a central principle primates is ancient among mammals, cumbed in far greater numbers; they of) progress defined to render the ap- and many other successful lineages arose were simply fortunate to possess a fa- pearance of something like human con- later than we did. vorable feature, evolved for other rea- sciousness either virtually inevitable or at We will not smash Freud’s pedestal sons, that fostered passage through the least predictable. The pedestal is not and complete Darwin’s revolution until impact and its sequelae. smashed until we abandon progress or we find, grasp and accept another way of Second, we all know that dinosaurs complexification as a central principle drawing life’s history. J.B.S. Haldane perished in the end Cretaceous event and and come to entertain the strong possi- proclaimed nature “queerer than we can that mammals therefore rule the verte- bility that H. sapiens is but a tiny, late- suppose,” but these limits may only be brate world today. Most people assume arising twig on life’s enormously ar- socially imposed conceptual locks rather that mammals prevailed in these tough borescent bush—a small bud that would then inherent restrictions of our neurol- times for some reason of general superi- almost surely not appear a second time if ogy. New icons might break the locks. ority over dinosaurs. But such a conclu- we could replant the bush from seed and Trees—or rather copiously and luxuri- sion seems most unlikely. Mammals and let it grow again. antly branching bushes—rather than lad- dinosaurs had coexisted for 100 million ders and sequences hold the key to this years, and mammals had remained rat- Parochial Evolution conceptual transition. sized or smaller, making no evolutionary PRIMATESAREVISUALANIMALS, We must learn to depict the full range “move” to oust dinosaurs. No good ar- and the pictures we draw betray our of variation, not just our parochial per- gument for mammalian prevalence by deepest convictions and display our cur- ception of the tiny right tail of most com- general superiority has ever been ad- rent conceptual limitations. Artists have plex creatures. We must recognize that vanced, and fortuity seems far more like- always painted the history of fossil life this tree may have contained a maximal ly. As one plausible argument, mammals as a sequence from invertebrates, to fish- number of branches near the beginning may have survived partly as a result of es, to early terrestrial amphibians and of multicellular life and that subsequent their small size (with much larger, and reptiles, to dinosaurs, to mammals and, history is for the most part a process of therefore extinction-resistant, popula- finally, to humans. There are no excep- elimination and lucky survivorship of a tions as a consequence, and less ecologi- tions; all sequences painted since the in- few, rather than continuous flowering, cal specialization with more places to hide, ception of this genre in the 1850s follow progress and expansion of a growing so to speak). Small size may not have been the convention. multitude. We must understand that lit- a positive mammalian adaptation at all, Yet we never stop to recognize the al- tle twigs are contingent nubbins, not pre- but more a sign of inability ever to pene- most absurd biases coded into this uni- dictable goals of the massive bush be- trate the dominant domain of dinosaurs. versal mode. No scene ever shows an- neath. We must remember the greatest of Yet this “negative” feature of normal other invertebrate after fishes evolved, all biblical statements about wisdom: times may be the key reason for mamma- but invertebrates did not go away or stop “She is a tree of life to them that lay hold lian survival and a prerequisite to my writ- evolving! After terrestrial reptiles emerge, upon her; and happy is every one that re- ing and your reading this article today. no subsequent scene ever shows a fish taineth her.” Sigmund Freud often remarked that great revolutions in the history of science MORE TO EXPLORE have but one common, and ironic, fea- Extinction: A Scientific American Book. Steven M. Stanley. W. H. Freeman and Company, 1987. ture: they knock human arrogance off Wonderful Life: The Burgess Shale and the Nature of History. S. J. Gould. W. W. Norton, 1989. one pedestal after another of our previous The Book of Life. Edited by Stephen Jay Gould. W. W. Norton, 1993. conviction about our own self-impor- The Structure of Evolutionary Theory. Stephen Jay Gould. Harvard University Press, 2002.